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  The
  Destinies of the Stars

  By
  Svante Arrhenius, Ph.D.

  President, Nobel Institute, Stockholm, Sweden
  (Recipient of the Nobel Prize in Chemistry, 1903)

  Authorized Translation from the Swedish
  By
  J. E. Fries
  Fellow A.I.E.E.

  _Illustrated_

  G. P. Putnam’s Sons
  New York and London
  The Knickerbocker Press
  1918




  COPYRIGHT, 1918
  BY
  G. P. PUTNAM’S SONS

  The Knickerbocker Press, New York




TRANSLATOR’S PREFACE


When Dr. Svante Arrhenius in the year 1903 received the NOBEL PRIZE in
Chemistry it came as a fitting reward of his achievements principally
in the electro-chemical field. It was natural, however, that a genius
of his calibre would not limit his interest to the “infinitely
small” but would gradually broaden it to encompass the “infinitely
large.” And “to take an interest” means with Dr. Arrhenius to push
the boundaries of the unknown and of the unexplored a little farther
away from man. His evolution in this respect runs parallel with that
of all the great men who stand out as leaders in the history of
science. Wrapt up in the solution of a particular problem and fired
with the divine yearning to reach ultimate causes they are inevitably
driven to ever widening circles of research until this whole material
universe, its whence and whither, becomes the overpowering passion
of their spirits. Thus the mere titles of the works of Dr. Arrhenius,
read in the sequence of their publication, give us, better than any
biography, the history of a soul, which, no matter what his unprofessed
philosophical faith may be, constitutes our strongest evidence in
favour of that theory of “purposiveness” in the universe which Dr.
Arrhenius so heartily abhors (and justly so) when resorted to in
natural science, but which theory nevertheless (and justly so) is so
dear to the philosopher:--_Researches in Regard to the Conductivity of
Electrolytes; Conductivity of Extremely Diluted Solutions; Chemical
Theory of Electrolytes; Textbook in Theoretical Electro-Chemistry;
Textbook in Cosmological Physics; Worlds in the Making; Infinity of the
Universe; Life of the Universe as Conceived by Man from Earliest Ages
to the Present Time_;--thus run the titles of a few of the works we
already have by Dr. Arrhenius’ hand. How were it possible for him NOT
to write _The Destinies of the Stars_? The volume came as inevitably
as fruition follows flowering. What remains to be seen is if Dr.
Arrhenius can withstand the tremendous temptation that must be at work
in his soul to lift, be it ever so little, the curtain that separates
natural science and philosophy; we hope he will give in; we admire in
this book how he reads “The Riddle of the Milky Way”; we certainly wish
to know how he reads--the riddle of the universe.

_The Destinies of the Stars_ met with unexampled success in Sweden.
Three editions appeared within two months when the book was published
in November, 1915. The American version has been somewhat delayed
principally due to war conditions. This, however, has not been wholly
a loss to the English-speaking world as Dr. Arrhenius by no means has
been idle in the meantime. Considerable additional subject matter,
including three new pictures, has been added, chiefly based on the most
recent astronomical discoveries some of which have been recorded as
late as 1917.

For valuable suggestions and for all the American equivalents of the
metric measures in the original, the reader as well as the translator,
is indebted to a member of the Publishing House that presents this
volume in such an attractive way, Mr. E. W. Putnam, himself an ardent
lover of astronomy and a writer on the subject.

Dr. Arrhenius is justly renowned for his lucid style and polished form.
All that is lacking in these qualities within the covers of this volume
is wholly due to the deficiencies of the translator, who however could
not resist the temptation of transferring to Anglo-Saxon soil this
monument to the genius of his former teacher, Dr. Svante Arrhenius.

                    J. E. FRIES.

  BIRMINGHAM, ALA., December 15, 1917.




PREFACE


Since I presented _Worlds in the Making_ and _Life of the Universe
as Conceived by Man from Earliest Ages to the Present Time_ to the
public--which received them with far greater interest and appreciation
than I could foresee--I have had repeated occasions to treat new
questions of a cosmological nature, questions largely arisen from
new discoveries and observations within the scope of astronomy. Vast
new vistas have been opened through the study of the relation of the
stars to the “Milky Way” and through observations of our neighbour
planets. The last mentioned give plain indications of the course of
planetary evolution and thus enable us to surmise the changing fate and
future position of the Earth. In an earlier German publication, _Das
Schicksal der Planeten_ (1911), I dealt with some of these problems.
As, further, the evolution of the solar system from the Milky Way
nebula, to which I have devoted several lectures at home and abroad,
may be considered as the pre-history of the evolution of the planets, I
have given this collection of cosmogenic articles the common title _The
Destinies of the Stars_. I offer as introduction a lecture delivered
before the Fourth International Philosophical Congress in Bologna,
1911, dealing with the “Origin of Star-Worship.”

Hoping that this little book will, to a considerable extent, fill the
gaps in my previous works, I present these treatises in remodelled form.

                    SVANTE ARRHENIUS.

  STOCKHOLM,
    _November, 1915_.




CONTENTS


  CHAPTER I
                                                                    PAGE
  ORIGIN OF STAR-WORSHIP                                               1

  Position and practical value of astronomy. Worship of stars.
      Chronology. The Australian negro’s conception of the stars.
      Day and night, summer and winter. Solar year. Sun-worship.
      Changing phases of the moon in chronology. The Mexican
      “Tonalamatl.” Moon-worship in Mesopotamia. Significance of
      the moon in astrology. The sun and the heat. Agriculture’s
      demand on chronology. Worship of the planet Venus by the
      Mexicans and the Babylonians. The Church Calendar. The
      Zodiac. The seven planets. The week. Correspondence and
      sympathetic magic. The Platonic-Aristotelian philosophy.
      Astrology and alchemy. Tycho Brahe. Occult sciences.
      Aristarchos from Samos. Kopernicus. The progress of
      Astronomy.


  CHAPTER II

  THE MYSTERY OF THE MILKY WAY                                        41

  Primitive conceptions of the Milky Way. Anaxagoras and
      Demokritos. Ptolemaios. Galilei. Cosmogenic speculations.
      Wm. Herschel’s statistical researches regarding the
      distribution of the stars. The Milky Way as the foundation
      of the stellar system. The Milky Way as a nebula.
      Classification according to age of the stars, their
      distribution and velocity. Motion in the Orion nebula.
      The planetary nebulæ. Kapteyn’s “star-drifts.” The origin
      of the Milky Way. Comparison between the Milky Way and a
      spiral nebula in “The Dogs of Orion.” A few details from
      the Milky Way. The infinitely great and the infinitely
      small. The magnitude and destiny of the Milky Way.


  CHAPTER III

  THE CLIMATIC IMPORTANCE OF WATER VAPOUR                             84

  The four elements of Aristotle. Humid-warm climates. The Congo
      and Amazon basins. The carboniferous age. The effect
      of cloudiness. Desert climate. Steppes. “Kevirs” and
      “Bayirs.” Sand dunes. The great Kevir. Climatic changes.
      Khanikoff’s description. Salt lakes. Deposits of salt
      through evaporation. Huntington about the arefaction of the
      earth. Humid period during the ice-age. Climatic changes
      during historic time. Africa, Asia, Greece, Italy, Sicily.
      West-Europe’s climate has grown more marine. Present
      conditions.


  CHAPTER IV

  ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES                       119

  Outer envelope of the stars. The large planets. Spectra. Mars,
      Earth, Venus, Mercury. Atmosphere impossible on the moon.
      The light from the Earth. The atmosphere of Mercury. The
      atmosphere of Venus and its Clouds. Composition of air and
      its change with height. Forced circulation. Troposphere
      and Stratosphere. Hydrogen in the highest strata of the
      atmosphere. Water vapour and carbonic acid in the air.
      “Geokoronium.” Influence of gravity on composition of the
      atmosphere. The air on Venus and Mars.


  CHAPTER V

  THE CHEMISTRY OF THE ATMOSPHERE                                    155

  Inhabited Worlds. Kinship of the stellar bodies. Presence
      of life. Importance of water and carbon. Importance of
      temperature. All life evolved from existence in water.
      Necessity of oxygen. Bacteria. Reducing substances
      preponderable in the World-matter. Volcanic gases and gases
      in solidified lava. Water, vapour, carbon dioxide, nitrogen
      gas, and sulphurous acid. Permanent gases and hydrogen. The
      poisonous character of the original air. Its purification.
      The importance of plant life and necessity of a solid crust
      in this process. Supply of carbonic dioxide and production
      of oxygen. The work of Koene. Silica. Cooling of the Earth
      and changing surface temperature. The ice-periods. Centres
      of collapse and lines of fissure in the crust. General
      survey of the gradual change of the air.


  CHAPTER VI

  THE PLANET MARS                                                    180

  The controversy about the habitableness of Mars. Humidity on
      Mars. Early observations. The spectra of Mars and of the
      Moon compared. Investigations by Campbell and Marchand. The
      work of Lowell. Measurements by Slipher. Calculations by
      Very. The temperature on Mars according to these sources.
      Campbell’s expedition to Mount Whitney in California.
      Oxygen on Mars. The cold on Mars detrimental to anything
      but the lowest forms of life. Cause of different results
      by Slipher and Campbell. Very’s answer to Campbell’s
      criticism. New measurements by Slipher. Campbell’s new
      method of measurement of 1910. Christiansen calculates
      the temperature on Mars from intensity of the Sun’s
      radiation. The sun-constant. Average temperature of Mars
      about forty degrees Centigrade. Possibly low plant life
      around the poles during summer. The canals on Mars are
      probably fissures in the crust. The length of the canals
      compared with that of the fissures in Earth’s crust.
      The double canals on Mars compared with the parallel
      fissures in Calabria. Emanations along the fissures. The
      canals as affected by increasing cold or heat. The polar
      snow. Thawing of the canals. Travel of the water vapour
      independent of the topography. The desert sand on Mars.
      Clouds and mists. Highlands and mountains on Mars. Sand
      filling of the canals. The seas on Mars. The straightness
      and uniform breadth of the canals an illusion. Light and
      dark spots. “New” canals. The fancies of Lowell.


  CHAPTER VII

  MERCURY, THE MOON, AND VENUS                                       228

  Fissures on Mercury. Lowell’s drawing. Centres of collapse.
      Absence of atmosphere. The climate on the Moon. W.
      Pickering’s belief in frost formations on the Moon. The
      mountains on the Moon. Volcanoes. Circular elevated rings.
      “Seas” on the Moon. The Crater “Linné.” “Sinuses” and
      “Streaks” on the Moon. The light matter of the streaks
      probably lava-scum. The colour of the Moon and of the
      Earth. Comparison between the Moon and Mars. Changes on
      the Moon. “Snow” and “Vegetation” on the Moon according to
      W. Pickering. The fate of Mars and of the Earth. Falling
      meteoric dust. The climate on Venus. Swamps like those of
      the carboniferous age. Abundant vegetation. Low organisms.
      “Culture” on Venus will proceed from the poles. The future
      of Venus. The claims of astrology in modern light. Tycho
      Brahe. The dreams of Giordano Bruno probably true.




ILLUSTRATIONS


                                                                    PAGE
  FIG. 1.--THE MILKY WAY                                              46
              Photo by Easton.

  FIG. 2.--PLANETARY NEBULA NO. 7009, NEW GENERAL CATALOGUE           52

  FIG. 3.--NEBULA NO. 4594, NEW GENERAL CATALOGUE                     66

  FIG. 4.--NEBULA NO. 101 IN MESSIER’S CATALOGUE                      70

  FIG. 5.--NEBULA NO. 51 IN MESSIER’S CATALOGUE                       71

  FIG. 6.--MILKY WAY BETWEEN CASSIOPEIA AND SWAN                      76
              According to Wolf.

  FIG. 7.--MILKY WAY IN EAGLE AND SAGITTARIUS                         76
              According to Wolf.

  FIG. 8.--THE “TRIFID” HOLE IN EAGLE                                 77
              According to Wolf.

  FIG. 9.--TARIM RIVER WITH LAKES AND BAYIRS                          96
              According to Sven Hedin.

  FIG. 10.--THE FORMER LAKE BONNEVILLE IN UTAH                       106

  FIG. 11.--JUPITER, 1909                                            122
              According to F. le Coultre.

  FIG. 12.--SATURN, 1909                                             123
              According to F. le Coultre.

  FIG. 13.--SPECTRA OF THE MAJOR PLANETS                             124
              According to V. M. Slipher.

  FIG. 14.--VENUS OBSERVED BY LANGLEY IN 1882                        136

  FIG. 15.--ROCK FISSURE AT HANGO, SWEDEN                            204
              Photo by I. I. Sederholm.

  FIG. 16.--EARTHQUAKE CENTERS IN CALABRIA                           205
              According to I. I. Sederholm.

  FIG. 17.--MAPS OF MARS BY SCHIAPARELLI AND ANTONIADI      _At the end_

  FIG. 17a.--MAPS OF MARS BY SCHIAPARELLI AND ANTONIADI     _At the end_

  FIG. 18.--PHOTO OF MARS BY LAMPLAND                                210

  FIG. 19.--MARS ON APRIL 8, 1909                                    210
              According to Quenisset.

  FIG. 20.--THE SOUTH POLE SPOT ON MARS, 1909                        210
              According to Jarry Desloges.

  FIG. 21.--SANDSTORM ON MARS, 1909                                  210
              According to Antoniadi.

  FIG. 22.--CLOUD ON EDGE OF MARS, THE 7TH OF MARCH, 1901            216
              According to Molesworth.

  FIG. 23.--MARS, JULY 11, 1907                                      216
              According to Lowell.

  FIG. 24.--MARS, OCTOBER 6, 1909                                    216
              According to Antoniadi.

  FIG. 25.--MERCURY                                                  230
              According to Lowell.

  FIG. 26.--THE MOON NEAR THE CRATER TYCHO                           230
              Photo from Yerkes Observatory.

  FIG. 27.--MARE SERENITATIS AND MARE TRANQUILLITATIS                231
              Photo from Yerkes Observatory.

  FIG. 28.--THE MOON NEAR ITS SOUTH POLE                             240
              Photo from Yerkes Observatory.

  FIG. 29.--THE LUNAR CRATER COPERNICUS AND VICINITY                 242
              Photo from Yerkes Observatory.




The Destinies of the Stars




CHAPTER I

THE ORIGIN OF STAR-WORSHIP


Astronomy occupies a rather unique position among the natural
sciences. While physics, chemistry, and the biological sciences form
the foundation of the extraordinary material development of our day,
astronomy, in the eyes of most people, is of little practical value.
What benefit could we derive from knowing whether a star lies a hundred
or a thousand billion miles from the Sun, or from understanding how the
stellar bodies have evolved in the course of billions of years? And yet
astronomy has not been as futile as is commonly imagined, neither is
it useless at the present time. This science is still of the greatest
importance to common life by fixing our standards of time and, before
the introduction of the compass, was invaluable also to navigation,
which art, moreover, depends now upon astronomy for determination
of geographical position on the open sea. Observations for these
purposes, however, are of such a simple nature, that they hardly fall
under the astronomical science proper, but rather under the applied
sciences. They have entered into daily life much as, in commerce, the
determination of the weight of a body is not considered as belonging to
the science of physics, although it depends on the use of a physical
instrument, the scale.

But we must not forget that what we now consider so commonplace that
it entirely has lost the grand aspect of science, once was the goal of
groping scientific endeavour. All natural science has grown out of the
needs of practical life.

Geometry is probably even older than astronomy. The name means: to
measure land, and the oldest geometry was, accordingly, devoted to the
measurement of distances on the Earth and later to the determination
of the area of land-holdings. This extremely important practical
application of geometry is of such a simple nature that it is not
mentioned in modern mathematical science, to which geometry belongs.
In this manner, the original theses of all our natural sciences have
become the possession of the public to such a degree that they are
looked upon as self-evident. This is the case also with those parts of
astronomy which, because of their practical importance at the outset,
gave rise to the science itself.

The growing knowledge about the stars, like all higher insight, became
among primitive peoples the private possession of their leaders, was by
these kept a secret and made a part of the venerable realm of religion.
We find that a majority of these old peoples rendered worship to the
stars, believing them to govern the fates of human beings. This may
indeed seem highly remarkable, as our everyday experience is that the
stellar bodies, with the exception of the Sun, exert no perceptible
influence on organic nature, and such conclusion is emphatically
confirmed by the systematic collection of all our experiences which we
call modern science. The Sun, as stated, is the exception as it reigns
over the entire nature, the living as well as the lifeless, by virtue
of the heat and light which abundantly flow from this autocrat of our
planetary system. Perhaps the Moon also plays some small active part as
it seems somewhat to affect the barometric pressure, the magnetic and
particularly the electric conditions of the atmosphere, which, in turn,
appear to influence several life processes. On the other hand we cannot
point to any influence upon nature traceable to the other stellar
bodies.

Obviously, primitive man devoted his thoughts only to such conditions
as affected his interests in a beneficial or detrimental way. On
the assumption that these conditions were governed by spirits who
resembled man and who in particular were endowed with will, our
ancestors endeavoured by sacrifice and exorcism to move the spirits
they feared to discontinue their harmful activity. Some such spirits
dwelt in beasts of prey and in other noxious animals, such as poisonous
snakes; others caused earthquakes, volcanic eruptions, snowstorms,
lightning ravages, fires, floods, heat, drought, etc. Against these
calamities religious exercises formed a protection. Religion sprang
mainly from fear of spirits. Later on thank offerings and hymns were
bestowed upon benevolent objects and phenomena in nature.

It is evident that this primitive, simple religion is of far greater
age than star-worship. The latter presupposes a comparatively high
degree of culture. The stars were of no benefit to man until it became
necessary to measure time intervals comprising a greater number of
days than might be counted on one’s fingers. How this growth in
all probability took place we shall endeavour to explain in the
following. Fairly certain it is that star-worship did not grow out
of man’s admiration for the sublime drama which at dawn of morning
commences at the eastern horizon, and proceeds in the course of a day
over the firmament in order to close before night just beyond the
western expanses, neither was it founded on man’s gratitude toward the
torch-bearers of night for their incessant battle against gloomy clouds
and all the other spirits of darkness.

Even tribes on a rather low stage have no doubt noticed the most
conspicuous among the stellar bodies. The attitude of the Australian
aborigines is significant in this connection. According to Spencer and
Gillen they possess legends about the Moon, which is male, about the
Sun and Venus, about the pernicious Magellanic clouds, and about the
Pleiades, who, like the Sun and Venus, are considered female. Eclipses
have, as is natural, attracted the greatest attention. While these
primitive men indulge in an incredible number of religious ceremonies
pertaining to conditions of their daily life, none of these exercises
are devoted to the stars, if we except stone throwing against the Sun
during eclipses. Even this treatment is left with considerable serenity
of mind for the medicine-men to perform. It is very significant that
all stellar bodies are of earthly origin, that the Moon is of male
gender while the Sun, Venus, and the Pleiades are female, which
indicates that the Moon is considered of greatest distinction. They
count time according to “sleeps,” _i. e._ the number of times they have
slept, or according to moons; longer periods according to seasons; they
have names for summer and winter. They can count to five, or perhaps
rather to four, as the term five also means “many.” Ideas of power
centred in the stellar bodies are evidently absent and therefore also
religious ceremonies pertaining thereto. A few tales exist relating to
the stars as to other objects within their observation. Thus conditions
would undoubtedly have remained but for the high value which the want
of a chronometry gave to the regularly changing light of the stellar
bodies.

The difference between day and night is of such a deeply fundamental
importance that it has left its stamp on the whole organic nature on
the surface of the Earth. Plants entirely change their life processes
in the course of twenty-four hours; during the day they add to their
growth under the stimulus of light; during the night they partly
expend the strength gathered in daytime. This cycle is so regular
that it functions automatically. The prominent botanist, Pfeffer, has
experimented with a Mimosa, which, as is well known, unfolds its leaves
during the day but curls them during the night. If left in a dark room
any arbitrary day, it nevertheless uncurls its leaves. By means of
electric light Pfeffer changed night into day for the plant kept in the
dark room. It took considerable time before the plant adjusted itself
to the new conditions so that it unfolded its leaves to the influence
of the electric light. Animals behave similarly. The night and day
period is instilled in their blood. In a certain sense they possess an
instinctive chronometry.

It is often stated that the assurance of the return of sunlight after
the darkness of night enabled humanity with greater equanimity to
acquiesce in the loss of light during one half of its existence and
that worship was rendered to the Sun in gratitude therefor. “A new
outlook upon life,” says Troels Lund, “awakens the moment the great
discovery is made that the night of sleep and the night of fear are
equally long and always followed by morning and subsequent day.” This
discovery, however, our predecessors made long before they reached the
human stage. Sun-worship by no means derived its origin therefrom.

Rather it is traceable to evidence of the Sun’s connection with
the changing seasons, although this change also is of domineering
influence in the vegetable world inasmuch as the plants store reserve
nourishment in the autumn, particularly, and on a large scale, during
fructification. Also, lower and higher animals, for example, bees and
squirrels, gather winter-stores. It is therefore small wonder if men on
a comparatively low stage lay in provisions for the recurring periods
of scant food supply.

But a true chronometry beyond five days is foreign to the Australian
negro as long as he can only count to four or five. He is aware that
Moon-phases reappear and that summer and winter return, but he has no
conception of the duration of the time passed between the recurrences
of these phenomena. Further progress was made by the people who took
the bold step to count the fingers, not on one hand alone, but on both
hands, and thus reached the number ten. This was utilized in reckoning
time so that the larger unit became a decade, _i. e._ ten days and
nights. This unit was original with the Indo-Europeans, Semites,
Hindus, Egyptians, and the islanders in the Pacific Ocean. Another
advance yet was made in Mexico where the number twenty was introduced
corresponding to the sum of all fingers and toes, and thus a unit of
time was obtained comprising twenty days and nights. But to rise from
these units to one of 365 days was a step exceedingly difficult for the
primitive peoples to take.

Thousands of years elapsed before the most intelligent among the
races established the length of the solar year. Those who lived in
regions where the Sun’s altitude notably changes, _i. e._ far from
the equator, undoubtedly reckoned time in years, without knowing its
length expressed in days. Imagine a nomadic tribe like the Lapps in
the north of Sweden. In the autumn their reindeer wander down toward
the coast in search of food and the Lapps go with them. In spring the
reindeer lead their masters back again to the mountains. It cannot very
well escape the observation of these nomads that the Sun shines almost
continuously during their stay in the uplands while dreary night reigns
nearly without interruption during their sojourn in the lowlands. They
are obviously forced to co-ordinate the beautiful summer with the
duration of sunlight. To them, therefore, the Sun’s extraordinary great
importance to life is unquestionable. The same holds true about all
people who live sufficiently far from the equator. As a consequence,
they become Sun-worshippers. It is not difficult to find examples of
peoples who have worshipped the Sun; only a few of the more important
ones will be mentioned here.

The people of the bronze age here in the North were zealous
Sun-worshippers thousands of years ago, as the many relics from this
period, and particularly the rock-carvings, bear witness. The Celts of
Western Europe have frequently symbolized the Sun as a cross, while the
worship of Moon and stars seems to have been foreign to them as well
as to the bronze-age people of the North. The Jewish Samson (Simson)
was a Sun-hero, the name being related to the Babylonian Shamash, the
Sun-god. In Hesiodos’ cosmogony the Sun (Helios) is mentioned before
the Moon (Selene). The old Germans worshipped both Sun and Moon,
particularly the former. The Slavs possessed a Sun-god Dazbogu, but no
deity identified with the Moon. Similar conditions obtained among the
Finnish forefathers. The Chinese Tao-priests light fires during the
vernal equinox as we do at Walpurgis and midsummer, and they sacrifice
rice and salt to the flames. “This is a remnant of the Sun-cult,”
says Solomon Reinach, from whom these data in regard to Sun-worship
principally are taken. In Japan, the Moon is of male sex, the Sun of
female sex, which indicates that there, as with the Australian negroes,
the Moon was originally considered more important than the Sun.
Nevertheless the Japanese are now Sun-worshippers; the Sun is placed
as emblem of the most high in their flag and the Mikado is known to
trace his lineage from the Sun. They have, therefore, long ago passed
from Moon-cult to Sun-cult. It is probable that this step was taken
even earlier in China, where the Sun furthermore is of male sex.
With growing civilization all people learn to understand, as have the
Japanese, the superior importance of the Sun. The Incas of Peru, who
reached a very high grade of culture, were sun-worshippers and called
themselves children of the Sun, although they lived near the equator
where the Moon-cult, as we presently shall see, owns its most faithful
adherents.

In the neighbourhood of the equator, winter and summer differ very
slightly with respect to temperature and altitude of the Sun. Rather,
it is the alternation of humid and dry seasons that is of deciding
importance. No sheet of snow covers the ground in winter-time, kills
the vegetation, or decimates the supply of nourishment for animals
and men. Indeed, contrary to our experience, a suppression of growth
often accompanies a high altitude of the Sun due to the drought
which simultaneously occurs. The altitude and luminosity of the Sun
change altogether too slightly in the course of the year to attract
the attention of primitive man. The light of the Moon, on the other
hand, varies from full intensity to nothing and this takes place in
periods so short that memory has no time to forget the cycle. Even
the low-standing Australian negroes utilize the phases of the Moon to
denote remote times. Chronometry in any true sense they do not possess,
unable as they are to state the number of days in a month. How much
more fortunate the peoples who could count to ten or better yet to
twenty and thus were able to use the single or the double decade as a
measure of time. For them it was easy to determine the time between two
successive phases of the Moon, seven and a half days apart.

The truth once grasped that four phases separate two new Moons the
important bridge could be established between the short measure, a day,
and the longer measure, a month. The latter was then found to be nearly
thirty times longer than the former. On a higher stage of culture, it
was discovered that this ratio was not exact and the discrepancy must
greatly have puzzled the people. The correct ratio is 29.53. At all
events the periodic return of full Moon and new Moon proved the most
reliable measure of time within their experience. This was something
entirely different from the irregular occurrences of earthquakes,
storms, lightning, and floods, not to mention the ravages of beasts and
foes. Spaces of time that hitherto appeared boundless could be surveyed
and computed. The idea of eternity dawned for the first time on
humanity. The Moon was the great master, measurer of all. In Sanskrit
the Moon’s name is _Mâs_, _i. e._ measurer, and “_mensis_” (Lat. month)
is closely related to “_mensura_” (Lat. measure).

With peoples who did not live too far from the equator, the Moon,
therefore, took precedence of the Sun. Among the Mexicans existed,
long ago, a peculiar unit for measuring long periods of time, called
“_tonalamatl_,” comprising 260 days. It was undoubtedly intended to
contain nine synodical months (reckoned from new Moon to new Moon).
But such a period would consist of 265.58 days and so could not be
made to agree with the double decade and was therefore shortened to
260 days, as we round off the solar year, in reality 365.24 days, to
an even 365 days. Elaborate studies have been made in order to explain
why the Mexicans chose nine synodical months instead of twelve, as
most other peoples, but the question has not been solved. This much is
certain, that “_tonalamatl_” has nothing to do with the solar year,
but only with the month. The high age of “_tonalamatl_” is proved by
the fact that the priests adhered to it in magic and horoscope casting
long after the solar year came into public use. A learned Mexican, de
Jonghe, has pointed out that “_tonalamatl_” was used by all the tribes
belonging to the Nahua-group, which tribes separated very long ago.
This unit of time, therefore, bears all the evidence of a very high
age, but is obviously younger than the synodical month.

Our information in regard to Moon-worship among the peoples of
Mesopotamia is even more explicit. The Moon (Sin) was rendered homage
far earlier than the Sun (Shamash). The following translation of a hymn
in cuneiform letters I quote from L. Bergström, who published a study
on “Semitic Moon-Worship” in _Nordisk Tidskrift_, 1909:

    Oh, Sin, thou who alone givest light,
    Thou, who bringest light to men,
    Thou, who showest favour to the dark-tressed ones,
    Thy light shines on the firmament,
    Thy torch illuminates like fire,
    Thy radiance fills the wide earth.
    Oh, heavenly Anu, whose insight and wisdom no one comprehends,
    Thy light is splendid as Shamash, thy firstborn,
    Before thee prostrate the great gods themselves in the dust
    For on thee rests the fate of the world.

Anu was god of the heaven and seems here to stand for god in general.
Sin was father to the daughter Shamash, who in this hymn already is
considered almost comparable to the father. Later on during the
Hammurabi-dynasty (about 2000 B.C.) the Sun, Shamash, was accepted as
supreme god, but the Moon remained the regulator of time for religious
purposes. For astrological forecasts the priests preferred to use the
Moon, and the “signs” in the Moon were the most important. This is true
of astrological prophecies also at the time of Tycho Brahe. “Oh, Sin,
thou tellest the oracles to the gods who pray them of thee,” reads an
incantation. From Babylon, the heart of civilization, Moon-worship
spread to Arabs and other Semites, and with the Hebrews, as Bergström
remarks, the Moon originally played a far greater part than the
Sun, although at the time of Christ the condition was reversed.
Nevertheless, the Moon has still retained its position as chronographer
in the Church Calendar. In Psalms 104:19 we read: “He appointed the
Moon for seasons.”

In general, we have built on the belief that the stellar bodies made
an impression on men by virtue of the light they radiate and it has
therefore been difficult to explain why the Moon ranked above the Sun.
It is frequently said that the Sun (in Babylonia) was considered an
enemy of mankind as its heat destroyed the vegetation. (It is true that
a period of severe drought occurs there in the height of the summer.)
As opposed to this, the moonlit nights would be considered refreshing
and salutary. Another explanation is essayed by Bergström, who says
that the luminous Moon with its ever-changing shape appealed to the
imagination of primitive people in a far higher degree than the fairly
constantly brilliant Sun. He is partly correct; the enormous variation
in luminosity from full to new Moon enables the observer to notice
the difference from night to night. The change by about one hour in
each twenty-four hours of the time for the rising and setting of the
Moon as compared to the disappearance and reappearance of sunlight,
and more important yet the short periods of the Moon phases which
leave the observations from each receding phase fresh in memory,--both
these phenomena contribute toward the high value of the Moon’s synodic
revolution as a measure of protracted time duration. The purely
practical need dominates, not, probably, any desire to visualize the
changes in legends. The Australian negroes use four different names
for the Moon according to its four quarters, showing that they, in all
probability, believe themselves confronted with four different stellar
bodies, just as the Greeks at the time of Homer considered the morning-
and the evening-star, _i. e._ Venus, as two separate planets.

On the other hand, no ground worth mentioning supports the theory that
the scorching heat of the Sun diminished the peoples’ inclination
for its worship. On the contrary, homage was generally rendered to
phenomena one feared. It is further not true that the Babylonians
themselves considered the Sun, Shamash, hostile, the Moon, Sin,
friendly. The Sun-god, Shamash, by virtue of his light, was believed
to give life and health. The scorching quality of the Sun was
attributed to another god, Nergal, prince of the underworld, demon of
war and slaughter, source of fever, and, pre-eminently, of plague. No
reason existed, therefore, why Shamash should take second rank after
Sin, who is said “to carry water and fire,” meaning, according to
Schrader, that he brought fits of ague and fever. When the Sun after
the oppressive day sinks behind the horizon, it is well known that
a sharp fall of temperature occurs, particularly in arid zones, but
also in humid regions within the tropics where this very phenomenon
is utilized for the production of ice. He who exposes himself to the
sudden cold of the night falls an easy prey to illness. Particularly
is this true under a clear sky--primitive people say “when the Moon
shines”--because of the strong radiation. Those who sleep in moonlight
are struck by delirium and madness according to primitive thought, an
opinion by no means dead among civilized nations--it is common with
seafaring men--and is no doubt the origin of the expression: moonstruck
(German: _mondsuchtig_, Ital.: _lunatico_, French: _lunatique_, Swedish:
_månadsrasande_, etc.). To this belief has probably contributed
the fact that epileptic fits frequently possess a period nearly
corresponding to the synodical month, which, as I have shown elsewhere,
most likely depends on a periodic change in the atmospheric electricity.

In this connection it might also be stated that the third among the
great star-deities, Ishtar, the queen of heaven (Astarte, Venus),
was the mild but potent, all-merciful sister in every affliction,
who delivered from sorcery and illness and gave pardon for sin and
guilt. This radiant goddess, who corresponds to the attractive figure
of the Catholics’ Virgin Mary, was, in spite of her solicitude for
the afflictions of men, placed third in the illustrious triad, Sin,
Shamash, and Ishtar.

The traveller in the desert is certainly often tortured by the severe
drought and a consequent insufferable thirst. This, however, was
justly ascribed to the scarcity of water and not to the Sun. The
Egyptians, therefore, wished that their departed ones on the journey to
their new dwelling-places would meet with refreshing springs where they
might quench their thirst and northerly winds that might cool the air.
It is well known that the Mohammedans have formed similar conceptions
of a paradise in life to come.

Entirely new conditions arose when the population grew until
agriculture became a necessity for the production of sufficient food.
The influence of the Sun now became so dominating that it must be given
the first place among the powers that affect the fates of men. The
plants have a decided annual period and so have the overflows of the
rivers which were of the utmost importance in those countries where
the cradle of civilization stood. The rainfalls themselves were of
annual recurrence as were also the intervening droughts. In Egypt the
great importance of the floods caused the introduction of the solar
year at a very early stage and its length was fixed at 12 months of 30
days, or 360 days, so that the beginning of the year occasionally had
to be adjusted. This was accomplished by resorting to observations of
the rising and setting of the Dog Star or Sirius. Thus we realize how
difficult it was to determine the exact length of the solar year from
everyday phenomena. The great reformer Amenhotep IV. endeavoured about
1400 B.C. to have the Sun-god recognized as sole master of all the
world. He met, however, with such great obstacles on the part of the
conservative priesthood, which as they largely were serving different
gods would have lost part of their power if the reform had prevailed,
that his successor was obliged to yield to the solid opposition.

In Babylon the local god Marduk, once representative of the planet
Jupiter, and among the star-gods ranking next to the three Super-gods,
became about 2000 B.C. elevated to the highest position among the gods
and assumed at the same time the dignity of Sun-god. It may here be
mentioned that Marduk also played a great part as healer of illness.
The evolution in ancient Rome followed the same course although at a
far later time. Emperor Aurelianus (270–275 A.D.) under the influence
of oriental Mithras-cult elevated the Sun-god to supreme god of the
whole Roman Empire, which then comprised almost the entire known world.

Especially significant is the fact that Venus with the Mexicans played
as important a part as did the Moon and the Sun. The luminosity of
Venus, unlike that of planets outside of the earth’s orbit, but like
that of the Moon, changes from a maximum, intense enough in the
tropics to throw a shadow, down to a minimum which approaches complete
darkness. Its period is closely 1.6 years. It falls short of this
figure by about two hours and the Mexicans therefore introduced a
correction similar to the bissextile day in our leap year, of one day
in every twelve years, which day, however, had to be deducted instead
of added. Observations of the changing luminosity of Venus and of its
position relative to the Sun obviously lent themselves admirably to
measurements of long periods of time and particularly to determination
of the length of the important solar year as five Venus-periods very
nearly equal eight solar years. The Mexican priests established the
fact that 104 solar years correspond to 65 Venus-periods or 146
“_Tonalamatl_.”

Star-cult was as strongly developed in Mexico as in Babylon. Its
main doctrine is stated by Alfredo Chavero thus: “The Father-Creator
was Heaven, Xiuhtecutli, or the Azure-blue master. The mother was
Omecihuatl, the Milky Way, or the dual mistress.” It is well known that
a large part of the Milky Way, from the “Swan” to the neighbourhood of
the Southern Cross, is divided in two parallel branches, which fact
probably is responsible for the title, the “Dual Mistress.” “Heaven
influenced the Milky Way through fire; from its cosmic matter the stars
were set free, the most prominent of which were Tonatiuh, the Sun;
Tezcatlipoca, the Moon; and Quetzalcoatl, Venus. These were made the
supreme gods. For the purpose of worship, they were symbolized in human
shape. Myriads of images, representing these star-gods, were modelled
in clay, wood, or stone.”

According to this remarkable picture the Mexicans should have
arrived at a far better solution of the world-riddle than even the
Babylonians did. While most other peoples assumed heaven and earth as
the original principles, they gave the high position of progenitress
of all to the Milky Way. From her the innumerable stars, with the
Sun in the lead, issued. This agrees to a considerable extent with
the present conception which we have arrived at during this very
last decade, principally thanks to the work of American astronomers.
Their investigations have shown how the stars are segregated from
the nebulous primeval matter of the Milky Way; how they add distance
between themselves and their matrix with age, while simultaneously
they develop an increasing individuality.

We have seen that Venus-Ishtar was honoured with membership in
the august triad of star-gods, also among the Babylonians. Their
successors, the Assyrians, retained the inherited traditions. Thus
their kings in the ninth century before Christ symbolized their divine
lineage by wearing a necklace with a moon-crescent in the middle, a
cross in a ring, emblem of the Sun, on one side, and on the other a
star, emblem of Venus (compare Montelius, _Nordisk Tidskrift_, 1904, p.
13, fig. 30). The Jewish synagogues are generally decorated with the
star-emblem.

The Mohammedans, like the Jews, utilize the position of the Moon for
determination of the Church calendar, and we employ the same means for
fixing Easter time. The Mohammedans reckon with a year of 12 synodical
months. Twelve such months contain only 354.4 days while a solar year
comprises 365.24 days, and as a rule, therefore, the synodical month
was rounded to 30 days instead of 29.53 and the solar year to 360 days.
Such was the arrangement in Egypt and originally also in Babylon.
Primitive men comprehend fractions only with extreme difficulty. In
order to correct the discrepancies, odd months were introduced about
every sixth year.

From this time we may trace the high reputation of the number twelve.
The Zodiac was divided into twelve houses in each of which the Sun
was to dwell a month at a time. Day and night were each divided into
twelve hours. The circle was divided into 360 degrees corresponding
to the number of days of the year, so that the position of the Sun
at noon should proceed one degree of the heaven from day to day. As
the Moon dominated chronometry, a complication which must have caused
considerable confusion was in many places introduced. We have seen
that the Australian negroes gave four different names to the Moon in
its four different phases. The great change in appearance of the Moon
from quarter to quarter makes such a division natural. The synodical
month was therefore made to consist of four parts, called weeks. As the
length of a month is 29.53 days, the nearest number evenly divisible by
four, namely 28, was substituted, and so seven days were allotted to
each week, thus introducing an error of not less than 5.5%.

To the establishment of this week the assumption of seven wandering
stars has no doubt largely contributed. The priests had discovered
that besides Sun, Moon, and Venus, four other stars shift their
position on the firmament with reference to the fixed stars, which
latter appear always to maintain their relative distances. These four
wandering stars were Mercury, Mars, Jupiter, and Saturn. Each day in
the week was dedicated to one of the seven wandering stars and received
its name. These names have been maintained to the present day, for
instance, Sunday, the Sun’s day, Monday, the Moon’s day, etc. The lunar
calendar, established through religious considerations, supplanted
the more rational one, which latter, however, survived in Egypt, and
was reinstated in the Occident during the French revolution, although
unfortunately only for a short time (1793–1805). As a result, the
synodical month, in order to suit the calendar, has been changed not
only with half a day to thirty days, but also with one and a half days
to twenty-eight days. If decades had been adhered to we would have had
months of even thirty days and either five leap months of thirty-one
days each (during leap year six) or half a decade interpolated at new
year.

Besides the seven wandering stars known to antiquity (at present
over eight hundred planets have been observed), several other stars
and constellations played an important part. The Magellanic clouds,
considered of evil nature, and the Pleiades appealed already to the
Australian negroes. In the northern hemisphere, where the opportunity
of observing the Magellanic clouds is small, situated as they are near
the South Pole, the Pleiades have attracted the greater attention
and the Phœnicians especially appear to have taken interest in this
constellation. From them, reverence for the Pleiades spread to a large
part of Africa, where we now to our surprise find this star cluster
reproduced along with symbols of Sun, Moon, and Venus. Homer also
mentions the Pleiades and a few other constellations, namely, the
Hyades, Orion, the Great Bear, and the stars Sirius and Arcturus. At
all events, the Pleiades have frequently occupied a unique position in
the old world. Sirius, the brightest star in the heavens, and Canopus,
the second in brightness, also belonging to the southern hemisphere
but only half as far removed from the South Pole as Sirius, have both
evoked the attention and the worship of the primitive people, in this
case the South Africans.

At length, the nations, particularly the Babylonians and the Mexicans,
acquired a wider knowledge of the different stars. As the most
important ones, Sun, Moon, and Venus, guided the seasons and hence
all natural phenomena, a certain mundane significance was naturally
ascribed also to the younger ones. Not only seasons, months, days, and
hours were each ruled by its star, but so was everything in nature;
different winds, provinces, trades, bodily organs, animals, persons,
each possessed its star and celestial protector. Comprehensive studies
of these correspondences and connections were made and the conclusions
were drawn from immaterial semblances and often wholly arbitrarily,--as
regards persons from the configuration of the stars at the time of
birth. Thus grew an enormously extensive collection of correspondence-
and sympathy-doctrines accompanied by a detailed symbolism, an entire
quasi-science, which must never be questioned as it originated with
the infallible priests. With the Babylonians, religion and science
completely melted together and even art was entirely subservient to
the same interests. Occasionally the loss of this blissful state draws
forth a sigh. Fortunately it is gone never to return.

The oriental wisdom was brought over to Greece and was there
amalgamated with the Platonic-Aristotelian philosophy. In this form
the Babylonian heritage held sway over the thought of mankind up to
less than 200 years ago. The most important branches of this fanciful,
so-called science were astrology and alchemy. Tycho Brahe himself made
it the object of his life to strengthen astrology by contributing new
material to it. Kepler is said not to have believed in astrology but he
nevertheless cast horoscopes not only for princes and persons of high
position in order to improve his economy, but also for his own family.
Probably traces of the old superstition clung to him, and presumably he
thought: “If it does no good, neither does it do any harm.”

In the same manner, alchemy was carried on by faithful adepts, but more
often by impostors, seldom averse to “occult” sciences. Astrologers and
alchemists exist even yet among the numerous devotees to occultism; at
high price many of them make their predictions or sell their secrets.
I have heard a Swedish engineer of very high standing state that their
prognostications agreed with events. Among the few alchemists in
Europe, most of whom seem to be religious visionaries, Strindberg is of
a certain interest to us. Correspondence-theory has played a very large
part in the speculations of the learned up to comparatively recent
time. It is utilized extensively in the later fantastic writings of
Swedenborg. Numerous traces are to be found also in the weakest works
of Strindberg.

The renowned French chemist, Berthelot, has given a valuable analysis
of the alchemist’s method of treating chemical phenomena. His general
conclusion is that the false principles which led the alchemists astray
revert back to Plato’s and Aristotle’s philosophical theories regarding
the composition of matter. Something similar can be said of astrology.
It plays with ideas of its own fabrication with hardly any foundation
in reality. The result is almost wholly without value.

The greatest astronomer in Babylon, Kidinnu (about 200 B.C.),
constructed tables of great accuracy giving the position of the stars.
In this work he utilized observations gathered over thousands of years.
These ephemerides were also intended as scripture source for reading
the fate of men and for determination of the auspicious moment for
the commencement of an undertaking. At all events, they placed great
revenue and power over souls in the hands of the ruling priesthood. It
does not appear that these priests were able to rise to an attempt of a
physical explanation as to the nature of the stellar bodies. That was
probably also considered dangerous. The stars were deities composed
of purer and more refined matter than found on Earth. It were not
improbable that the gods would inflict vengeance on the presumptuous
one who dared to intrude upon their secrets and pass judgment on their
peculiarities.

Fortunately, there existed in Greece another tendency in philosophy
besides the scholastic and the Platonic-Aristotelian. But this was
mainly represented in southern Italy, Sicily, and later in Alexandria.
Already the followers of Pythagoras had made important progress toward
a solution of the stellar problems. The crowning point was reached by
Aristarchos from Samos, who lived in Alexandria about 2100 years ago.
He established 1700 years before Copernicus the heliocentric system.
It is often said that his work was of little value, as Copernicus
nevertheless must do it over again. It is then forgotten that
Copernicus himself cites the philosophers of antiquity who expressed
theories in agreement with the heliocentric system and expressly states
that he was bold enough to advance his hypotheses because so many
prominent authorities could be mentioned who favoured them. Copernicus
did not dare entirely to break away from the Ptolemaic system, and was
inconsistent enough partly to use it in his calculations of the motions
of the stars.

We have lately advanced farther along the road of Pythagoras and
Aristarchos, of Copernicus and Galileo, and we have perfected their
methods to a high degree. Progress in astronomy and kindred sciences
is nowadays made at a dizzying speed if measured with the standard
of antiquity. Occasionally we hear a warning voice asking us to
show more deference to a philosophy directly descending from the
Platonic-Aristotelian. He who is at all familiar with the history of
natural science will understand us when we answer: “We have had more
than enough thereof.”

That non-naturalists sometimes have a peculiar conception of the
present status of astronomy is well illustrated by the statement of one
of our foremost theologians in a review of a popular astronomical work
where he remarked that the astronomer of today had not advanced much
beyond those of ancient time who also could forecast eclipses of the
Sun. The predictions were then founded on the recurrences of eclipses
after regular intervals much as the new Moons were foretold, with the
difference only that the latter occur much more frequently.

Our knowledge of the stellar bodies at present and fifty or sixty
years ago are a world apart and the same is true of the latter and
that of antiquity. But we must not therefore forget that our brilliant
star-science today is derived from men’s desire to measure time, and
particularly from their need to foresee the food supply in coming days.




CHAPTER II

THE RIDDLE OF THE MILKY WAY


During dark but starlit nights, the gorgeous firmament is decorated
with an irregular band of light that describes a winding path across
the heavens. It continues also in the quarters hidden from our sight so
that it may be said to surround the firmament like a girdle. This band,
which is most luminous in the Northern Hemisphere, is called “The Milky
Way.”[1] It forms an angle with the equator of about 60° and divides
the firmament in two nearly equal parts--the northern, however, is
slightly larger.

    [1] The literal translation of the Swedish name is “The Wintry
        Way.”

The Milky Way, no less than other stellar phenomena, attracted the
early attention of the people. The Dieri Tribe in Central Australia
says that the Milky Way is the stream of heaven and the Mexicans
consider it the source of all that is. Tradition endeavoured to explain
its origin. Its milky appearance caused the Romans to call it “_Via
Lactea_,” a name that is retained in translated form in most modern
languages. This name is coupled with the legend of the Hercules-child,
who sucked the breast of Juno and when it was pushed away by the
incensed goddess, the milk was spread across the sky.

Nevertheless, the human race, until about two hundred years ago, had
little conception of the extraordinary importance of the Milky Way.
Anaxagoras and Democritos surmised, however, that it consists of a
collection of exceedingly minute and densely clustered stars each of
which has the nature of our Sun. Ptolemy described, nearly two thousand
years ago, its position on the firmament and his observations are
valid today as far as determinations with the naked eye suffice. When
Galileo introduced the telescope, the conception of the Milky Way as
made up of innumerable stars was verified. Not quite two hundred years
ago Swedenborg, in his cosmological speculations, considered our solar
system as a part of the Galaxy. Wright, Kant, and Lambert further
amplified these theories.

The first important forward step was taken by the great William
Herschel in his statistical researches. He demonstrated that the
stars lie closer to each other the nearer the Milky Way they are
located. This is mainly true about the small stars invisible to the
naked eye while the more luminous ones are more evenly distributed
over the heavens. In certain parts of the Milky Way the stars are
over one hundred times more crowded than at its poles--that is, the
points farthest removed from the Galaxy. Herschel’s investigations
were continued and elaborated by Struve, and later by numerous other
scientists.

Through these researches, it has been determined that the Milky Way
is, so to speak, the foundation upon which the star system, visible
to us, is built. All kinds of stellar bodies have been studied and
their distribution has proved to be symmetrical with reference to the
plane of the Galaxy. The majority is greatly concentrated around the
Milky Way. To these belong the new stars which occasionally blaze into
existence, as the well-known new star in Perseus, 1901, and nearly all
of which have appeared in the Milky Way or in its immediate vicinity.
We also find there the irregular nebulæ, enormous, vastly diffused
volumes of gas, among which the best known is the Orion nebula, and
which seem to constitute the primeval matter out of which the stars are
born. We might further mention the star-clusters, dense, ball-shaped
agglomerations of stars, and the so-called planetary nebulæ, which--at
least in their visible outer shell--also consist of gas accumulations
with a spherical or ellipsoid conformation. The numerous spiral nebulæ
on the other hand, those strange stellar bodies to which we later
shall have occasion to return, are beyond comparison more frequent in
the regions surrounding the poles of the Galaxy than in the rest of the
heavens.

Many astronomers have considered the Milky Way itself a nebula. The
most common theory doubtless is that it closely conforms to a spiral
nebula--an opinion that has found a particularly warm advocate in
the Dutch astronomer, Easton (see Figure 1). A few years ago Prof.
Bohlin expressed the view that it is most akin to a planetary nebula,
or more precisely to a ring nebula which is supposed to grow out of
a planetary ellipsoid nebula by the gaseous matter being driven from
its poles toward its equator. It is of a certain interest that this
theory lends itself to the support of Swedenborg’s--nevertheless
improbable--hypothesis about the origin of the planets in the solar
system. As we later shall see the Easton conception has the better
reasons in its favour.

If classified according to age the stars are again distributed with the
Milky Way as a reference point. Thus, let us consider their evolution,
which for various reasons is assumed to take the following course.
We may commence when the star-matter existed on the nebula stage. It
then radiated the light characteristic of certain incandescent gases,
principally the lightest two, hydrogen and helium, and further of an
otherwise unknown gas called nebulium (nebula-substance). These gases
were later condensed and dark spectral lines commenced to appear
beside the bright lines of the aforesaid gases. Stars on this stage,
named after their discoverer Wolf-Rayet stars, occur only in the
immediate vicinity of the Milky Way. A later stage in their evolution
is represented by the so-called helium stars in whose spectrum the dark
helium lines preponderate. They are considerably concentrated around
our Galaxy. Somewhat more evenly distributed and yet of decidedly
greater frequence in the neighbourhood of the Milky Way, the hydrogen
stars appear, characterized by strongly developed hydrogen lines and
somewhat retreating helium lines. These stars are more developed than
the helium stars and form with them the group of white stars so named
after the colour of their light. Next in evolution follow the yellow
stars, to which our Sun belongs. Dark metal lines appear in their
spectrum. They are more evenly distributed than the groups mentioned
before. Still further is this true about the red stars whose spectra
contain the characteristic bands of chemical compounds and therefore
betray comparatively advanced cooling. They are fairly uniformly spread
over the heavens but are still somewhat more numerous in the vicinity
of the Milky Way than further therefrom.

[Illustration: Fig. 1. The Milky Way, pictured as a spiral nebula by
Easton]

These facts are demonstrated in the statistics by E. C. Pickering,
Director of the Harvard Observatory, who divided the firmament in
four equal zones, the first of which is nearest to the Milky Way (and
includes it) and the last of which contains the Galactic poles. His
table shows the percentages of different stars in each of the four
zones.

                 |         |           | _White |         |
    _Galactic    | _Helium | _Hydrogen | Yellow | _Yellow |  _Red
    Latitude_    |  Stars_ |   Stars_  | Stars_ |  Stars_ | Stars_
  ---------------+---------+-----------+--------+---------+-------
      ±8.1°      |  51.2   |    37.4   |  29.7  |   29.4  |  26.7
     ±21.6       |  31.7   |    28.6   |  27.9  |   26.7  |  27.6
     ±39.8       |  11.9   |    18.3   |  21.1  |   21.9  |  23.6
     ±62.3       |   5.2   |    15.7   |  21.3  |   22.0  |  22.1
     Number of   |         |           |        |         |
  stars observed |   716   |    1885   |  1329  |   1719  |   457

The difference is most pronounced in the two first groups; in the
three last it is small but unmistakable. An even distribution would
correspond to 25 per cent. in all four divisions of the heavens.

These comprehensive statistics, embracing 6106 stars, seem to indicate
that the stars in their first stage were within the Milky Way but
subsequently drifted away with increasing age. This leads us to the
thought that they originated from the irregular, nebulous accumulations
which occur in the Milky Way and in its vicinity, or more correctly
from similar formations which formerly existed in these regions but
which now have clustered into stars. This agrees very well with
another observation. With the help of the spectroscope the motion
of different stars has been determined with reference to the point
where the sun now is. The velocities have been found greater the
older the stars are as shown in the table below taken mainly from the
investigations of the renowned astronomer, Campbell.

  _Mean velocity of_:

  Irregular nebulæ       0   Km. ( 0   Miles) per sec.
  Wolf-Rayet stars       4.5  ”  ( 2.8   ”  )  ”   ”
  Helium stars           6.5  ”  ( 4.0   ”  )  ”   ”
  Hydrogen stars        11    ”  ( 6.8   ”  )  ”   ”
  Yellow stars          15    ”  ( 9.3   ”  )  ”   ”
  Red stars             17    ”  (11.5   ”  )  ”   ”
  Planetary nebulæ      25    ”  (15.5   ”  )  ”   ”

To these figures a few remarks founded on recent observations might be
made. The average distance between us and the stars in each group is
different and the yellow stars, to which indeed our Sun belongs, are
those nearest to us in space. They are therefore easier to observe than
stars in the other groups. Campbell’s statistics include also for this
reason a smaller number of stars in this class than in the others. It
is conceivable and by the astronomer Halm held to be true that the mean
velocity of the smaller stars is greater than that of the larger ones.
This is the condition existing in a mixture of different gas molecules,
with which the brilliant French scientist, Henri Poincaré, compared
the throng of stars, inasmuch as the heavier molecules possess the
slower motion. To confirm this W. S. Adams of the Carnegie Observatory
on Mount Wilson compared stars of equal velocity in their own orbits.
Such stars are considered to be on the average equally far removed
from us. He found the theory of Halm confirmed. The mean velocity of
the hydrogen stars was reduced from 11 km. (6.8 miles) to 7.5 km. (4.7
miles), that of the yellow stars from 15 km. (9.3 miles) to 9.2 km.
(5.8 miles), and that of the red stars from 17 km. (11.5 miles) to 14
km. (8.7 miles), while that of the helium stars remained unchanged. The
sequence of the stars arranged according to velocity in the line of
sight is evidently not modified by this new calculation.

In regard to the motion of the planetary nebulæ it should be mentioned
that Campbell in this connection has carried out a great number of new
determinations according to which the mean observed velocity of these
large bodies in the line of sight must be increased to not less than 42
km. (26 miles) per second.

Campbell and Moore contribute the following interesting data in regard
to Nebula N. G. C. 7009 (Fig. 2):

“Measures of the rotational velocity of the nebula enable us to draw
some interesting conclusions concerning its mass. On the most plausible
assumption as to the location of the axis of rotation the orbital speed
of the nebular materials lying at a distance of 9 seconds of arc from
the centre is about 6 km. (3.7 miles) per second. If we provisionally
assume the mass of the central nucleus to equal that of the Sun,
Kepler’s law connecting the periodic time with the distance from the
nucleus tells us definitely that the nebula is distant from us only 8.9
light years. This must be regarded as an improbably small value, in
view of other evidence bearing on the question. For assumed distances
of 100 and 1000 light years, which we have reason to believe are more
probable orders of nebular distance, the masses of the nebula would be
respectively 11.3 and 113 times that of the Sun, and the corresponding
periods of rotation 1371 and 13,710 years. From these considerations
it seems certain that the mass of the planetary nebula N. G. C. 7009
is several times that of the Sun. The nebula is therefore competent,
from the point of view of its mass, to develop into a system more
pretentious than is our solar system.

“A few speculations concerning this nebula may not be without interest
and value.

“The faint extensions to the east and to the west of the elliptical
figure suggest an encircling ring of materials whose principal plane,
passing through the nucleus, passes also near our (the observers’)
position in space. These extensions terminate in condensed nuclei at
equal distances from the nucleus and on exactly opposite sides of the
nucleus. The faint extensions and condensations may be and probably are
largely the effect of the edgewise projection of such a ring, as in the
case of Saturn’s rings when the observer is in the plane of the rings.
The forms of the two terminating condensations, and especially the wing
extending up and out from the east condensation, suggest that we are
not precisely in the plane of the assumed ring.

“The form of the main nebula appears to be ellipsoidal and not chiefly
elliptical.

“The space immediately surrounding the central nucleus appears to be
relatively vacuous. Aside from the nucleus, the principal mass of
visible nebulosity exists in the brilliant ring, roughly elliptical
as to its inner and outer boundaries, which occupies the region about
midway between the nucleus and the outer edge of the nebular structure.
The brilliant ring is probably in reality an ellipsoidal shell; the
projection of such a shell upon a plane at right angles to the line
of sight would naturally show a relatively dark central area, but the
projection principle may not be the only one involved.

“If this nebula is in process of development into a solar system, the
indications are for a system having certain resemblances to our solar
system. Our four outer planets have a combined mass 225 times as great
as that of the four inner planets. Similarly in N. G. C. 7009, there
is apparently a paucity of material to form planets near the nucleus
and an abundance of material for planets at greater distances from the
nucleus.”

[Illustration:

  Fig. 2. Planetary nebula N.G.C. 7009 (composite drawing, from
          Curtis’s photographs of the nebula made with the Crossley
          reflecting telescope. The scale is in seconds of arc).

From _Proceedings of the National Academy of Sciences of U. S. A._]

Interesting observations have been presented recently also with
reference to the largest among the irregular nebulæ, namely the Orion
nebula. Three astronomers in Marseilles, Bourget, Fabry, and Buisson,
found that parts of this nebula, in the neighbourhood of the so-called
trapeze and very close to each other, moved with different velocities
and that this difference might amount to 10 km. (6.2 miles) per
second. The south-easterly part approaches us while the north-easterly
recedes from us. Consequently a violent whirl-motion undoubtedly
takes place in this region. This observation has been verified by
the well-known Chicago astronomer, Frost, who employed a different
method of investigation than his predecessors. He noted differences in
velocity amounting to 11 km. (6.8 miles) per second between points not
over two seconds of arc distant from the trapeze.

If therefore we say that the irregular nebulæ on the average possess
no motion, this statement does not preclude important local deviations
from the rule within the nebulæ, intimating a transformation which
probably leads to the concentration of the nebulous matter toward the
centre of the whirl.

Leaving out, to begin with, the planetary nebulæ, it appears that the
original matter of the stars stands still in space, that their average
velocity increases with increasing age and approaches a mean value
of about 18 km. per second or roughly 1000 times the speed of the
ordinary passenger train. Our Sun, in particular, moves with a velocity
of 20 km. (12.4 miles) per second toward a point in the constellation
Hercules 30 degrees north of the equator.

What force then shall we say it is that causes the motion of the stars?
As far as we know none but gravitation. It appears therefore as if
the gaseous primeval substance of the stars were not governed by this
force. It might prove hazardous, however, to make this assumption
as gases also possess weight and even the most rarefied strata of
the Earth’s atmosphere exert barometric pressure by virtue of their
attraction to the mass of the earth. Rather the immobility of the
nebulæ is due to the frequent collisions between the molecules in any
quantity of gas even if it be attenuated to such a high degree as in
the nebulæ. Thus, the molecules strike a balance, as it were, against
each other so that the different parts of the gas accumulations
shortly are brought to rest relative to each other. The irregular
gas mists around the Milky Way form therefore a continuous whole. A
different condition obtains in regard to condensed stellar bodies such
as the stars. They may in the densest throng move during billions of
years before they collide; but they might on the other hand enter
nebulous masses and thereby suffer gradual retardation. We now refer
to stars moving outside of the vapour clouds. They are therefore
unrestricted and the longer they have obeyed gravitation without
impeding encounters with nebulous matter, in other words the longer
the time elapsed since they emerged from the gas accumulations which
gave them birth, the swifter is their motion. Their (average) velocity
can of course not exceed a certain limit which in our parts of the
universe appears to be about 18 km. (11.2 miles) per second. Campbell’s
measurements show that for the youngest stars (all except the red) the
velocity is greatest in the plane of the Milky Way, a natural enough
condition as the attracting matter here is most abundant.

The planetary nebulæ possess a greater velocity although they, as
consisting of mist vapours, are in the first stage of evolution. Faster
yet do the spiral nebulæ move according to measurements by Wolf of
Heidelberg. This shows that they are of a different nature from the
irregular nebulæ, which form the matrix of the Milky Way. A closer
examination of the few--thirteen in all--planetary nebulæ, determined
by the American astronomer Keeler, convinced me that they approach the
Galaxy from its poles with a moderate speed, and subsequently under
the influence of its attraction curve their orbit, rapidly gain in
velocity, and finally rush into the nearest part of the Milky Way with
a very high speed.

A great number of them are no doubt caught in the mists or star-throngs
of the Milky Way after exposure to numerous collisions and sweeping
away all matter in their course. Such clean-swept traces are very
common in the area of the Milky Way. One of the most beautiful examples
is the so-called Cocoon nebula in the constellation Cygnus (the
Swan). It has left in its wake a dark rift, in whose bottom, however,
exceedingly small and evidently very remote stars are visible according
to the German astronomer Wolf (see _Worlds in the Making_, page 172,
Fig. 55).

The great mean velocity of the planetary nebulæ indicates that they
originally did not belong to the Galactic system, a conclusion also
reached by Bohlin, but for other reasons. They are nevertheless more
abundant in the neighbourhood of the Milky Way than in other parts
of the heavens. This fact, if viewed superficially, might lead to
the belief that they are indigenous to the Galactic system, but is
explained by their concentration in obedience to gravitation toward the
Milky Way.

Quite recently (1917) Van Maanen determined the distance of one of
these highly interesting celestial bodies, tabulated in the New
General Catalogue as No. 7662. Its distance was found to be only about
140 light years. This is about sixteen times the distance of Sirius and
the mean distance of a star of the fifth magnitude. This circumstance
agrees very well with the idea that this nebula is captured by the
Galactic system to which it has approached from very distant parts of
the space outside of the Galactic system.

One of the most remarkable astronomical discoveries in recent years
was made by Kapteyn, who thereby as well as by other achievements has
gained perhaps the highest rank among astronomers of today. He has
shown that the stars rushing forth in the neighbourhood of the Sun
belong to two great groups, one coming from the constellation Orion,
and the other at nearly a right angle (100°) from the constellation
Scorpio. In the former, we find nearly all the helium stars hitherto
studied. We have previously seen that these stars stand almost still
with reference to the Galaxy while the irregular nebulæ possess no
motion at all relative to the same reference point--and the Galaxy is
the natural datum-line for all astronomical measurements--so that the
motion of the first-mentioned star-group toward the Sun is principally
due to the Sun’s own motion. This group, according to Kapteyn, obeys
the law of relative star velocities even better than the combined world
of all stars; thus with reference to the Sun, the helium stars are the
slowest, the yellow stars the fastest, while the hydrogen stars occupy
a middle position, all a self-evident consequence of their own velocity
with reference to the Galaxy which increases from helium stars to
yellow stars.

Kapteyn has shown another regularity in this group which is easily
explained. We have previously mentioned that the yellow stars are most,
the helium stars least removed from their place of birth in the Milky
Way. The result is that the yellow stars appear (on the average) to
come from a point farther from the Galaxy than the apparent origin of
the hydrogen stars and more remote yet than that of the helium stars.
On account of the relatively high velocity of the yellow stars in
different directions, their stream appears to be more divergent than
the stream of hydrogen stars, and helium stars move in almost parallel
paths (nearly directly opposed to the Sun’s true motion with reference
to the Galaxy).

Similar regularities have been found by Kapteyn in the second
star-drift which would lead us to think, as indeed Kapteyn assumes,
that these stars also developed from an original nebulous mist, which
arrived in our neighbourhood from the unknown distant, but is now used
up in the formation of corresponding stars. Here again the yellow stars
should have departed farther from their matrix than the white hydrogen
stars. Helium stars are very rare in this drift, so that no reliable
statistics have as yet been made for them.

It has been one of the most difficult problems of cosmogony to form
a theory to account for the origin of the Galactic system. We may,
almost yearly, witness how new stars blaze into existence only to fade
rapidly and in a few years return to their old insignificance--that is,
they become invisible to the naked eye although through powerful lenses
we may frequently discover an exceedingly faint star in their position.
As a rule, a nebula of the planetary type is formed in the course of a
few months. Somewhat later the nebula is transformed into a Wolf-Rayet
star. It is interesting to note that Wright found the central bodies in
certain planetary nebulæ to be Wolf-Rayet stars. We have good reasons
to assume this blazing forth into light to signify the collision of two
faintly luminous or possibly extinct stars. The new lights appear also
in stellar regions where the star density is very great, particularly
in the Milky Way or its vicinity.

We see therefore repeatedly how mists with enclosed central stars
originate. They remind us to a certain extent of the Galaxy with its
clouds and stars and along the road thus suggested trials have been
made to reach the solution of the riddle. The difficulty lies in the
fact that the orbs whose collision create “new stars” are small,
probably smaller than our Sun, while the mass gathered in the Milky
Way most likely is trillions[2] of times greater than that of the Sun.
It is true that we know a few unique stars, such as Arcturus, which
exceed our Sun in size several tens of thousand times, but not even two
such stars would account for the mass of the Galaxy, and furthermore
the probability that two stellar bodies of such rare dimensions would
collide is so very small that it must be left out of account.

    [2] American and French numeration; billions acc. to Swedish
        and English usage.

Kapteyn’s star-drifts, containing many thousands or probably millions
of stars, appear to furnish the bridge that leads to the solution
of the riddle of the Milky Way. These drifts were once enormous
gas-clouds, in mass probably several million times greater than that
of the Sun. They also had an extension equal to trillions of stars.
The probability for the meeting of two such gas-drifts is comparatively
large and should not be much smaller than for the entrance of a
star-drift into the Milky Way, an occurrence which actually has
happened as shown by Kapteyn.

When two such enormous gas-clouds meet, each with a cosmic velocity of
about 20 km. per second, a long time would not elapse before the gas
molecules in the region of interpenetration would be retarded in their
original motions. An extraordinarily strong concentration and heating
would occur in this territory, which is surrounded by the comparatively
cold and heavy masses which remain unaffected because outside of the
impact-area. A certain degree of equalization would naturally take
place in the layers adjoining the boundary between active and inactive
parts and the former would, furthermore, be set into a rapid spin
around an axis perpendicular to the plane containing the two original
motions. On account of the great viscosity of gases, particularly
at high temperature, the central part would rotate as a coherent
unit. Thus it would form a disk of gaseous matter. This disk would be
thickest in the middle and would become thinner toward the edge where
centrifugal force acts most powerfully.

[Illustration: Fig. 3. N.G.C. 4594. Exposure = 2 hours. 1mm = 6”7.

From _Proceedings of the National Academy of Sciences of U. S. A._]

Such a discous nebula has been investigated by the astronomer F. G.
Pease of the Carnegie Observatory on Mount Wilson. By means of the
spectroscope he has studied the motion of Nebula No. 4594 in the New
General Catalogue (see Fig. 3). This body is believed to be a spiral
nebula like those in Figs. 4 and 5 but viewed from the side so that
the spirals appear as a band. As the picture shows, this band is
coursed through by a thick dark line, owing, it is believed, to a cold
non-luminous dust-cloud outside of the spiral. The bright band is
broadest in the centre. The curvature of the dark middle line in the
shape of an arc, whose apex points downward, combined with the fact
that the major portion of light falls above this arc, indicates
that we do not view the nebula exactly on edge but from the upper
(north) side of a plane through the arms of the nebula. The nucleus
speeds away from us at the dizzying rate of 1180 km. (730 miles) per
second. The east edge, _i. e._ the left on the picture, departs with
the still higher velocity of 1630 km. (1100 miles) per second, while
the west (right) edge retreats at the rate of only 800 km. (495 miles)
per second. According to Pease, the nebula rotates as a solid disk so
that the difference between the velocity of any point and that of the
centre increases in the same proportion as the distance of the point in
question from the centre. It is probable that we have been prevented
from observing the outer parts, corresponding to the spiral arms
proper, by the ring of dust which encircles the nebula. The visible
portion occupies an arc of 2¼ minutes on each side of the centre. Its
spectrum corresponds to that of star-group F-5 among the yellow stars
in the Harvard classification. Therefore, it is not the light of the
coherent gas-cloud which preponderates but rather that of the stars
consolidated within the cloud, and corresponding to the stars in the
Milky Way. This star-light is so bright that it entirely suppresses the
radiation from the gas-cloud itself.

Such parts of the gas-aggregation as are most removed from the place of
collision continue in their course through space little affected by the
attraction of the central mass because of the great distance involved.
The portions nearer the point of impact receive orbits curved by this
same attraction and the curvature becomes the sharper the nearer the
axis of rotation. One result of the mutual gravitation between the
central mass and the particles in the outer sections of the nebula is
also that the velocity in the spiral arms becomes smaller the farther
the section in question is located from the centre, just as comets in
the solar system move slower the farther they are removed from the Sun.
But in all portions outside the central region the attraction is too
weak to give circular orbits to the gaseous matter. All substance in
these localities, therefore, departs ever more from the centre. As the
spiral arms stretch out into straight lines such matter finally leaves
the central disk altogether. It is possible that only the disk itself
remains in the nebula computed by Pease.

Another astronomer on Mount Wilson, A. Van Maanen, has investigated
a nebula, No. 101 in Messier’s catalogue (Fig. 4). This nebula lies
nearly at a right angle to the line of vision which consequently almost
coincides with the axis of rotation. The motion of the different parts
of this nebula has been calculated with the help of photographs taken
in the years 1899, 1908, and 1914, whereby its changed position with
reference to surrounding fixed stars has been recorded. Out of 87
points in the spirals only 9 moved in the direction of the hands on a
clock while the other 78 moved in the opposite sense. The mean angular
velocity is 0.022 seconds of arc per year, corresponding to 85,000
years for one complete revolution at 5 minutes’ distance from the
centre. The absolute velocity 2 minutes of arc from the centre is 1.5
times as great as at 7.5 minutes’ distance.

Fig. 4 is reproduced from Van Maanen’s original. It shows clearly the
general regularity in the motion of the component parts as well as
the numerous exceptions to the rule. Such exceptions may be caused by
perturbations due to invading masses, which impart their own motion to
the entangling matter. These foreign bodies have probably condensed
surrounding vapours and this created the bright knots which stud the
nebular spirals. The upward motion amounts on the average to 0.007
seconds per year. While the points of condensation describe half a
revolution around the centre they depart therefrom to about twice their
original distance. More than a million years is therefore likely to
elapse before the outer portions of the nebula are so far removed from
the nucleus that the spiral form of the nebula is no more apparent.

[Illustration:

  Fig. 4. Internal motions in Messier 101. The arrows indicate the
          direction and magnitude of the mean annual motions. Their
          scale (0”1) is indicated on the plate. The scale of the
          nebula is 1mm = 10”5. The comparison stars are enclosed in
          circles.

From _Proceedings of the National Academy of Sciences of U. S. A._]

[Illustration:

  Fig. 5. Spiral nebula No. 51 in Messier’s Catalogue; situated in
          Canes Venatici, and photographed February 7 and 8, 1910,
          from Mount Wilson Observatory in California. Scale: 1
          millimeter = 5 sec. arc.

  Lyran Herkules
  Ormbäraren Örnen Skölden
  Svanen Räven
  Pilen

  Ormbäraren Skytten
  Skorpionen
  Södra Kronan Kolsäck
  Syd-Korset
  Skeppet Argo Kölen

  Skeppet Argo Styre Segel
  N
  Enhörningen

  Enhörningen
  Kusken
  Cepheus
  Perseus
  Kassiopeja
  Svanen
]

It is clear that the Milky Way may have been formed through the
collision of two immense nebulous gas-aggregations in the manner just
described. Subsequently and by virtue of the magnitude of the Galaxy
great quantities of wandering cosmic matter and minor stellar bodies
have been gathered in occasionally accompanied by larger clusterings
such as the planetary nebulæ referred to.

How well justified we are in looking upon the Milky Way as a spiral
nebula is apparent from the picture (Fig. 5) reproducing a photograph
of the familiar regular nebula in the Dogs (Canes Venatici). It shows
a wealth of detail hitherto not surmised. The feat was accomplished in
the Carnegie Observatory on Mount Wilson in Southern California with
the help of optical resources vastly superior to all earlier means.
The Milky Way has previously been compared to this nebula, but due to
deficient enlargement their striking similarity has not been fully
recognized until now.

Assume the Sun located in the point marked “S” in Fig. 5 and some
distance above the plane of the picture, then the nebula if viewed
from this point would appear somewhat as the Milky Way appears to
us. In the middle we behold the substantial nucleus and on its left
side a cleft between the two branches of the inner spiral. Farther
to the left, we see only the outer spiral, first broadening toward
the left where it approaches “S,” then narrowing only to spread again
on account of the great clustering in the lower right part of the
spiral. The axis of the nebula corresponds to the densest parts of
the Galaxy in Cygnus (the Swan), the loop in the inner spiral again
to the empty space between Cepheus and Cassiopeia, the narrow part
of the outer spiral branch resembles the constriction at Algenib,
the following diffusion corresponds to the broad section in Auriga
(Charioteer) and Monoceros (Unicorn). At the subsequent narrow place
we see the outer nebula-clump corresponding in certain respects to the
Magellanic clouds on our firmament although these are farther removed,
and apparently not indigenous to the Milky Way. There follows in the
nebula a massive section in our system represented by the well known,
far less compact, yet brightly luminous tract containing the Southern
Cross. Here, from the star Alpha in Centaurus--the nearest bright fixed
star[3] to our Sun, “only” 4.5 light years or about 25 trillion[4]
miles distant--commences a bi-section of the spiral and strangely
enough the nebula is similarly forked. Now the outer spiral stretching
in a faint line upward from the “clump” begins to show as a weak band,
while the inner spiral stands forth powerfully above “S” corresponding
to the brilliant section of the Galaxy in Scutum (Shield) and Aquila
(Eagle). The partition in the nebula between these two branches is
the counterpart of the 110° long “prong” in the Milky Way between the
constellations Norma and Lyra. Numerous faint bridges join the two
branches in the nebula as well as in the Galaxy, according to Wolf.

    [3] Up to a short time ago Alpha Centauri was considered the
        fixed star nearest to the Sun. By comparing old photographs
        of the firmament with such of recent date the renowned
        astronomer Barnard found that a very small fixed star--of
        the magnitude 10.5 and therefore far from visible to the
        naked eye--in the constellation Ophiuchus (Right ascension
        17 h. 58 m. 44 s., North declination 4° 27´.4 January 1,
        1917) possesses a very large proper motion. It traverses in
        a year 10.3 seconds on the firmament. The distance to this
        star, which has the largest proper motion so far known,
        was later determined to be 3.3 light years or 3/4 of the
        distance from Alpha Centauri to our Sun. Hence its velocity
        at right angle to the line of vision is computed to be
        49 km. (32 miles) per second. Spectroscopic measurements
        show that it approaches us with a velocity of 91 km. (56.5
        miles) per second along the line of vision. The combined
        velocity, therefore, is 103 km. (63 miles) per second, an
        unusually high value. The value of 3.3 light years used in
        this calculation was determined by the French astronomer
        Gonnessiat, who found it by the study of old photographs
        from Algeria. He also calculated the parallax of this star
        to be 1 second. According to later measurements, given in
        the Harvard Bulletins 616 and 617, its parallax is only
        0.7 seconds and consequently its distance 4.6 light years
        and its speed perpendicularly to the line of vision 70 km.
        (43.5 miles) per second. Campbell, in the Lick Observatory,
        had determined its radial velocity and found that it
        approaches the Sun at the rate of 128 km. (79 miles) per
        second. Its total velocity is, according to these two last
        determinations, 146 km. (91 miles) per second.

        It is by no means improbable that similar discoveries will
        be made in the future, so that the Sun will be found to have
        more stars in its “immediate” vicinity than previously
        assumed.

    [4] American and French numeration; the Swedish and English
        usage is billion.

The correspondence is indeed surprisingly good. Proportions are, of
course, somewhat different--in particular is the central part of the
Galaxy not so predominating, which fact has been troublesome to the
adherents of the nebula theory. Probably it was originally denser but
has become attenuated through star-formation, explaining, for instance,
the great gap between the constellations Lyra and Vulpecula.

To give a better idea of the structure of the Milky Way, two
photographs are here reproduced as taken by Wolf, the German astronomer
in Heidelberg, who has done particularly meritorious work in this
department. One (Fig. 6) shows a section of the Galaxy in Cygnus
(the Swan) with the star Deneb in the centre and to the left the
“Northamerican-nebula” so named from its shape. Above Deneb is the dark
“hole” in Cygnus and below another chasm not quite so black. Left of
the “hole” is the winding canal enclosing the so-called Cocoon nebula.
(See _Worlds in the Making_, page 172.)

The following picture (Fig. 7) contains, in the upper left, the bright
star Altair in Aquila (the Eagle) located close to the powerful arm of
the Galaxy in this constellation. Farther to the right is the fainter
arm in Ophiuchus (the Serpent-holder). The lower half contains the
most brilliant part of the Milky Way in the constellations Scutum (the
Shield) and Sagittarius (the Archer). Bright stars are infrequent but
the fainter ones are innumerable: “They are crowded into dense clusters
and between them the most delicate star dust is scattered.”--“We behold
how the star-ribbon dissolves into detached tufts which intertwine into
the strangest patterns. These clouds of stars reach their greatest
splendour in the lower part of the map.”

[Illustration:

  Fig. 6. The Milky Way between constellations Cassiopeia and Swan
          from photo by M. Wolf of Heidelberg. A little to the left
          of the middle the beautiful nebula America appears.
]

[Illustration:

  Fig. 7. The Milky Way in constellations Eagle (upper half) and
          Archer (lower half). In the upper left the bright star
          Altair appears. Photo from M. Wolf of Heidelberg.
]

We also reproduce (Fig. 8) from M. Wolf a photograph in larger scale
from the region of Gamma (lower part of Fig. 7) in the Eagle
with its “trident-hole,” so called from its peculiar shape, and in
whose vicinity mists and star-clouds abound. This picture is a more
complicated counterpart of the flatter photograph by Wolf of the
“Cocoon nebula.” It appears as if three or four stellar bodies here
had stepped in from without, swept away the stars in their way, and
left clean “streets” behind. Probably other “empty” spots in the
neighbourhood have been formed in a similar manner. Another theory is
that such dark places are caused by opaque mist-formations which shield
the light of the stars behind from our sight.

[Illustration: Fig. 8. The trisected hole (Trifid-hole) in Eagle. Photo
from M. Wolf of Heidelberg.]

Through these pictures we gain a conception of the manner in which the
present stars in the Milky Way have clustered out of the original misty
chaos. We cannot avoid the idea of great exterior similarity between
the lumps formed in curdling or souring milk and those which we observe
in the Milky Way. The renowned French scientist Duclaux says in his
micro-biology: “In milk, commencing to sour, but yet entirely liquid,
we observe under the microscope a precipitation of tiny particles.
To begin with they are seen with difficulty and are discovered only
by slightly displacing the plane of vision. Later they develop into
distinct grains, characterized by Brownian movements, just like small
particles of clay.... Still later the phenomenon appears as a steady
molecular agglomeration. The grains have the tendency of the clay
particles to lump and precipitate.”

The first condensation-nuclei in the mist-clouds are no doubt cosmic
dust entering from without and perhaps also larger clusterings such as
meteorites and comets. At the existing low temperature, surrounding
gases condense into fluid state on the dust particles which by
virtue of these moist shells are cemented into aggregations of such
size that gravitation overcomes the repelling radiation pressure.
Gravitation assisted by the retarding vapours further mass these
aggregates together. This process of coalescence is accompanied by
heat production. Finally, small stars are formed, then groups of such
stars, while the spaces between, now comparatively devoid of matter,
appear dark much as the whey between flocks of curd. As yet, the small
stellar bodies are surrounded by quantities of dust and gas, which,
however, with continued condensation become ever more rarefied. Even
yet the big stars in Pleiades, belonging to the helium group, appear on
the photographic plates interspaced with great patches of dust-clouds.
These are now, however, so unsubstantial that they offer little
impediment to the procession of the mighty stars. The condensation
process may be greatly accelerated through the invasion of voluminous
gas nebulæ similar to the Cocoon nebula. At last all gases in the new
star condense, that is to say the shell of tenuous vapours and dust
contracts to such an insignificant thickness that it cannot be seen
except possibly from the immediate vicinity. Small bodies ingathered
through friction against the remnants of the original extended
wrapping wander as planets around the new sun, sweeping away the last
traces of unattached matter. The condensation on the new orb leaves a
“hole” in the nebulosity which in this way is transformed into stars
and their satellites which emerge from the mist and scatter on the
firmament.

The Milky Way appears to be in a rather advanced stage of this
evolution.

The “infinitely small” presents occasionally surprising likeness to the
“infinitely large.”

In this manner we can form a conception of the growth of the wonderful
structure which has brought forth the majority of the stellar bodies
that we discern. The spiral nebulæ visible at the Galactic poles are
similar formations but probably of far more modest dimensions. They may
compare to the Milky Way as the smaller planets to the Sun. According
to recent investigations the spiral nebulæ seem also to possess an
enormous velocity and they have probably invaded the Milky Way from
without.

As previously stated, an exceedingly remarkable conception of the
Milky Way exists among the Mexicans. To them it is the Matrix of all
and gave birth to the stars, the most important of which are the Sun,
the Moon, and Venus. This idea evidently agrees very well with the
results of investigations in the last few years.

Finally, a few words about the extent of the Milky Way. So far we have
not been able to measure it; only rather uncertain approximations are
possible. Wolf estimates the diameter, that is the distance between
the two spirals at the point where the Sun now is, to be about 10,000
times the distance from the Sun to the nearest fixed star, Alpha in
Centaurus, which distance on the other hand is about 10,000 times the
distance from the Sun to the remotest known planet, Neptune, or 300,000
times that from Sun to Earth. Expressed in the usual units, we arrive
at 40,000 light years or 400,000 trillion kilometres (240,000 trillion
miles). Lord Kelvin makes another estimate of 6000 light years, that is
seven times smaller. The mean diameter of the nebula proper might be
about five times larger, in round figures one hundred thousand light
years, or one billion billion kilometres (600 million billion miles).

Like a monstrous octopus, the Milky Way swims in the fathomless
ether-sea. Its dimensions are about as many times larger than those
of the earth as that globe is larger than an atom. This has caused
the gifted Irish physicist Fournier D’Albe to consider the celestial
globes as atoms, out of which systems of the order of the Milky Way
are constructed in the same manner as the earth and other stellar
bodies are upbuilt with atoms, invisible to us and yet measured with an
incredible degree of accuracy.

Fournier D’Albe goes further still. In poetic flight he does not
hesitate to endow the Milky Way organism with life. Its evolution
cannot be denied a certain similarity to the processes of life. The
great nebula owes its origin to the union of two individuals, two
nebulosities, who met in their course through boundless space. There
floated the newborn extending its tentacles in the cool ether-waves
and gained substance and strength through the smaller beings which
the surging billows brought within its reach. It has now attained
the zenith of its evolution, and is breaking up into molecules, or
solar systems, which again are composed of stellar bodies, or atoms
within the molecule. In violent exuberance of youth these rush through
space in fulfilment of their individual life. Many will in due time
undoubtedly become dust again and then serve to nourish a new youthful
nebula. Others succumb to a freezing-death but will be restored to life
in collision with a nebulosity or some other stellar body and give
form to “new stars” or planetary clouds. Again and again shall the
starry mists go through the cycle of existence and after a lifetime,
whose duration stands in proportion to their dimensions, _i. e._ may be
estimated to billions of billions of years, give rise to new celestial
beings. Thus shall it for ever continue in an eternal rhythm.




CHAPTER III

THE CLIMATIC IMPORTANCE OF WATER VAPOUR


When Aristotle, for two thousand years our leading savant in
cosmography, about twenty-three centuries ago stated the foundation of
his natural science, he laid down as the important principles: moisture
and heat and their opposites; because the four elements out of which
everything was made, were: earth, characterized by dryness and cold;
water, which was moist and cold; air, which combined moisture with
heat; and lastly fire, which stood for dryness and heat. Undoubtedly
he was considering the requisites of life, which may be designated as
humidity and heat. We have seemingly agreed that all life originates
in the sea, so that moisture can be considered the first requirement
for its appearance on the earth. As to heat, life is destroyed by
frost and favoured by increased warmth, at least to a certain point,
about 35° to 40° C. (95° to 104° Fahrenheit), which temperature is
most propitious to the development of life, while a further increase
is detrimental, so that already below the boiling point of water life
suffers more harm than at temperatures below freezing. In fact, the
geologists have found that the different epochs in earth’s evolution
are best characterized by their humidity or dryness. To arrive at a
clear conception in these matters we shall briefly survey our present
knowledge of the importance for the evolution of life on earth that we
should attach to the humid and to the dry periods or localities.

We are all familiar with the heavy, moisture-laden warmth which meets
us when we enter a hothouse. It is particularly favourable to the
growth of plants and to the prosperity of the lower animals. To the
higher animals and to man, the humid heat is not so beneficial. In the
open such hothouse air exists only in the tropics. Particularly the
Congo region and the parts of Brazil adjoining the Amazon River are
remarkable for their humid heat and for their fabulously luxuriant
vegetation. From our greatest living climatologist, Julius Hann, I have
borrowed the following description of such a clime:

“The changes of temperature between the coldest and the hottest month
are very small in the Congo, from .5° to 5° C. (.9° to 9° Fahrenheit),
with an average of about 3.5° C. (6.3° F.). The difference between day
and night reaches nearly thrice this value, or 9.5° C. (17.1° F.).
The dry season becomes shorter the more we approach the equator
and in Equatorville and Bangala it shrinks to nothing. During the
rainless months, a dense humid fog settles morning and evening over
the savannahs. Low hanging clouds of uniform thickness frequently hide
the Sun for weeks at a time. It is during the rainy season alone that
we see a clear sky between the showers. This season opens and closes
with magnificent thunderstorms coming from the east. In Luluaburg,
lightning occurs during not less than 106 days in the year. In the
dry season the wind carries with it clouds of dust which falls to the
ground. The cloudiness is enormous in the Congo basin, so that there
are veritably no months with a clear sky in this part of the world. In
Vivi, the number of overcast days averages 74 per cent., fluctuating
between 63 per cent. in August and 83 per cent. in November. The
humidity is very high, varying in Vivi from 70 to 79 per cent., with a
mean value of 75 per cent., and in Bolebo the mean itself reaches 79
per cent. During the rainy season, the heat is sometimes unbearably
oppressive; suffocating fumes rise from vegetable matter which rapidly
decays in the excessive humidity. The annual precipitation does not
reach very startling figures; it varies between 120 and 180 cm. (47 to
71 inches). In Gabun, close by, the sky is almost continuously covered
with clouds during the dry season.

“Corresponding regions in South America are in parts characterized by
an even higher humidity. In Iquitos by the Amazon River, it reaches not
less than 83 per cent. of saturation. The annual change of temperature
is only about 5° C. (9° F.); in Para (1.08° south latitude on the
coast) it shrinks to 1° or 1.5° C. (1.8° to 2.7° F.). In the course
of twenty-four hours the variation is considerably larger. The sky
is remarkably clear between showers during the rainy season. In the
interior of Guiana, the rains continue from the end of April well into
July or even into August. Abundant dew is common during the rainless
part of the year, thus maintaining the humidity. Sun and Moon are
rarely visible, and gigantic lightning storms announce the arrival of
the rainy period.”

Similar conditions apparently prevailed during the carboniferous
period, which was characterized by a luxuriant vegetation. The mighty
tree-trunks of that time fell into the water-covered marshes out of
which they had grown and their decay was thereby prevented. Instead
they turned into coal like the peat in the mosses of today. This
was for some time thought to indicate that the temperature was not
particularly high--Frech estimated about 12° C. (53.6° F.) (1910). But
since the discovery and subsequent description by Keilhack (1914) of
peat-beds on Ceylon, where the average yearly temperature is 26° C.
(78.8° F.), a return is to be expected to the older conception,
which held that the vegetation during the carboniferous period is
evidence of a very warm climate. Judging by the appearance of fossil
plants, the temperature should have been nearly the same all over
the globe. Carthaus remarks that the air was stirred by only feeble
winds because the trees of that age with their great dimensions but
frail root-systems could not have withstood a fresh breeze. The sky
was hidden behind a continuous thick cover of clouds which only let a
faint light sift through to the ground. The motionless air was almost
saturated with moisture. The luxuriance of the vegetation, transcending
anything existing today, indicates a favourable high percentage of
carbonic acid in the air. This combined with the humidity and the dense
clouds caused the heat radiation from the Sun to be almost entirely
absorbed by the upper strata of the atmosphere in which thereby a
strong circulation was maintained. As a result, the heat was nearly
equalized between the poles and the equator and under the cloud cover
an almost constant temperature reigned day and night, summer and
winter. The damp air stood wellnigh still and was filled with dense fog
at the smallest changes in temperature. Lack of light prevented the
development of flowers, and the thriving plants belonged mainly to the
ferns and to the horsetails. Pine and fir trees were yet comparatively
few. The conditions in the swampy regions where plant life flourished
were nearly identical with those in a hothouse if we were to draw a
dense veil in front of the windows in walls and ceiling so that a
continuous twilight would prevail.

In this uniform climate, plant life developed enormously faster than
animal life. The dense clouds could store considerable quantities of
heat in the equatorial belt through evaporation in their upper layers
and the violent wind storms above the clouds would carry the aqueous
vapours to colder regions where the heat would be liberated through
new cloud formations. Currents in the oceans now largely attend to
this heat transportation and give for instance to the coast of Norway,
and indeed to the whole of Western Europe, its remarkably mild, and to
life and civilization, propitious climate, but in the carboniferous age
humid air currents fulfilled the same task. They moved considerably
faster and more evenly than the ocean currents, were not checked
or deflected by coasts or islands, and could therefore produce the
extraordinarily uniform temperature and the marine climate all over
the globe. Such a heat distribution takes place also in our days at a
height of about 10,000 m. (6.2 miles) in the so-called “stratosphere,”
but the temperature here is very low, about -60° C. (-76° F.), so that
the vapour suspended is hardly worth mentioning, and cannot give rise
to cloud formations. The quantities of heat carried in these higher
strata of the atmosphere are too insignificant to influence the masses
of air below, whose temperature, therefore, is almost entirely governed
by that of the sun-heated surface of the earth, except where the
ocean currents equalize matters, as for instance in the almost wholly
water-covered latitudes south of the 30th parallel on the Southern
Hemisphere. Even during the carboniferous period at its height, there
existed, of course, a temperature difference between pole and equator,
but it was very small, some 10° C. (18° F.) perhaps. Undoubtedly, the
formation of coal beds was mainly confined to those regions where the
climate was most uniform all the year around.

The opposite extremity, the dry desert climate, is far more pronounced
in the present time. This condition is well known in all continents
except Europe, where we hardly can claim a desert but instead have
steppes, with a vegetation abundant after the spring rains but fast
disappearing with the arrival of the burning summer heat. A particular
type of plant life has adapted itself to this periodic change from rain
to drought, from bitter cold during the winter to parching sun during
the summer. Perennial plants, and particularly trees, can rarely endure
the rigors of such climatic upheavals. Animal life on the other hand
has proved fairly adaptable and displays considerable wealth.

This steppe climate is only an intermediate stage towards the desert
climate proper, which is hostile to all life. Its temperature is
subject to enormous changes in the course of the day and the year.
The annual variation is less pronounced near the equator and the
daily variation less on the approach to the poles, on account of the
small changes in the sun’s radiation during corresponding periods.
The difference between day and night in Sahara is frequently 30°
to 40° C. (50° to 70° F.). The lowest temperature observed by
Foureau-Lamy, 1898–1899, was -20° C. (-4° F.) or nearly the same as on
the Scandinavian coasts. The highest amounted to 48° C. (118.4° F.) or
a total variation of nearly 70° C. (126° F.). In Upper Egypt (21.9°
N. Lat.) the mean temperature changed from 16.3° C. (61.3° F.) in
January to 34.1° C. (93.2° F.) in July, and nearer the equator in
Central Africa (8.1° N. Lat., 23.6° E. Long.) the difference amounted
to only 6.9° C. (44.6° F.), 22.7° C. (72.5° F.) in December, 29.6° C.
(85.1° F.) in April, while in Kiachta (50.4° N. Lat., 106.5° E. Long.)
in Siberia, the yearly change reaches 45° C. (81° F.), -26.6° C.
(-15.7° F.) in January, 19.1° C. (66.2° F.) in July. The average daily
variation at continental stations is about 12° C. (21.6° F.). All this
refers to the temperature of the air, while the surface temperature in
the course of twenty-four hours may change 50° C. (90° F.) and in the
desert even more. Frost occurs in the Sahara as late as May when the
maximum temperature may reach 50° C. (122° F.). While in Scandinavia
the diurnal difference between highest and lowest temperature averages
only 6° to 7° C. (11° to 13° F.), a maximum in July of 10.4° C.
(18.7° F.) and a minimum in November of 4° C. (7.2° F.), Hedin on his
journey in Tibet, 1899–1902, observed a daily variation of 19° C.
(34.2° F.) and no appreciable difference with change in altitude.

The result of such a violent temperature change in the course of a day
is a breaking up of the rocks which subsequently and gradually are
ground to fine dust by unobstructed winds wherever vegetation does not
bind the soil. In this manner the sand deserts are formed. The arid
wastes of Asia have lately been vividly described by Sven Hedin. The
mountains eroded by the sandstorms resemble dilapidated ruins, standing
as monuments of an ancient highland. The sand in East Turkestan is
reduced to such a fine powder that it can float in the air for several
days after a storm, revealing itself in gorgeous sunsets. Winds sweep
the sand into long dunes, which shift in the direction of the blast.
It is ferruginous and therefore red or if powderized reddish-yellow.
When moistened it assumes a brown to black shade. After rain, the water
descends toward the valley, carrying with it the sand in the form of
silt. This, through evaporation, is transformed into a plastic black
dough, slides like a glacier slowly down the hillsides, and finally
comes to rest in some broad hollow which it gradually fills. Such a
silt aggregation is called in Persia a “Kevir.” Its surface dries,
but the interior remains moist. As evaporation continues it becomes
richer in salt so that white crusts of this substance are formed during
dry periods. In other districts, as in the basin of the Tarim River,
the water occasionally appears in the lowest parts, the so-called
“Bayirs” (see Fig. 9), formations similar to the Kevirs, or in salt
lakes between the sand dunes. Sand carried by the winds quickly fills
these lakes so that they too move in the direction of prevailing
winds. They lie with their longest dimensions parallel to each other
and at right angle to the course of the Tarim River. The sketch map
taken from Hedin’s work shows the Bayirs strung out in line with the
lakes somewhat like panels in a tapestry pattern. This reticulation
of the landscape is the result of the dune formations. The main dunes
with steep western slopes run in the direction N.N.E.-S.S.W. They
stand at right angle to the prevailing winds. Nearly perpendicular to
their crest lines dunes of smaller height are thrown up by winds in
another common direction but less frequent than those which raised the
fundamental dunes. This system brings to mind the cloud formations
called mackerel sky, clouds rippled in two directions frequently almost
at right angle to each other. They owe their peculiarity to two series
of wave motions propelled by winds from two different directions in
the upper strata of the atmosphere. The cloud patches correspond to
the wavecrests on a surging sea. The map of the Bayirs suggests a
chessboard with squares somewhat elongated and irregular.

[Illustration: Fig. 9. Tarim river with adjacent lakes and Bayirs,
drawn by Sven Hedin.

  (solid arrow) Predominating direction of winds.

  (dashed arrow) Direction in which the river is shifting.

  (hollow box) Vegetation.

  (dotted box) Sand.

  (horizontal lines in box) Lakes.

  (vertical lines in box) Bayirs.
]

We may now return to a closer study of the largest of these
formations, the great Kevir in Persia. This mud-lake, with a dry
surface, measures 500 km. (310 miles) in length and 200 km. (124
miles) in width over its largest dimensions. Hedin estimates its area
at 55,000 sq. km. (21,142 sq. miles) or the same as that of the great
Lake Michigan on the boundary between the United States and Canada.
Due to the continuous growth of the salt proportion through the inflow
of Kevirs and through superficial evaporation a salt crust of varying
thickness is formed near the surface of the lake. Hedin caused a hole
to be cut with an iron bar. He first encountered a 10 cm. (3.9 in.)
deep covering of clayey paste, and then the salt crust about 7 cm. (2.8
in.) thick resting on a semi-dry layer of clay with a depth of 15 cm.
(5.9 in.). Farther below, softer strata of clay followed, becoming more
watery the deeper he went. The iron bar carelessly wielded would have
disappeared in the mire. Another investigator, Buhse, examined a piece
of the crust, which when dry is fairly solid and of a yellowish-grey
colour. One half consisted of sand (probably quartz-sand), one sixth of
limestone, 6.1 per cent. oxide of iron (causing the yellow colour), 5.3
per cent. common salt, 2.5 per cent. sulphate of sodium, and 2.1 per
cent. clay. Rain converts this surface layer into a plastic mass, which
persistently sticks to the clothes of the traveller or to the bodies of
the camels if they should slip and fall into the mud. Not the slightest
trace of vegetation or of any life exists. On the shore of the mud-lake
small flat elevations and depressions may be found; otherwise, the
surface is as level as that of an ordinary lake.

The Kevir battles with the drifting sand as does the water in East
Turkestan. The sand appears to gain in the contest. After storms, vast
portions of the Kevir are covered with yellow desert sand. “If the
climatic change in Persia continues in the present direction,” says
Hedin, who, however, is dealing with large spans of time inasmuch
as in his opinion no appreciable alterations have taken place since
the invasions of Alexander the Great, “then it may be taken for
granted that the slough of the Kevir will lose in moisture and afflux
of water and in time will become more solid and that the drift sand
with greater ease will gain foothold and territory. The final outcome
of the physico-geographical transformation now in progress will no
doubt be to convert the entire Kevir into a sand desert of the kind
predominating in East Turkestan. And conversely we may infer that
East Turkestan, once a part of the central Asiatic Mediterranean sea,
in the course of time was filled with the finely divided products of
disintegration, such as we now find in the Kevir, and further that its
expanse of watery mud and clay finally dried and hardened to such an
extent that it could support the load of the encroaching sand. That the
extension of the sand formerly was smaller, is also borne out by the
archæological discoveries in East Turkestan which several travellers
besides myself have made. The hardpan laid bare in the ‘Bayirs’ of the
Cherchen desert strongly reminds of the Kevir-ground. In both cases
the same dark, fine powder forming a nearly plane surface. In both
cases this material when mixed with water is transformed into a slough
in which one hopelessly sinks, but in East Turkestan the water has
receded to a greater depth and as rains are extremely rare travel all
over the smooth ‘Bayir’-ground may be undertaken with impunity.”

These formations are of the greatest interest because they picture
the changes taking place on a slowly desiccating planet. In 1858 the
Geographical Society in Petrograd despatched an expedition under
the command of Khanikoff to visit these regions. From Hedin’s work,
_Overland to India_, from which the preceding quotation is taken, we
borrow the following vivid description by Khanikoff: “At last, in
the morning of April fourth, and during the most oppressive heat, we
reached Bala-haus. At this place remnants of a ruined reservoir, long
since bereft of its water, could be seen. The desert had here assumed
the perfect character of the ‘the accursed land,’ which name it bears
among the natives. Not the smallest tuft of grass, not a sign of animal
life gladdened the eye, not a sound interrupted the deathlike, awful
silence but that of the marching caravan.

“Because of the slow procession of the camels and the delay suffered
when we lost our way, we only covered 25 km. (15.5 miles) in the night
stage. After four hours’ rest we resumed our march and directed our
steps toward the hills, called Kellehper and situated 20 km. (12.5
miles) from Bala-haus; they were plainly visible but positively seemed
to take flight on our approach. I was somewhat ahead of the caravan and
sat down at the foot of this sand-elevation; and never can I describe
the feeling of weariness and of depression that I was unable to ward
off as I looked into the ghastly solitude that engulfed me. Scattered
clouds intercepted the rays of the sun, but the air was hot and heavy;
the diffused light lent a monotonous and disconsolating hue to the
greyish, burning surface of the desert; hardly a single variation
of colour gave relief to the immense expanses that the vision
embraced. The absolute immobility of every point in this mournful
landscape, combined with the complete absence of any sound, produced
an overwhelming depression of the spirit; one felt as set upon a place
that had been struck inanimate for ever, a place to which organic life
would never return safe through some terrible catastrophe of nature.
One witnessed the beginning, so to speak, of the death-agony of our
planet.”

If a drying out has taken place in these regions,--which seems probable
from Hedin’s observation that the water in a Tibet lake, Lakker-tso,
formerly reached a 133 m. (435 ft.) higher level than at present,--such
a process is nevertheless not so obvious here as in the salt inland
lakes, for instance the Great Salt Lake in Utah, the Dead Sea, and
the Caspian Sea, where the saltness has greatly increased due to
evaporation. Concerning the Great Salt Lake we know that even at a
comparatively late time it had a much wider extension than now. Its
water contains 22 per cent. common salt besides other compounds. The
Dead Sea holds 25 per cent. salt. A very variable percentage is to be
found in the Caspian Sea. Near the mouth of the Volga it is of course
low, only 0.15, and increases southward to 1.32 at the peninsula
Apsheron and to 5.63 in the bay of Kaidak. In the gulf of Karaboghaz on
the Asiatic side it reaches 28.5 per cent. It has been computed that
this gulf receives annually from the inrushing waters of the Caspian
Sea 350,000 tons of salt which is partly deposited on its shores and
bottom.

This desiccation, however, is a mere trifle when contrasted with the
process by which the mighty salt deposits in Germany were formed. It
took place, we believe, in a shallow bay extending southward from the
Arctic Sea. As the salts, first gypsum, then common salt, and lastly
more soluble potassium and magnesium compounds, crystallized by degrees
new water masses entered from the sea. At the same time, the bottom of
the gulf slowly receded, giving room for fresh evaporating floods. The
salt layers deposited in this manner sometimes reach a depth of more
than 1000 m. (about one half a mile). We can thus gain a conception
of the immense quantities of water evaporated and the enormous time
required therefor. The deposits would long ago have been carried away
from their original place were it not for the fact that they finally
were covered with a layer of slime nearly impervious to water. The most
soluble salts, such as chloride of magnesium, have nevertheless been
extracted to a great extent.

The extremes of aridity or humidity have of course not occurred
during the brief time known to history. Of a special interest is the
question in what direction the climate at present is tending. In this
connection, Huntington has aroused great attention by propounding the
theory that the earth is now in a period of rapid desiccation.

Judging by the testimony of geology, it seems beyond doubt that an age
of humidity prevailed simultaneously with the ice-period in northern
Europe, over several parts of the globe, in fact everywhere as far as
we know except in Australia. This is clearly borne out by the higher
levels of the lakes and their consequent greater extension in earlier
days. So far as Tibet and Central Asia are concerned we have already
mentioned this fact. But in America and Africa the humid period was
even more patent. The Great Salt Lake has covered an area many times
greater than at present as testify the picturesque terraces in its
surroundings (compare Fig. 10). According to the researches by Passage,
this period was also strongly pronounced in Africa. A large fresh-water
body occupied the Congo basin, the Tsad lake had a far greater
expansion than now, and mighty rivers traversed Sahara.

[Illustration: Fig. 10. Extension of the great Lake Bonneville in Utah,
of which the Great Salt Lake is a remnant.]

It is often assumed that the climate of Africa has been more humid
even in historic times. The geographer Leo Berg in Petrograd, however,
is emphatically opposed to this theory. He points out that the
writers of antiquity, Diodoros, Polybios, and Pausanias, have given
descriptions of the rivers on the North African coast which nearly
agree with conditions today. The location of two ancient cities on
the shores of Lake Chott-el-Djerid in Tunis (Lacus Tritonis of old),
which lake, it is claimed, reached a level very much higher than now
500 years B.C., plainly demonstrates that the shore-line then ran very
close to its present position. Students of ancient Egypt are unable to
find evidence of any distinct difference in the climate of that country
from the earliest times to our days. It is true that marshes in the
Nile delta have changed into splendid meadows--but this is the work of
man. The humid period must have ended long before history commenced.
A few of the old writers, such as Herodotos, Aristophanes, and Philo,
assert that rain never falls in Egypt, but this must be classed as an
exaggeration when contrasted with references to rain, snow, and hail
in this section made by other ancient authors, for instance Plutarch,
Pliny, and Ælianus. At any rate it seems as if precipitation was as
rare an occurrence in the land of the Pharaohs as in the land of the
Nile of today.

Against the statement by Huntington that the climate of Palestine
has become very much more torrid during historic times, stands the
assertion by Hilderscheid, who has made a thorough study of these
questions, that no reason whatever exists for such a conclusion.

Of greatest interest to us in this connection are perhaps Italy and
Greece. Huntington believes that the river Alpheios, which flooded
Olympia and covered it with a 4 m. to 5 m. (4 or 5 yards) thick
sediment, carried a far greater volume of water formerly than today.
This flood, however, was caused by an earthquake accompanied by a
fall of rocks whereby the river was dammed. There is no necessity for
assuming a greater abundance of the waterflow. According to Strabo,
the streams Kefissos and Ilissos, between which Athens is situated,
dried out during the summer then as they do now. If we are to believe
Pausanias, the brooks which traversed the Argive plain behaved
similarly, and so they behave today. From all we can judge, the climate
of Greece has not changed perceptibly since the days of Homer.

In regard to Sicily, it is stated that several of its rivers were
navigable in the Middle Age, while such is not the case now. But this
is explained by the devastation of the forests which formerly equalized
the flow of these rivers, and perhaps also by the size of the vessels
of that time. Cultivation in these regions has sharply declined since
antiquity. As a consequence, the loose soil, which formerly was
planted, has been washed away and the dams and retaining walls, built
to prevent a too rapid drain of the water, have disappeared. Thus
the country has grown increasingly arid. Large cities, as Palmyra,
existed in desert regions where lack of water now prevents habitation.
But water was brought to the metropolitan cities of old through long
magnificent aqueducts, the ruins of which partly remain today. We
have all reasons to believe that the marked decrease in cultivation
and population, laid to changes in conditions of humidity, depended
altogether on man’s interference with nature. On certain rocks in
Morocco, carvings have been found portraying in a simple way large
mammals such as elephants, rhinoceroses, and giraffes, which do not
exist in these regions now because of lack of nourishment. But these
rough works of art, resembling those of the bushmen of today, date
from prehistoric time, the so-called paleolithic era, when the climate
admittedly was more humid in these regions than it is now.

Similar conditions obtain, according to Hedin, in Central Asia and in
Persia. The climate there has without doubt been more humid but not in
historic times. The march of Alexander toward India took place under
as adverse conditions as now are found in these regions (Baluchistan).
Their cities, now in ruins, received their water supply through
conduits from rivers some of which were then adjacent to the cities,
although they later have changed their course as pointed out by Leo
Berg.

In Western and Central Europe numerous marshes and morasses have
indeed been drained and rendered available for cultivation, but this
does not prove that the climate is become more dry. On the contrary
all observations, for instance those made by Tycho Brahe on the
island Hven, indicate that the difference between summer and winter
temperature has decreased during historic time; that is the climate is
become less continental, or more humid, than formerly. Furthermore,
many circumstances, such as the occurrence of hazel and of water
chestnut in far more northerly latitudes and the higher altitude of
the timber line in earlier time, prove that the summer in prehistoric
ages was warmer than now. Simultaneously it was dryer. A study of the
lacustrine pile dwellings in Switzerland shows that the levels of the
lakes then were not higher than now but very nearly the same, which
demonstrates that the precipitation has not changed perceptibly in
Switzerland since these buildings were made; the period in question
occurred, we believe, about 7000 years ago.

While great climatic changes have taken place since man’s first
appearance on earth, presumably before the end of the ice period,
historic time is too short to record any distinct modifications.
Local ones may be in evidence such as West Europe’s transition to a
less continental climate. A variation of this nature has been found
not longer back than since thermometrical observations commenced.
Thus the winters in Berlin during the period 1746–1847 were colder
and the summers warmer than during 1848–1907. The difference for
January amounted to -1.5° C. (-2.7° F.) and for May to +0.6° C.
(+1.08° F.). The tabulation below, quoted from Ekholm, shows the mean
temperature in Stockholm, Lund, London, and Paris, during winter
(December-February), spring (March-May), summer (June-August), and
autumn (September-November) and for the following periods:

  -------+-------------------+------------------
         |    _Stockholm_    |      _Lund_
         |-------------------+------------------
         |  _1799– |  _1849– |  _1753– | _1799–
         |  1848_  |  1898_  |  1798_  | 1898_
  -------+---------+---------+---------+--------
  Winter |+25.5° F.|+26.8° F.|+30.2° F.|30.9° F.
         | -3.6° C.| -2.9° C.| -1.0° C.|-0.6° C.
  -------+---------+---------+---------+--------
  Spring | 37.9° F.| 37.9° F.| 41.2° F.|41.5° F.
         |  3.3° C.|  3.3° C.|  5.1° C.| 5.3° C.
  -------+---------+---------+---------+--------
  Summer |   60° F.|   60° F.|   61° F.|60.2° F.
         | 15.6° C.| 15.6° C.| 16.1° C.|15.7° C.
  -------+---------+---------+---------+--------
  Autumn | 43.9° F.| 43.5° F.| 45.9° F.|45.9° F.
         |  6.6° C.|  6.4° C.|  7.7° C.| 7.7° C.
  -------+---------+---------+---------+--------
  Year   | 41.9° F.| 42.1° F.| 44.6° F.|44.6° F.
         |  5.5° C.|  5.6° C.|  7.0° C.| 7.0° C.
  -------+---------+---------+---------+--------

  -------+-----------------+-----------------
         |    _London_     |     _Paris_
         |-----------------+-----------------
         | _1799– | _1849– | _1806– | _1849-
         | 1848_  | 1898_  | 1848_  | 1898_
  -------+--------+--------+--------+--------
  Winter |38.5° F.|39.2° F.|37.9° F.|37.9° F.
         | 3.6° C.| 4.0° C.| 3.3° C.| 3.3° C.
  -------+--------+--------+--------+--------
  Spring |48.2° F.|  48° F.|50.5° F.|50.3° F.
         | 9.0° C.| 8.9° C.|10.3° C.|10.2° C.
  -------+--------+--------+--------+--------
  Summer |61.9° F.|62.2° F.|64.6° F.|64.8° F.
         |16.6° C.|16.8° C.|18.1° C.|18.2° C.
  -------+--------+--------+--------+--------
  Autumn |50.7° F.|50.5° F.|54.2° F.|51.8° F.
         |10.4° C.|10.3° C.|11.3° C.|11.0° C.
  -------+--------+--------+--------+--------
  Year   | 9.8° F.|  50° F.|51.3° F.|51.3° F.
         | 9.9° C.|10.0° C.|10.7° C.|10.7° C.
  -------+--------+--------+--------+--------

The difference is not great. For Stockholm the winter has grown warmer,
the autumn colder, for London the winter warmer and so slightly also
the summer, but spring and autumn a trifle colder, and for Paris the
summer a little warmer while the autumn is considerably colder. Lund
shows the least variation. The winter has grown 0.4° C. (0.72° F.)
warmer and the summer colder by the same amount. The annual mean
remains nearly constant, only slightly increased, but the climate is
become more marine. (This is hardly apparent from the figures cited as
far as Paris is concerned.)

From Tycho Brahe’s observations of the number of days when snow or rain
fell in the place where his observatory was situated on the island
Hven in Öresund not far from Copenhagen, Ekholm has calculated that
the temperature there during the time 1582–1597 was 1.4° C. (2.5° F.)
lower in February and 1° C. (1.8° F.) lower in March than in later
years (1881–1896). On the other hand the first autumn frost occurred at
the same time as now and the same was the case with the last frost in
spring, so that the temperatures on these dates in autumn and spring
were nearly identical at the end of the sixteenth century and now.
Ekholm drew the conclusion that the climate is become more marine.

Hildebrandson makes the objection that Tycho Brahe’s observations
were confined to an abnormally cold period judging by the tables
Speerschneider has prepared showing the ice formation in Danish
navigable waters. Nine out of the sixteen years in which Tycho Brahe
gathered his data were notable for abnormally cold winters, while only
nineteen out of the hundred years composing the sixteenth century were
characterized by equally severe winters.

Thus, the conclusion is not warranted that the winters of the sixteenth
century as a whole averaged colder than those of the nineteenth. Later
investigations (in 1917) in regard to the dates when the ice would
break up in Lake Mälar at Vesterås, in Neva River at Petrograd, and
in Dwina River at Riga, have led Ekholm to believe that he has found
a periodicity in winter temperatures of not less than 212 years, a
conclusion which would agree with Speerschneider’s statistics. If
so, we are at present living in a period remarkable for its mild
winters while a series of extremely severe winter seasons occurred at
the time of Tycho Brahe. This law would also have a bearing on the
preceding table of temperatures in Stockholm, Lund, London, and Paris
as a succession of severe winters came around in the beginning of the
nineteenth century while the reverse is true towards its end. On the
whole climatic variations during historic time have been insignificant,
if present at all, provided we extend our comparisons over two or more
centuries. Such is also the opinion of Hildebrandson.

The idea of a slow deterioration of the climate due to increasing
desiccation is of old lineage and is most likely related to the
venerable conception of a bygone golden age. Aristotle even at that
early date believed that a gradual arefaction of the Earth took
place. In recent times this faith has been particularly fostered by
Huntington in a number of treatises where he endeavours to prove that
Asia, represented for instance by Palestine, Syria, and Persia, and
further Africa and North America are subject to a rapid exsiccation
clearly traceable through historic time. The contrary, however, is true
about Western Europe. It was often said that Southern Russia in recent
times suffered a slow arefaction manifesting itself in the formation of
steppes. This led to careful investigations showing the assertion to
be erroneous and culminating in the work of Leo Berg. Rather, a slight
shifting in the opposite direction is detectable as the forest region
has expanded at the expense of the steppes in conformity with the
development toward the end of prehistoric time. The renowned American
astronomer, the late Dr. Lowell, embraced the idea of increasing
aridity which he observed himself in Arizona where his observatory
is located. The drying out of Arizona undoubtedly took place in
long bygone prehistoric time. The disappearance of a high culture in
Syria and Mesopotamia was the result of hostile devastation of their
waterworks; a compensation is now offered in the reclamation of the
deserts along the Nile, in California and Arizona, and in numerous
other places.




CHAPTER IV

ATMOSPHERE AND PHYSICS OF THE STELLAR BODIES


In a certain sense, we are justified in speaking of an atmosphere of
suns and stars. These bodies consist mainly of a comparatively dense
mass surrounded by a layer of very attenuated gas. The density of our
Sun is about 1.4 times that of water. In other stars it is considerably
lower, in some cases only a few hundredths of the density of water.
This applies particularly to those stars of variable magnitude, the
Cepheid type, named after their longest and best known representative,
the mysterious star Delta in the constellation Cepheus, and in general
to young stars. In any case the stars are gaseous throughout on account
of their high temperature. An exception must be made for the clouds
of matter precipitated by easily condensed vapours, such as gaseous
carbon, which clouds float in the outer strata, and are responsible for
the bright astral light.

The stars just mentioned belong to the comparatively young stellar
bodies, while the Sun in common with other yellow stars is considerably
older. Correlated with their age is undoubtedly the greater mean
density of the yellow stars. Around many of the young stars, for
instance around the brilliant Altair, the principal member of the
constellation Aquila, a gas shell of great expansion has been observed,
usually consisting of hydrogen but frequently also of helium. These
extensive gas appendages may be considered as a kind of atmosphere
surrounding the stars in question. Their density is no doubt
exceedingly small. Our own central orb, the Sun, is also endowed with
rarefied gases outside of the luminous clouds. By their absorption of
light they cause the dark lines in the Sun’s spectrum, after their
discoverer named Fraunhofer lines. The greatest height from the
surface of the Sun is reached by hydrogen, mixed with a small quantity
of helium and with a gas, unknown on Earth, which we call coronium
because it has been observed in the Sun’s corona. These gases may be
looked upon as the atmosphere of the Sun.

Similar conditions obtain no doubt on the major planets, which possess
a density not essentially different from that of the Sun. They have,
moreover, practically the same period of revolution around their own
axis, Jupiter 9.9 hours, Saturn 10.3, and Uranus (probably) 10.8 hours.
Judging by their density, they are, in all probability, like the Sun,
gaseous throughout except for the heavy cloud formations which appear
to constitute the outer limit of these stellar bodies. Their interior,
like that of the Sun, may contain comparatively sluggish gas-masses,
as certain peculiar patches appear on their exterior, similar to the
sunspots and persisting for long intervals, sometimes over a year. The
best known example of the kind is the so-called red spot on Jupiter,
which has remained since 1878, although it is not so pronounced now as
during its early days (see Fig. 11). Characteristic for these planets
are certain bands of a marked delineation and running parallel with the
equator (see Figs. 11 and 12). They are caused by the rapid peripheral
motion of these planets, with Jupiter 28 and with Saturn 24 times that
of the Earth.

[Illustration:

  Fig. 11. The planet Jupiter in 1909 in Mercator’s projection by
           F. le Coultre of Geneva. The “red spot” over which the
           clouds are dispersed, is found at 355° Long. and 20° S.
           Lat. in the outcurving of the dark band. South direction
           is upward, as on all pictures obtained with astronomical
           tubes.
]

Which gases should we expect to find in the atmosphere of these
planets? According to the Kant-Laplace hypothesis, a theory generally
credited with a sound kernel, the planets were segregated from the
Sun’s substance at a time when the latter was expanded so as to include
the orbits of these planets and beyond. Naturally, therefore, their
atmospheres would originally be composed of the very gases that formed
the outmost part of the Sun’s atmosphere, notably hydrogen. Slipher,
who has photographed the spectra here reproduced of the outer planets,
believes that certain strong absorption bands in the spectra of
Neptune and Uranus correspond to the distinctive F and C lines of
hydrogen, using Fraunhofer’s denotation (see Fig. 13). But because
the bands in question, as shown on the figure, are very broad it is
difficult to identify them with certainty. Also other gases of unknown
origin enter into the vapour envelopes outside of the clouds and cause,
as apparent from their spectra, a strong absorption of the sunlight
reflected from the clouds below. The absorption increases with the
planet’s distance from the Sun; thus, it is most pronounced on Neptune
and least on Jupiter.

[Illustration: Fig. 12. The appearance of Saturn September 30, 1909,
according to F. le Coultre of Geneva.]

At any rate, the gas appendages of the heavenly bodies just considered
differ in one essential respect from the atmospheres of the inner
planets: Mars, Earth, Venus, and Mercury. On the Sun and on the outer
planets the atmosphere gradually merges into the interior gas-masses so
that no distinct boundary can be found between the rarer and the denser
layers. Widely different conditions obtain on the Earth. Here the range
of air is sharply defined below by the Earth’s solid crust or by the
oceans. In such case alone may we speak of an atmosphere proper, of
the kind that enters into our commonplace conceptions. Similar are the
conditions on all stellar bodies with a solid or a liquid surface.

[Illustration:

  Fig. 13. Spectra of the major planets, compared with that of
           the moon. The latter corresponds to the spectrum
           of sunlight reflected from a planet which lacks a
           light-absorbing atmosphere. Photos by V. M. Slipher of the
           Lowell Observatory.
]

But it is not certain in every case that all such planets possess an
atmosphere. Observations of the Moon when passing some star show that
the air envelope, if present, is unable to deflect the light beam from
the star, or in other words it has no perceptible power of refraction.
From this we also infer that its density is very small, corresponding
at most to one or two mm. (.04 to .08 inches) barometric pressure. But
we have good reasons to believe that the Moon has been detached from
the Earth, carrying away parts of its lightest substance, which theory
is supported by the fact that the Moon’s mean density (3.3) is only
six-tenths of the Earth’s (which again is 5.53 times that of water),
and we might therefore have expected that the Moon in parting
should have shared in the very lightest constituents of the Earth,
namely its air-covering. Such was unquestionably the actual procedure,
but in the course of time the Moon has lost its originally no doubt
considerable atmosphere. The reason is that the molecules in a gas are
in a continuous rapid motion, which is the swifter the lighter the gas
and the higher its temperature. In hydrogen, the lightest gas known,
the velocity amounts to 1.84 km. (1.15 miles) per second at 0° C.
(32° F.). The parts of the Moon exposed to the strongest sunlight are
heated to about 150° C. (300° F.). At that temperature, the average
velocity of hydrogen molecules is 2.29 km. (1.43 miles) per second.
But a body departing from the surface of the Moon at a rate of 2 km.
(1.24 miles) per second, or more, cannot be retained by the attraction
of that globe and therefore never retraces its path but speeds away
for ever. In the same manner a bullet ejected from a cannon with an
initial velocity of 11.2 km. (7 miles)--a velocity not even approached
by present artillery--would fly away from Earth barring the resistance
of the air. Thus we see that we as yet are far from the realization of
the dreams of Jules Verne in his _A Voyage to the Moon_. At any rate,
gravity on the Moon is too weak to retain hydrogen over the hottest
point of the surface. This part of the gas flies away, new supplies
rush in from the sides, and in a short time all traces of hydrogen have
disappeared from the Moon. Probably it was mainly gathered in by the
Sun, where a velocity of 613 km. (380 miles) per second is necessary if
the molecules are to overcome the Sun’s attraction, while their actual
velocity there amounts to only about 8 km. (5 miles) per second.

In a similar manner we find that the second lightest gas, helium, at
a temperature of 150° C. (300° F.), possesses a molecular velocity of
1.62 km. (1.1 miles) per second. This is less than the 2 km. (1.24
miles) per second necessary to leave the Moon’s sphere of attraction.
But all helium molecules do not move at the same speed; some are
faster and some slower than the average. Those moving at a higher rate
than 2 km. (1.24 miles) per second constitute a considerable fraction
of the total. This fraction disappears. Equilibrium is soon restored
so that in less than a second the same fraction of helium molecules is
ready to depart. In this manner the Moon lost its helium atmosphere
speedily, although not quite as rapidly as its hydrogen.

More slowly yet vanished the gases which are most abundant in our
atmosphere, nitrogen and oxygen, but these too were not fettered
for ever by the limited gravity on the Moon. The same fate befell
aqueous vapour, which is nearly twice as light as oxygen. The loss of
water, however, was long delayed, as we later shall learn, because
new vapour masses were discharged from the lunar volcanoes. In these
considerations, we should also bear in mind that the Moon no doubt was
a fluid molten mass when separating from the Earth and its substance
resembled the lava from our volcanoes. In this condition it remained
until its exterior temperature had fallen to about 1200° C. (2200° F.).
At that point, the average velocity of oxygen molecules is about 1 km.
(.62 mile) per second, with variations in both directions, so that a
few per cent. of them reach a sufficient velocity to leave the Moon for
ever. Such gas molecules of medium weight return probably to the Earth
which, as experience tells us, is ponderous enough to hold them in
bonds.

All gases, that constitute any considerable fraction of the Earth’s
atmosphere, and which, therefore, most likely were divided with the
Moon in its parting from us, have again left that globe. The same
unquestionably holds true for other stellar bodies of equal or smaller
size, such as all the minor planets and for the great majority of the
satellites to the major ones. Only the very largest of Jupiter’s moons,
and possibly Neptune’s lonely companion, whose size is not known with
certainty, might possibly surpass our Moon in ability to retain gases.
Our reasoning with respect to the Moon applies also to Mercury. It is
true that the molecules there must possess a velocity one and a half
times as high as on the Moon if they are to leave the planet. But at
the same time the temperature on Mercury’s hottest point, always turned
toward the Sun, is far higher, about 400° C. (750° F.), so that the
molecules there move 1.26 times as fast as similar molecules over the
Moon’s hottest point. Mercury is consequently better able than the Moon
to retain gases, but the difference is slight. Direct observations (see
below) also lead us to believe that Mercury is very similar to the
Moon in these respects. We might possibly imagine that certain gases,
which on the Moon would condense into fluids or solids, on Mercury
might remain volatilized on account of the high temperature and thus
form an atmosphere. Such assumption, however, would be erroneous. The
investigations by Schiaparelli and by all his successors show that
Mercury in turning around its axis always presents the selfsame side
to the Sun. The opposite side, never reached by a ray of sunlight,
must assume an extremely low temperature, very close to the absolute
zero (-273.7° C. or -460.6° F.) and far below any cold existing on the
Moon. To this side, all bodies with an appreciable vapour pressure
must distil and freeze to solid lumps or frost-coverings without
perceptible vapour pressure. For these reasons, Mercury cannot possess
any atmosphere to speak of. There remain in the whole series of planets
and satellites in our solar system only two bodies besides the Earth
which are endowed with an atmosphere in the original sense of the
word--namely, Mars and Venus.

We reach the same conclusion when investigating the ability of the
planets to reflect the sunlight falling upon them. The bodies which
possess an atmosphere hold also suspended therein clouds of water or
ice, and also of dust, whirled up from below. These floating particles
reflect light far more efficiently than the solid or fluid surface of
a planet. The Moon can now reflect 7.3 per cent. of the sunlight and
Mercury 6.9 per cent. (H. N. Russell, _Proceedings Nat. Acad._, 1916).
These numbers lie so close that they may be considered practically the
same within errors of observation.

It is therefore probable that Mercury is as devoid of an atmosphere as
the Moon. The opposite extreme is represented by Venus, which reflects
not less than 59 per cent. of the sunlight received, according to
H. N. Russell. Terrestrial clouds were found by Abbot to return 65
per cent. We believe from astronomical observations that the entire
surface of Venus is hidden behind a thick opaque cloud-covering.
The slight difference between O.65 and O.59 may be due to errors of
observation, but also to a small absorption of light in those parts of
Venus’s atmosphere which are outside of the clouds. Saturn and Jupiter
are very similar to Venus in this regard with 63 and 56 per cent.
respectively. The gases above the clouds on these planets extinguish
to a considerable extent the sunlight reflected from the clouds, as
apparent from their spectra. (Compare Fig. 13.) Hence the value 0.63
given by Russell for Saturn is probably too high. Concerning Jupiter
it has been observed that its red light becomes deeper when the
sunspots are few, but whiter when the spots are numerous. The sunspots
have been found to favour the formation of high clouds, such as cirrus,
and this would seem to apply also to Jupiter; when spots are plenty,
the clouds are high, and consequently the absorbing layers above, which
cause the red colour, are thinner, so that Jupiter will then shine with
a whiter--less red--lustre than when the sunspots are rare.

The two outmost planets, Uranus and Neptune, return, according to
Russell, 63 and 73 per cent. respectively of the sunlight received.
These figures are probably too high. They do not agree well with
Slipher’s records of their spectra (Fig. 12).

There now remains Mars. This planet approaches the Moon inasmuch as
it reflects only 15.4 per cent. of the sunlight arriving to the orb.
Everything points to the conclusion that the atmosphere of Mars is very
thin. Lowell estimates, on somewhat meagre grounds, however, that on
each square metre of the planet rests only 22 per cent. of the mass of
air supported by each square metre of the Earth’s surface.

It would naturally be very interesting to ascertain the amount of
sunlight our Earth throws back into space. This we cannot measure, as
we cannot place our instruments outside of the Earth’s cloud-mists
nor can we read them there. Not less than 52 per cent. of the Earth
is covered with clouds, whose whiteness (Latin: _Albedo_) is 65. Thus
the clouds alone return 0.52 × 0.65 = 0.338 parts of the sunlight. Of
this portion a fraction amounting to about 4 per cent. is extinguished
in the air above. The remainder is 0.325. Atmosphere and suspended
dust reduce the sunlight over the cloud-free part, _i. e._ 48 per
cent. of the Earth, by 60 per cent, half of which returns to space,
while the other half reaches the ground in the form of light from the
sky, and of this fraction again about 4 per cent. is reflected into
space; these two items added give 0.15. Finally, the 40 per cent.
sunlight directly received by the Earth’s surface is reflected to the
extent of 6 per cent. by the oceans and by the generally moist ground;
deserts and bare rocks reflect about twice as much, but their total
area is comparatively small; of this 6 per cent. reflected light, 70
per cent. reaches outside space; thus we obtain 0.48 × 0.40 × 0.06 ×
0.70 = 0.008. In all, therefore, the amount of reflected sunlight is
0.338 + 0.15 + 0.008 = 49.6 per cent. If the air were free from clouds,
the reflexion-number or Albedo would be 33 per cent., or considerably
higher than that of Mars. When now half or a little more (52%) of the
Earth’s surface is overcast with clouds and this portion therefore has
the whiteness of Venus, the figure 49.6 (Russell calculates the figure
45) for the entire Earth naturally falls closer--almost 3.6 times--to
59, the figure of Venus, than to 15.4, the figure for Mars. We may
also compare the value 33 per cent., which applies to the cloud-free
portion of the Earth, with the value 15.4 per cent. for Mars, which is
almost without clouds, and with the value 7.3 per cent. for the Moon,
which has neither clouds nor dust, because it lacks an atmosphere. We
can then conclude that the atmosphere of our Earth holds almost three
times as much dust suspended over each square metre as does Mars, and
this in spite of the smaller gravitational force on Mars, which is
about 37.5 per cent. of that on Earth. Taking proper account of the
low temperature on Mars we may easily compute, by means of a formula
given by Stokes, that a particle of dust should sink 2.3 times slower
on Mars than on Earth. When, nevertheless and in spite of frequent
but thin mists, so few particles of dust float in the atmosphere of
Mars, the conception inevitably comes to our mind that the air on
that planet must be extremely rarefied so that the wind-puffs have
little power to raise the dust from the ground. Lowell estimated the
barometric pressure at the surface of Mars to be about 64 mm. (2.52
inches), and Proctor gives about twice this figure. There appears to
be ground for considering already the former value too high; both are
very uncertain. If we accept that of Lowell, we find that each square
metre of the surface of Mars supports a column of air, whose mass is
only about one-fifth of the mass resting on each square metre of ocean
surface on Earth.

[Illustration:

  Fig. 14. The planet Venus, with sunlit atmosphere (to the
           left), as observed by Langley at the Venus passage
           December 6, 1882.
]

The dense clouds which float above Venus have long ago led to the
assumption that the atmosphere of that planet must be far deeper than
that of the Earth. Its strong refractory power has also contributed
to this belief. When Venus is close to the sun-disc the dark body
stands forth surrounded by a ring of light (see Fig. 14). It is,
however, recognized that this phenomenon requires no greater air
density than that on Earth for its appearance. In this connection we
should remember that the inside limit of the vapour shell which we
in this manner observe, is the cloud-wall, not the ground. And these
clouds, we have every reason to believe, float on account of the heat
prevailing at a great height in the atmosphere, so high in fact that
they form an impenetrable wall already where the cirrus clouds
appear in our sky. If these suppositions are correct, the light-ring
mentioned is caused by a quarter only of the air-masses on Venus, and
its total air-covering must be far deeper than that of the Earth.
The latter occupies probably in this respect as well as in reference
to position in space a middle ground between Mars with its extremely
thin and Venus with its comparatively dense atmosphere. If so, we
might expect the atmosphere on Mercury to be denser yet, while we
already have seen that it is almost wholly lacking on that planet.
The explanation is that Mercury has lost its spin around its own axis
and therefore always presents the same side to the Sun--just as the
Moon and probably all other satellites turn one side only toward their
respective central bodies--hence the opposite side becomes so cold
that all gases are there condensed to fluids or solids except the two
most volatile ones, hydrogen and helium, which on the other hand leave
the planet on its hot side. If Venus, therefore, as held by several
astronomers from Schiaparelli to Lowell, always turned one side only
toward the Sun, this planet also would be without any perceptible
air-covering. According to investigations by Bjelopolsky of Venus’s
spectrum, which investigations, however, are in complete disagreement
with corresponding measurements by Slipher, that planet has a period
of rotation on its own axis of about 29 hours. This figure is very
uncertain and a new determination is therefore highly desirable.

In order to understand the atmospheres of the planets, it is of great
interest to ascertain the composition of the air that surrounds the
Earth. Our knowledge in these matters has grown considerably of late.
We shall in the main follow the presentation by Dr. Wegener of Marburg.

We know at present with considerable accuracy which gases enter into
the air. Besides the previously well-known nitrogen and oxygen which
contribute the bulk, 78.1 and 20.9 per cent. respectively, of the total
volume at the earth’s surface, we find water vapour in proportions
changing with localities and times, and it is for this reason left
out when fixing the various percentages; further, carbon dioxide 0.03
volume per cent. and the rare gases discovered by Rayleigh and Ramsay,
argon, 0.932 per cent., neon 0.0012 per cent., helium 0.0004 per cent.,
krypton 0.000005 per cent., and xenon 0.0000006 per cent. Each one of
these constituents diminishes in quantity with height in accordance
with the so-called barometer-formula and the rate is the more rapid
the heavier the gas. Krypton and xenon, therefore, which are two and
one half and four times heavier than oxygen, occur mainly in the lower
strata. The percentage of helium on the other hand, a gas eight times
lighter than oxygen, should increase rapidly with height. If the air
consisted of a mixture of oxygen and helium at 0° C. (32° F.) the
former would decrease to one half at an elevation of 5 km. (3.1 miles)
but the latter not before we had ascended 40 km. (25 miles) (eight
times higher than for oxygen, as the weights are in the ratio of 1 to
8). At that altitude, oxygen would have decreased in the proportion
1:2^8 = 1:256. When, as actually is the case, there is 50,000 times
more oxygen than helium at the surface this ratio should decrease
in the proportion 128:1 at a height of 40 km. (25 miles). Ninety
kilometers (56 miles) above the surface helium should overbalance
oxygen and thereafter rapidly gain in preponderance. This holds true
provided no agitation takes place in the form of vertical currents of
air.

Similar laws apply to all light gases which do not turn into fluids or
solids at low temperatures. Aqueous vapour, on the other hand, which
when cooled condenses to clouds, diminishes much faster than the nearly
twice as heavy oxygen, because the temperature rapidly decreases as we
move upward or at a rate of about 5° C. per km. (14.5° F. per mile)
up to 2.5 km. (1.5 miles) and of 8° C. per km. (23° F. per mile) at a
height of 8.5 km. (5.3 miles). The quantity of water vapour shrinks to
one half at 1.9 km. (slightly more than a mile) above ground. Carbon
dioxide again follows the barometer-formula applicable to other gases
because it occurs in such minute quantity that it never condenses to
clouds. In fact, it is water vapour alone which must be treated as an
exception. Carbon dioxide is nearly one and one half times heavier than
the other gases of the atmosphere on an average. It should therefore
diminish in the proportion 1:2^{1.5} = 1:2.8 in a vertical distance of
5 km. (3.1 miles) while the density of the air decreases only in the
ratio 1:2. Several determinations of the presence of carbon dioxide
in the atmosphere as high up as 3.8 km. (2.33 miles) have been made,
by S. A. Andree among others, but the percentage of this gas remains
constant within the errors of observation. The same holds true to a
height of 7 km. (4.35 miles) for the proportion between oxygen and
nitrogen, although we might have expected a perceptible change as
oxygen is 14 per cent. heavier than nitrogen. How shall we explain this
fact which seemingly contradicts the theory just advanced?

The explanation is quite simple. The preceding statements hold true for
a mass of air at perfect rest. But, if the air is violently agitated,
the composition becomes homogeneous all through. We know that in the
barometric cyclones and anticyclones strong rising and descending air
currents flow. The composition of the atmosphere, therefore, becomes
the same as far up as this mixing action prevails. These currents
produce another effect, namely, a fall of temperature with rising
height. Because when a gas is transported upward the surrounding
pressure decreases, resulting in expansion and consequent cooling. It
is well known that a gas is heated when (rapidly) compressed, a quality
formerly made use of in the pneumatic fire-tool to ignite tinder. It is
evident that conversely a gas must cool off when expanding. If now the
mixing of the air were extremely rapid the thermometer would fall very
close to 10° C. (18° F.) with each km. (.62 miles) rise in elevation.
If, on the other hand, the air stood perfectly still in a vertical
direction, the temperature would remain constant at all heights
over the same point. Between these two extremes, we find the actual
condition, inasmuch as the temperature of the atmosphere decreases
upward 5° to 8° C. per km. (14.5° to 23° F. per mile) as observed
during balloon ascensions.

This applies to the so-called “troposphere”--mixing-zone. One of the
most remarkable discoveries in recent times, made by Teisserenc de Bort
and Assman, is the fact that the decrease of temperature with height
does not continue indefinitely but only up to a certain elevation,--in
middle Europe about 11 km. (7 miles), in Lapland about 7 km. (4.5
miles), and at the equator about 15 km. (10.5 miles)--and above this
point the temperature remains constant. We now meet the peculiar
condition that the temperature of this upper layer, which is called
stratosphere--“film-zone”--is lowest over the equator, because it
commences at a great height there, and lowest over the polar regions,
where it extends farther down. The stratosphere has received its
name from the fact that it consists so to speak of lamellæ almost
parallel to the earth’s surface and moving in a horizontal direction
while vertical motions are absent. The winds in these strata have a
marked westerly direction (_i. e._ they are east winds) and they become
stronger the higher the stratum--at an altitude of 83 km. (52.5 miles)
their velocity is about 100 m. (330 ft.) per second. In the troposphere
on the other hand west winds predominate. The wind direction in the
stratosphere was observed on the so-called luminous night-clouds which
were found as high as 80 km. (50 miles) above the Earth. These strata
consequently revolve slower around the earth’s axis than the solid
body of the planet itself. At an elevation of 80 km. (50 miles) the
rotational speed has decreased to 65 per cent. of the angular velocity
of the earth’s surface. We have reason to believe that the very highest
strata stand still, that is do not take part in the earth’s rotation on
its axis. This would follow if outside space were not entirely devoid
of vapour so that our atmosphere would merge imperceptibly into the
exceedingly attenuated gas masses of interplanetary space.

As high as the mixing-zone extends, so high also is the composition of
the air constant and like that at the surface of the earth. But above
this limit--in Scandinavia, we might say above an elevation of 10 km.
(6.2 miles)--commences a rapid tapering of the heavy gases, while the
percentage of the light ones correspondingly rises. Foremost among the
latter is hydrogen, with only half the weight of helium. The presence
of hydrogen in the atmosphere has been shown by Boussingault, and
the proportion in which it occurs has later been measured by Armand
Gautier. It is about one three hundredth part of one per cent. It
increases extremely rapidly with height in the stratosphere so that 80
km. (50 miles) above the earth and upward hydrogen is more abundant
than all other known gases of the atmosphere at the same altitudes.

We reproduce below a somewhat revised table by Dr. Wegener of Marburg,
who has made the most recent computation of the percentages of the
various constituents of air at different heights. Consideration has
been given to the fact that the composition of the air does not change
except as regards the percentage of moisture within the troposphere,
which is assumed to reach a height of 10 km. (6.2 miles). As usual in
similar cases the percentages refer to volume.

  -------------+-----------------+----------+--------+----------
    _Height_   |   _Pressure_    |          |        |
  -----+-------+--------+--------+          |        |
  _in  | _in   |  _in   |  _in   |_Hydrogen |_Helium |_Nitrogen
   km._| miles_|   mm._ | inches_|    2_    |   4_   |    28_
  -----+-------+--------+--------+----------+--------+----------
     0 |   0   |760     |29.9    |   0.0033 | 0.0005 |   78.1
  -----+-------+--------+--------+----------+--------+----------
    10 |   6.2 |197     | 7.75   |   0.0033 | 0.0005 |   78.1
  -----+-------+--------+--------+----------+--------+----------
    30 |  18.6 |  8.95  |  .352  |   ----   |  ----  |   85
  -----+-------+--------+--------+----------+--------+----------
    50 |  31.0 |  0.45  |  .0177 |   1      |  ----  |   88
  -----+-------+--------+--------+----------+--------+----------
    70 |  43.5 |  0.045 |  .00177|  13      | 1      |   80
  -----+-------+--------+--------+----------+--------+----------
    90 |  55.8 |  0.0157|  .00062|  68      | 5      |   26
  -----+-------+--------+--------+----------+--------+----------
   110 |  68.2 |  0.0116|  .00046|  94      | 5      |    1
  -----+-------+--------+--------+----------+--------+----------
   130 |  80.6 |  0.0097|  .00038|  96      | 4      |    0
  -----+-------+--------+--------+----------+--------+----------
   210 | 130.2 |  0.0055|  .00022|  99      | 1      |   ----
  -----+-------+--------+--------+----------+--------+----------
   310 | 192.6 |  0.0032|  .00013| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------
   410 | 254.2 |  0.0021|  .00008| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------
   510 | 316.2 |  0.0016|  .00006| 100      |  ----  |   ----
  -----+-------+--------+--------+----------+--------+----------

  -------------+-----------------+--------+-------+---------+-------
    _Height_   |   _Pressure_    |        |       |         |
  -----+-------+--------+--------+        |       |_Carbon  |
  _in  | _in   |  _in   |  _in   |_Oxygen |_Argon | Dioxide |_Water
   km._| miles_|   mm._ | inches_|   32_  | 39.9_ |   44_   |  18_
  -----+-------+--------+--------+--------+-------+---------+-------
     0 |   0   |760     |29.9    |  20.9  | 0.937 | 0.03    | 1.41
  -----+-------+--------+--------+--------+-------+---------+-------
    10 |   6.2 |197     | 7.75   |  20.9  | 0.937 | 0.03    | 0.14
  -----+-------+--------+--------+--------+-------+---------+-------
    30 |  18.6 |  8.95  |  .352  |  15    | 0.29  | 0.0064  | 0.5
  -----+-------+--------+--------+--------+-------+---------+-------
    50 |  31.0 |  0.45  |  .0177 |  10    | 0.10  | 0.0014  | 1.7
  -----+-------+--------+--------+--------+-------+---------+-------
    70 |  43.5 |  0.045 |  .00177|   6    | 0.05  | 0.0005  | ----
  -----+-------+--------+--------+--------+-------+---------+-------
    90 |  55.8 |  0.0157|  .00062|   1    | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   110 |  68.2 |  0.0116|  .00046|   0    | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   130 |  80.6 |  0.0097|  .00038|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   210 | 130.2 |  0.0055|  .00022|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   310 | 192.6 |  0.0032|  .00013|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   410 | 254.2 |  0.0021|  .00008|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------
   510 | 316.2 |  0.0016|  .00006|  ----  | ----  |  ----   | ----
  -----+-------+--------+--------+--------+-------+---------+-------

Under the name of each gas its molecular weight is given as a measure
of corresponding specific weight. The quantity of water vapour was
not included when the percentages of the other gases were calculated,
because it changes considerably with locality and time. The number
given in the table for water is the mean for the entire globe--it
corresponds to 11.4 grammes per cubic metre (.31 oz. per cu. yd.)--or
the amount present in air saturated with moisture at 16.5° C.
(61.7° F.). The bulk of the water vapour forms a layer strongly
concentrated toward the surface of the earth. Carbon dioxide also
tapers rapidly with increasing height because its density is 1.5
times greater than that of air. This is apparent from the molecular
weight 44 stated under carbon dioxide, while the average molecular
weight of air is 29. Faster yet do krypton, with a molecular weight of
83, and xenon, with a molecular weight of 131, decrease as we ascend
in the atmosphere. These gases, like neon, whose percentage first
increases slightly with height, and argon, which decreases upward as
shown in the table, do not perceptibly influence the processes of
nature. The reverse is true about water vapour and carbon dioxide,
which nourish the plants and also protect the Earth against a too
rapid heat radiation into space. We well remember how abruptly the
temperature changes in the course of the day in the dry desert
climate, while corresponding variation is comparatively slight in
humid climates (compare page 86). This is the result of the ability of
water vapour to arrest the radiation from the Earth. Carbon dioxide is
about evenly distributed over the globe--although somewhat sparser
over highlands--and its heat-conserving and equalizing influence is,
therefore, not so manifest as that of moisture. Only by the most
accurate investigations has this influence been demonstrated.

In Wegener’s table a gas is included, called Geocoronium, whose
existence in the air has not been directly proved. Conspicuous is,
however, the green light displayed at great altitude by the Northern
Light arches, a green color which does not, as far as we are aware,
belong to any known constituent of the air. It is true that the
corresponding spectral line (557 µµ) lies very close to a line
belonging to krypton, but the latter is a heavy gas which cannot occur
to any traceable extent in the high strata, more than 300 km. (186
miles) above the earth, where occasionally the Northern Light arches
appear--their favoured height according to measurements by Störmer
is about 120 km. (75 miles). Wegener assumes, therefore, that this
green line belongs to an hitherto unknown substance, Geocoronium,
which should be five times lighter than hydrogen. Recent researches
present great difficulties to the acceptance of this assumption, and
for this reason further discussion of the problem will be omitted.
Above a height of 210 km. (130 miles), this gas, according to Wegener,
would preponderate. If such postulated gas does not exist, hydrogen
completely dominates in these regions and down to 85 km. (53 miles)
above the Earth. Because hydrogen is so light, the density of the
air in the range of a barometric pressure below 0.02 mm. (.0008
inches) increases but slowly as we descend toward the Earth. This
uppermost part of the atmosphere may appropriately be designated as
the hydrogen-zone. Even within this range, the “shooting stars” meet
sufficient resistance to flame into light at a height of about 120
km. (75 miles) and dissolve into dust which turns dark about 85 km.
(53 miles) above the Earth. E. C. Pickering recognized the spectrum
of hydrogen in the light of meteors passing at great height, but
decomposed water vapour might possibly be its source. Meteors crossing
lower strata show the spectrum of nitrogen. Nitrogen becomes important
from a height of about 85 km. (53 miles) downward and from 75 km. (46.5
miles) to the surface of the Earth it predominates. As a consequence
the pressure increases rapidly as we approach the ground. In these
regions or up to 80 km. (50 miles) floated the highest luminous night
clouds, observed by Jesse, indicating that here commenced a new range,
the nitrogen-zone. Only the heavy meteorites are able to penetrate
into the nitrogen sphere, which checks their speed and causes them to
explode, and thereafter the remnants fall with a velocity compatible
with the air resistance they meet. To these parts descend also the
lowest rays of the Northern Lights, the so-called draperies--Störmer
observed them once at a height of 37 km. (23 miles). Finally, water
vapour presents itself in appreciable quantities at an elevation of
about 10 km. (6.2 miles), where the troposphere commences. We now meet
the highest clouds, cirrus (with the exception of the “luminous night
clouds” observed only in the years 1883–1892 after the eruption on
Krakatoa). To these heights, reach the vertical air currents which are
essential to cloud formations. Only light clouds, however, float at
these altitudes; the heavy clouds (alto-cumulus) do not rise above 4
to 5 km. (2.5 to 3.1 miles) and the rain clouds proper (cumuli) occur
only below a height of 2 km. (1.25 miles). This is the result of the
downward concentration of water vapour within the troposphere.

If gravity decreased in intensity, the effect would be the same as if
the gases were lighter. On Venus, the intensity of gravity is eight
tenths of that on Earth. The difference is slight. If everything else
were similar the various air-zones would reach 25 per cent. higher on
Venus than they do on our globe. But one essential condition is varied
by the far higher temperature on our neighbour. The proportion of
moisture in the air is thereby vastly increased. The dense clouds rise
to much greater heights than on Earth. If there be ten times as much
water in the air on Venus as there is in the air on Earth--which might
fairly represent the actual condition--the heavy rainclouds would there
rise to a height of more than 10 km. (6.2 miles), and their smaller
weight on Venus would also contribute to their buoyancy. The light
cirrus clouds should appear as high as about 30 km. (18.5 miles) above
the ground. Under such circumstances, we cannot expect but that the
planet must be entirely hidden from our sight as well as from the rays
of the Sun.

On Mars, the intensity of gravity is 2.68 times smaller than on Earth.
In consequence barometric pressure falls 2.68 times slower with
increasing height there than here. The same ratio holds for decrease in
temperature and for shrinkage of proportion of moisture when comparing
conditions on the two planets. The strong cold precludes anything but
insignificant quantities of water vapour. The air on Mars is similar
to the atmosphere on Earth in and above the cirrus-region. The clouds
existing there are not only extremely thin and transparent--it is well
known that cirrus clouds throw no shadows--but they are confined to
small fractions of the planet’s sky. They are replaced by light mists.

We shall later return to the consequences of these peculiarities.

With help of the spectroscope we have ascertained that the gases on
the Sun are, in the main, also arranged according to specific weights,
so that the lightest reach the greatest heights. Somewhat similar
conditions obtain in the gas-appendages of the stars (compare page
119).




CHAPTER V

THE CHEMISTRY OF THE ATMOSPHERE


Of a very particular interest is the question of the atmosphere of
the planets. The great problem of habitability of the latter is most
intimately connected therewith. Primitive fancy, very early, populated
the stellar bodies, especially the stars and the Sun, with beings
similar to those on Earth. Gradually, however, the insight awoke that
these bodies are incandescent and therefore unfit to shelter life of
the kind with which we are familiar. Attention then turned to the
planets, as they and the Earth belonged to the same order of heavenly
bodies. Perhaps they furnished abodes for our kin. And the stars, suns
like our Sun, should not they be surrounded each with its throng of
planets, gravitating around their central source of light and heat?
This beautiful thought vied with the conception of Earth as the centre
of the universe and as wholly set apart from the other stellar bodies,
whose prime object it were merely to furnish light and to indicate time
for the inhabitants of the Earth. Much to be regretted, it was the
latter far less attractive theory which gained firm hold on the Church,
although a few of its unbiased men, like the renowned Cardinal Nicolaus
Cusanus (1401–1464), declared in favour of the contrary opinion, and
did so unmolested. But times grew harder, ironclad orthodoxy triumphed,
and Giordano Bruno, whose defence was that he simply accepted the
theory of the great Cusanus, was burned at the stake to expiate his
fearless assertion that other worlds, no less than ours, might be
blessed with the presence of living beings.

Undoubtedly, other planets are upbuilt of the same material which
enters into the composition of the Earth--as held already by Leonardo
da Vinci. Spectral analysis teaches us that the same constituents
form all the suns, including our own. If we agree that the suns have
furnished the original substance of the planets revolving around them
it is a natural conclusion that this matter should combine into similar
chemical compounds on planets equally advanced in their evolutionary,
that is cooling, process. And we know, indeed, that the same elements
compose the Sun and the Earth, and that the samples brought to us from
other worlds, _i. e._ the meteors, are of a composition which strongly
reminds us of the rocks in the interior of the Earth. We seek in vain
only for indications of the action of water, which substance has left
such obvious traces on the surface of our globe and in the immediate
strata below. But it should be remembered that the water, in the form
of vapour, as previously set forth, has left all the minor stellar
bodies, and the meteorites manifestly belong to the small or smallest
among the wanderers of the heavens.

There is then no reason to doubt that the material of which the
planets are built essentially is the same throughout the universe.
Their interior should, like that of our Earth, consist of the heavy
metals, principally iron,--strongly prevalent also in the Sun and in
the meteors,--and this metallic nucleus should be clothed with the
silicates, oxides, carbonates, sulphides, and hydrates of all metals,
particularly aluminum, and among the metals we may also count hydrogen.
The melting points of these exterior and lighter substances lies above
1000° C (1800° F.). No life could exist in such a molten mass, so
that not until a solid crust had been formed through cooling was the
possibility of life at hand.

Life is, at least on Earth, tied to certain so-called compounds, in
which carbon is the essential common element, while hydrogen, nitrogen,
and oxygen together with sulphur, phosphorus, iron, magnesium, and a
few other less important elements also enter therein. No substance but
carbon possesses this quality of being a prerequisite of life. Silica
is a close kin to carbon and a substitute in certain organic compounds,
but protoplasm, the main constituent of the living cell, cannot be
built without carbon. In the inorganic world, however, silica by
virtue of its affinity, which is kindred to that of carbon, plays a
rôle somewhat similar to the latter in the almost infinitely variably
silicates. Protoplasm cannot endure in a temperature above 60° C.
(140° F.) or thereabout--certain algæ, it is sometimes stated, thrive
in hot springs up to 80° to 90° C. (176° to 194° F.) but certainly not
over 100° C. (212° F.). At these temperatures--strictly speaking at
all temperatures between O° C. (32° F.) and 365° C. (689° F.)--water
can exist in fluid state and this too is a prerequisite of life. We
may say therefore that life is confined to a small temperature range
between the freezing and boiling points of water. But wherever water
occurs, except in a vessel which it completely fills, there exists also
in the adjacent space, if unoccupied by fluids or solids, water vapour
of at least 4.6 mm. (.18 inches) pressure. There is, therefore, always
an atmosphere of aqueous vapour on any planet whose surface is partly
covered by water.

The palæontologists have agreed that all life commenced in the water.
The manifold living beings which now inhabit the solid crust of the
Earth all descend from ancestors which floated in the waves of the
ocean, the cradle of all organisms. It is not absolutely certain
that oxygen is necessary to all living beings but many biologists
hold that opinion. Certain bacteria are able to draw the oxygen they
require for their development from compounds in which oxygen is bound
sometimes in a very intimate manner, as in sulphates. But these
bacteria are considered degenerate plants, and free oxygen is certainly
indispensable to the existence of the animals and probably also to the
plants with the exception just mentioned. As we shall see later, free
oxygen cannot be present on the planets until a solid crust has been
formed. We may therefore state that the conditions for the existence of
life on a planet are fulfilled when a true atmosphere containing oxygen
and water surround its body.

If we are to understand these conditions, we must study the processes
whereby oxygen is supplied to the atmosphere. As the planets are
segregations from the Sun, they should originally have the composition
of the Sun, particularly that of its outer layers. Here metals occur in
greatest abundance, but there are also a few oxides, especially those
of titanium and magnesium (according to Fowler), hydrogen in great
quantities, oxygen, carbon, cyanogen, and carbon monoxide. It may seem
strange that free oxygen exists side by side with a surplus of hydrogen
and sodium, strong so-called reducing substances which bind oxygen.
But at the high temperatures prevailing on the Sun compounds of oxygen
and the reducing substances, for instance hydrogen, _i. e._ water, are
largely dissociated into their constituents. But, if the temperature
should drop to about 1200° C. (2200° F.), at which point crust building
does not yet take place, the oxygen would be entirely absorbed in the
formation of the compounds mentioned. The compound substances of the
Earth, like those of the Sun, are also strongly reducing, so that we
must infer that free oxygen did not enter into the gas covering of
the Earth at the time when its solid crust was formed. We may gain
a conception of the gases which then existed in the outmost layers
of the Earth by studying the gases on the Sun and on other stellar
bodies, particularly the comets, and also by investigating the gases
absorbed by the molten interior of the Earth. Previous to the crust
formation, the entire mass of the Earth except the gases in its outmost
layers were of the same character as possessed by its molten interior
now. This molten mass, or magma, when in contact with the surrounding
gases partly imbibed them through the process called absorption. An
investigation of the gases present in the magma will therefore give
us an idea of those existing in the original vapours surrounding the
Earth. The magma occasionally comes into view in volcanic eruptions,
and the confined gases are then partly given up into the air, but they
are also partly retained in the solidifying lava, or volcanic rocks,
whence they may be driven off by high temperature and subsequently
analyzed. The direct gas emanations from the craters may also be
gathered and analyzed. Such investigations have been carried out on
a large scale by Albert Brun, Frenchman, Arthur Day, American, and
his co-labourers Shepherd and Perret. Brun reached the surprising
conclusion that water vapour, hitherto considered the most important of
the volcanic gases, in reality was not one of them but originated in
the crust of the earth. This theory, however, was completely refuted
through the investigations by Day and his associates. As an example,
we give the analysis (the mean of several determinations) of volcanic
gases from the crater Halemaumau on the volcano Kilauea in Hawaii:

           _May, 1912_
      _Per cent. of Volume_

      Carbon dioxide    55.4
      Carbon monoxide    4.3
      Hydrogen           7.7
      Nitrogen          29.6
      Sulphurous acid    2.9

        _December, 1912_
      _Per cent. of Weight_

      Carbon dioxide    42.9
      Nitrogen          25.8
      Water             27.5
      Sulphurous acid    3.7

In the latter case, it has been shown that air had gained entrance to
the volcanic gases, which air might have carried a quantity of water.
But this quantity could not have been large judging by the amount of
nitrogen present which corresponds to not quite 3 per cent. water by
weight. In the former case water was not included in the analysis. At
any event, a high percentage of water has frequently been observed in
volcanic gases.

When the gases were left in contact with water a considerable part was
absorbed thereby, particularly compounds of chlorine and fluorine and
also ammonia and sulphurous acid. An analysis of such water showed 10
per cent. more fluorine than sulphurous acid and two fifths as much
chlorine as fluorine. The ammonia amounted only to half of one per
cent. of the chlorine. None of the rare air gases were present, which
indicates that the nitrogen originated entirely from the magma and not
from the air.

Brun has analyzed lavas from different volcanoes. The gases extracted
therefrom naturally do not give as reliable information about the
original atmosphere of the Earth as do the gases directly emanating
from the volcanoes. As examples we quote the composition of the vapours
in lavas ejected March 4, 1901, from Stromboli, and from Vesuvius in
the well-known eruption of 1906. They show in percentages of volume:

                       _Stromboli_    _Vesuvius_
  Free chlorine           12.8            0
  Hydrate of chlorine      2.0            6.6
  Sulphurous acid          4.5           12.0
  Carbon dioxide          60.2           73.8
  Carbon monoxide         11.5          traces
  Hydrogen                 0.5            7.6
  Nitrogen                 6.9          traces
  Marsh-gas                1.6            0
                         -----          ------
                         100.0           100.0

It will be noticed that the composition varies considerably. Free
chlorine cannot very well be primigenial as it, like oxygen, combines
with reducing substances. Chlorine may be produced by heating chloride
of calcium with silica and ferro-silicates which are present in the
magma. At all events, carbon dioxide constitutes the bulk. Next
in importance are sulphurous acid and hydrate of chlorine. Carbon
monoxide, hydrogen, and nitrogen may occur in quite considerable
quantities but are sometimes almost wholly absent.

Day and Shepherd reached the conclusion that the gases emitted by the
hot lava in the Halemaumau crater are nitrogen, water, carbon dioxide,
carbon monoxide, sulphurous acid, hydrogen, sulphur vapour, also
small quantities of chlorides, fluorides, and possibly ammonia. Such,
at least approximately, should also the original composition of the
Earth’s atmosphere have been when the crust was newly formed. Nitrogen,
water, and carbon dioxide were the most important ingredients; in
the high strata hydrogen was present. Oxygen was totally lacking and
reducing gases such as hydrogen, sulphurous acid, and carbon monoxide
abounded instead. If we further note the composition of comets and
meteorites we find that cyanogen, carbohydrates, and carbon monoxide
also are present in the former, argon and helium in the latter. It
is therefore probable that these substances, although absent in the
emanations from Kilauea, yet belonged to the primary atmospheres of the
planets. The rare gases of the air especially should originally have
come from the exterior parts of the sun, as did the nitrogen.

An atmosphere of such a composition would be utterly unendurable to
living beings. It must, if organisms are going to thrive therein,
be purified from such poisons as carbon dioxide, gaseous sulphur,
cyanogen, and sulphurous acid. We know that such a process has taken
place and that the sunlight has been the great chemist who produced
oxygen and carbon from the carbon dioxide. The just mentioned poisonous
gases were subsequently oxidized through electrical discharges. We
all know that the plants up-build their framework under the influence
of sunlight, consuming in the meantime carbon dioxide, water, and a
little ammonia. In the process, oxygen is formed, as are also starch,
cellulose, sugars, and albuminous substances with the aid of the
green colouring matter in plants, chlorophyll, which considerably
accelerates the action. Subsequently these new substances, which all
(except the albuminous ones) belong to the carbohydrate group, are
converted into principally carbon and water. The final result is that
carbon dioxide through the agency of sunlight is split into its two
constituents, carbon and oxygen. This process, which is comparatively
rapid in the presence of chlorophyll, should also, although more
slowly, take place without that medium; and in recent trials the
chemists--particularly Daniel Berthelot--have, indeed succeeded in
imitating without chlorophyll this important function of the plants
through the application of light of a short wave length. In the course
of the many millions of years which geology has proved necessary
for the evolution of our planet, the carbon dioxide in the air was
gradually converted into oxygen and carbon. As long as reducing gases,
such as the poisonous ones mentioned, or any considerable quantities
of carbohydrates and hydrogen yet remained in the atmosphere, the
oxygen was consumed in their combustion. If a solid crust had not
existed to prevent the oxygen from entering into the interior molten
mass, it would also have found its way there and would have oxidized
the reducing substances in the magma. The separation of the interior
from the surrounding gas shell is therefore a necessary requisite for
the existence of free oxygen in the air. Another condition is that the
combustible gases escaping from the volcanoes must be added to the air
at a sufficiently slow rate not to consume all of the simultaneously
formed oxygen. A third requirement is that the liberated carbon should
not in a renewed process of oxidation bind the oxygen just recovered.
As long as the air still was reducing, this last condition was no doubt
fulfilled for that very reason. At all events, the crust once formed
and the original violent volcanic activity somewhat abated, the time
finally arrived when free oxygen existed in the air. The previously
present reducing gases were, except for small fractions, burned into
water, carbon dioxide, and sulphuric acid and the nitrogen compounds
had no doubt yielded free nitrogen to the stores of that gas already
a part of the atmosphere. The time was now ripe for the first plants,
probably low forms of algæ, which, in the oceans, commenced life on our
planet. The carbon dioxide and hydrochloric acid of the air as well as
the newly formed sulphuric acid were absorbed by the running water and
caused a rapid disintegration producing silica and acid silicates. As
plant life developed and extended the formation of oxygen increased.
Falling vegetable matter was imbedded in slime which prevented the
access of oxygen during decay and in this manner the fossil fuels were
deposited. Koene of Brussels first pointed out that the carbon and the
sulphuric compounds accumulated in the Earth would suffice to bind the
oxygen of the air. Later investigations lead to the conclusion that the
carbon alone is sufficient for the purpose. It would therefore appear
that all the oxygen of the air is derived from carbon dioxide belonging
to the original atmosphere or contributed thereto by the volcanoes.

The reason why carbon dioxide and water continuously are liberated from
the magma is undoubtedly that the acid silicates are lighter than the
basic and therefore accumulate in the exterior parts of the magma. A
great surplus of silica exists there. Compounds containing water and
carbon dioxide, _i. e._ hydrates and carbonates, are also light and
should therefore congregate to the same strata where silica abounds,
there partly to be dissolved by the free silica and thus setting
water and carbonic acid free. The latter are, in contrast to silica,
volatile and evaporate therefore into the air leaving the silica
behind. This process is yet in evidence wherever the magma emerges as
through the volcanoes. But also a few other acids in the magma are
highly volatile, as sulphurous, thiosulphuric, and hydrochloric acids.
These also belong to the volcanic gases, are dissolved by the water,
and partake in the processes of disintegration. The carbon dioxide
and hydrochloric acid form carbonates and chlorides. The former are
extracted from the sea water by crustacea, sometimes also by plants,
and form part of our sedimentary strata; the latter are soluble and
remain in the water, chiefly as sodium chloride or common salt. The
thiosulphuric acid, probably a product of ferrous sulphide and acids in
the magma, has entered into numerous insoluble metallic sulphides found
in the Earth. Partly, it has also been oxidized, like the sulphurous
acid, into sulphuric acid and has then assisted in the processes
of disintegration, forming gypsum which has been deposited in the
sedimentary rocks.

The geologists believed formerly that the Earth gradually and
continuously has grown cooler. This theory, however, struggled with
the difficulty that certain cold time intervals, ice periods, were
succeeded by warmer epochs. To begin with efforts were made to
surmount this obstacle by assuming that an ice period on the north
hemisphere was counterbalanced by a warm period on the south hemisphere
and vice versa. In this manner, the mean temperature for the entire
surface of the globe might possibly have been continuously decreasing
although fluctuations had occurred on the two hemispheres. But this
view has proved untenable, because the ice period has left traces also
within the tropics, near the equator, as on Kilimanjaro, in New Guinea,
and so on. It is now practically agreed that the last great ice period
was characterized by a temperature between 4° to 5° C. (7° to 9° F.)
below the present all over the surface of the Earth. This determination
has been accomplished by measuring the difference in height between the
terminals of the glaciers at present and the lowest points where their
grinding action has left obvious traces. The ice coverings of North
Europe, North-East America, South America, along the coast of Chile
and in Argentina, as well as on the southern island of New Zealand,
all appear to have existed simultaneously. Also during earlier eras,
for instance during the Algonkian and the Permian epochs, ice periods
have occurred. The latter, which was felt in Australia, India, and
South Africa, is called the Gondwana-time. This period was formerly
supposed not to have caused any temperature drop except in the tracts
mentioned. Later investigations lead us to believe, as asserted by
Holland in his presidential address to the geological section of the
British Association at its 1914 meeting, that also this ice period
simultaneously embraced the entire globe.

As the Algonkian time belongs to the oldest epochs of geological
history it appears that the temperature on the Earth as long as life
has existed on our planet on the whole has been nearly constant,
with important alternations, however, of warm and cold periods. For
an explanation of these fluctuations, our well-nigh only recourse is
the assumption that the heat-conserving quality of the atmosphere has
changed by virtue of a varying composition. Warm periods occurred when
carbon dioxide was abundant in the atmosphere due to volcanic activity,
cold periods again accompanied a paucity of carbon dioxide. With rising
temperature, the percentage of water vapour in the air also increased,
affording further protection against radiation loss of heat.

Thus, it would seem as if the mean temperature of the Earth’s surface
hardly had changed to any extent worth mentioning during immense spans
of time estimated to about 500 million years. Nevertheless, a slow
process of cooling proceeding toward the centre of the planet probably
takes place. Ever growing quantities of matter are transported from the
interior of the earth through volcanic action. Sedimentary deposits
increase continuously while the interior becomes hollow. As a result
the crust must gradually settle, causing large cracks in the process.
For these weakened places the volcanic products show a special liking
and the craters are strung out in lines along such fissures. In other
places, where volcanic action is less pronounced, hot springs appear
instead, generally emitting carbon dioxide in abundance, occasionally
also sulphurous acid and sulphuretted hydrogen. The dislocations in the
crust also take place along these cracks accompanied by earthquakes.
The study of these various phenomena has enabled us to map out the
fissures, which generally radiate in nearly straight lines from one
point, the so-called centre of collapse, as the cracks in a pane of
glass issue from the point of breakage caused by a swift blow. We shall
later see that such breakage lines and centres of collapse are common
on all stellar bodies which possess a solid crust and are observable
from the Earth.

We may now easily form an idea of the general trend in the development
of the atmosphere. The gases originally present were all, except the
hydrogen, the nitrogen, and the rare gases, strongly absorbent of light
and in particular of heat. It is, therefore, natural that the planets
which have not formed a solid crust possess a strongly absorbing
vapour-shell, as indeed is the case with the large planets (compare
Fig. 13). The crust once formed and the air gradually purged of these
gases, thanks to the sunlight, so that mainly nitrogen and oxygen,
small quantities of the rare gases, and carbon dioxide besides water
remained, the temperature fell rather rapidly. Carbon dioxide formed
the last effective heat-conserving ingredient. As the crust grew
thicker, the supply of this gas diminished and was further used up
in the processes of disintegration. As a consequence the temperature
slowly decreased, although decided fluctuations occurred with the
changing volcanic activity during different periods. Supply and
consumption of carbon dioxide fairly balanced as disintegration ran
parallel with the proportion of this gas in the air. But evolution on
the whole can only proceed in one direction toward a final cooling of
the Earth. This must occur if for no other reason because the store of
energy in the Sun and therefore its radiation must slowly decrease.
With deepening crust and disappearing carbon dioxide vegetation must
ebb, and with it the production of oxygen. This gas also partakes in
the general disintegration through oxidation of iron protoxides in the
mineral rocks. The oxygen portion of the air must therefore finally
reach its maximum and start on the decline. Calculations point to the
conclusion that the carbon dioxide of the air would be consumed in a
few tens of thousand years if new supplies were not furnished from the
interior. Water is also absorbed in the processes of decay as hydrated
compounds are formed, increasing in quantity with falling temperature.
As the amount of water in the ocean is immensely larger (about 50,000
times) than the stores of carbon dioxide in the air and in the seas
the lack of the latter will undoubtedly first become serious. But a
slow desiccation of the planet must subsequently take place, and will
proceed at an accelerated rate with the continued cooling of the Earth.
Then the vapour in the air and consequently precipitation will wane.
Then, as during the ice periods, mighty ice caps will cover the poles
and impound a large portion of the water in the ocean. Finally, the
entire planet, perhaps after having harboured life during trillions
of years, becomes an ice waste with a few cracks in its hard crust
through which warm and acid vapours rise and create small melted areas
characterized by a darker colour than the desert and ice-landscape in
general. Organic life lacks the conditions for existence and ceases
therefore to cheer the planet with its interesting variations. The
planet is dead but continues in obedience to gravitation to describe
its orbit in space.




CHAPTER VI

THE PLANET MARS


Through the works of Schiaparelli, Flammarion, and Lowell the vivid
interest of the general public has been directed toward our neighbour
planet Mars. Several investigators, Flammarion and Lowell among them,
assert with full confidence that Mars has intelligent inhabitants, who
have built and maintain the curious “canals,” which, it is stated,
could not have been created except by intelligent beings far superior
to man.

Air, water, and sunshine exist there, says Flammarion in his well-known
great work, _La Planète Mars_ (1902, page 515). “It appears incongruous
to us to condemn a world like Mars, where all the conditions for life
exist, to such a fate” (to be a dry desert). No doubt sentiment and the
desired result play a part in all such deductions, as indeed the words
chosen by Flammarion would indicate.

As contrasted to the Earth, Mars is, on the other hand, considerably
further removed from the Sun, whose radiation therefore on Mars
possesses only 43 per cent. of its intensity on Earth. Judging by this
fact, the mean temperature of Mars should fall far below that of the
Earth and considerably below the freezing point of water and under such
conditions it is hard to imagine a vegetation near the poles of Mars
as Lowell does in his volume, _Mars as the Abode of Life_ (1909), or
even in the neighbourhood of the canals anywhere on the surface of the
planet as assumed by Flammarion.

With such ideas in vogue we can well understand that the astronomers
would point their lately extremely sharpened instruments toward our
ruby-coloured neighbour in the sky when, in 1909, it passed very close
to the earth under conditions particularly favourable to accurate
observations, more so in fact than they had been during the seventeen
preceding years.

Numerous astro-physicists, among them the world’s foremost
representatives of their discipline, have repeatedly turned their
spectroscopes toward Mars in order to ascertain whether water vapour
was present there or not. In the spectrum of the Sun we find several
so-called “rain-bands” due to the fact that the light before it reaches
the apparatus has passed through the moisture of the air. The more
humid the air the more strongly developed these rain-bands are. If we
direct the instrument first on the Moon, which lacks an atmosphere
and therefore also moisture, and then on Mars, which for the sake of
simplicity we assume standing close to the Moon-disk, a difference
should appear in the spectra of these two bodies, provided moisture
is present in the atmosphere of Mars. The rain-bands ought to be more
pronounced in sunlight that has passed the atmosphere of Mars (passed
it twice as the light is reflected by the surface of the planet) than
in light reflected from the naked Moon. The bands appear of course
in both spectra as the light on its final stage to the spectroscope
passes the atmosphere of the Earth, which never is free from moisture.
In this manner Huggins and Janssen, scientists of world-wide fame,
believed that they had demonstrated the presence of water vapour on
Mars. Campbell, on the other hand, the prominent director of the
Lick Observatory, made similar investigations of the planet in 1894,
and so did a French astronomer, Marchand, in 1896 and 1898, both
under unusually favourable circumstances as the former installed
his instrument 1283 m. (5200 ft.) and the latter 2860 m. (9370 ft.)
above sea level, but neither found any indication of moisture in the
atmosphere of Mars.

It is evident that the observations would be far more accurate if we
could eliminate the moisture from the atmosphere of the Earth, in which
case no rain-bands would appear in the spectrum of the Moon. It would
then be unnecessary to compare the two spectra; we would only have to
determine whether rain-bands were present in the spectrum of Mars or
not. We can never entirely avoid the water vapour of the air, but its
influence may be greatly reduced by making our observations from high
mountains or in desert climates where the air is comparatively dry,
that is free from water vapour. Investigations undertaken where the
air is dry deserve therefore more confidence than those handicapped by
greater humidity. The observations by Campbell and Marchand fall in the
former category, and it would, therefore, appear that the presence of
water vapour on Mars to any extent worth mentioning is highly doubtful.

In later trials, Campbell and Keeler have employed an improved method,
using photographs of the spectra on sensitive plates, but neither has
succeeded in discovering any water vapour in the atmosphere of Mars.

Obviously photography offers a great advantage over direct ocular
observation. The two pictures may be placed side by side and very
accurate measurements may be made at leisure. We may also choose the
moments for exposure when the two stellar bodies stand equally high
over the horizon so that the sunlight reflected from them traverses
equal distances in the humid atmosphere of the earth.

It now devolved upon Lowell to test his theories by means of the
magnificent resources at his disposal in Flagstaff observatory in the
desert of Arizona 2200 m. (7200 ft.) above sea level. In the months of
January and February the dew-point there is about -7° C. (+19.4° F.)
_i. e._, each cubic meter (1.3 cu. yds.) of air contains 2.8 grammes
(43.25 grains) of water vapour while saturated air at zero temperature
(32° F.) holds nearly twice this amount or 4.8 grammes (74 grains)
per cubic meter (1.3 cu. yds.). Slipher, working in this observatory,
pushed the sensitiveness of his plates to the utmost then obtainable,
and photographed the spectrum of Mars in January and February, 1908.
He found that the most important rain-band always was more prominent
in the spectrum of Mars than in the spectrum of the Moon photographed
later during the same night. Peculiarly enough, it was only the
rain-band designated “A,” and located in the red spectral field, that
was of a marked difference in the two spectra. Other bands gave no
indication of the presence of water vapour on Mars. This result did not
directly contradict the conclusions reached by Campbell and Keeler,
also by means of photography; they had investigated other bands than
“A.” The “A”-line might therefore possibly be more sensitive to water
vapour than the others.

Slipher’s discovery was considered so valuable that it must be employed
to the limit. The well-known physicist Very was therefore called in
consultation; he made careful measurements of the intensity of the
“A”-lines on the various plates and calculated that the atmosphere of
Mars contained 1.75 times as much water vapour as that of the Earth at
the point of observation. If we desire to determine the proportion
of water vapour in the air at the surface of Mars from this statement
we may figure in the following manner. The amount of water vapour in
a vertical column of air one square meter (1.2 sq. yds.) in section
is, according to Hann, 2500 times the amount in a cubic meter (1.3 cu.
yds.) at the surface of the earth. At the time of the observation, the
latter amount was 2.29 grammes (35.4 grains); on each square meter (1.2
sq. yds.) of the ground rested therefore 5725 grammes (12 lbs. 11 oz.)
of water vapour. That the quantity of water is not larger, although
the depth of the atmosphere far exceeds 2500 m. (1.5 miles), is due
to the fact that the temperature rapidly decreases with distance from
the ground. On Mars, the temperature ought not to fall so quickly with
change in height because the intensity of gravity there is 2.68 times
smaller than on the Earth. The temperature drops there 2.68 times
slower with ascent in the atmosphere, and a column of air on Mars one
square meter (1.2 sq. yds.) in section should therefore contain 6680
times as much water vapour as a cubic meter (1.3 cu. yds.) at its
surface. As Mars did not stand in zenith, the distance traversed by the
light-ray in the atmosphere was greater--in fact 1.43 times greater
than if such had been the case. A column of air in the direction of
the light-ray, one square meter (1.2 sq. yds.) in section contained
therefore 8175 grammes (18 lbs. 3 oz.) water vapour. In the atmosphere
of Mars which the light passed in a vertical direction there was, if
we are to believe Very, 1.75 times as much, or 14,300 grammes (31
lbs. 8 oz.) and in a cubic meter (1.3 cu. yds.) at the surface of the
planet, consequently 6680 times less, or 2.14 grammes (33.1 grains).
The corresponding dew-point is then, according to this determination,
-10.3° C. (+13.5° F.). It is agreed upon that a desert climate
prevails on Mars. It might at the time of the observation conform to
the extremely dry climate at Salt Lake City in the height of summer
when the humidity there is only 31 per cent. of saturation. Under
such conditions saturated air at noon in the equatorial belt on Mars
should contain 7 grammes (108 grains) per cubic meter (1.3 cu. yds.)
corresponding to a temperature of 5.3° C. (41.5° F.).

It must be admitted that this was not very encouraging to Lowell. If
the temperature in the middle of the day, when the sunlight falls
perpendicularly on the surface of the planet, rises only to about 5° C.
(41° F.), the mean temperature for twenty-four hours, even in the midst
of the summer, must in this entirely clear, light air be far below
freezing and vegetation on Mars is therefore not very well conceivable.
In spite of this, Lowell saw in Slipher’s measurements a confirmation
of his theory that Mars is the abode of an intelligent race that
utilizes, in their wrestle with existence, a verdant vegetation pushed
even into the polar regions.

Campbell, however, went one step further than Slipher. In August and
September, 1909, Mars occupied a position in the sky particularly
favourable to observations. Campbell decided to benefit thereby.
With the support of a rich patron of science, a Mr. Crocker, who
on several occasions has made magnificent contributions toward
astronomical research, Campbell equipped an expedition to Mount Whitney
in California, 4425 m. (14,502 ft.) high and the loftiest peak in the
United States. He was accompanied by an able scientific staff, the most
prominent of which were Dr. Abbot, head of the observatory belonging
to the Smithsonian Institution, and a well-known German astronomer,
Albrecht. The members of the expedition were affected by mountain
sickness and suffered many severe hardships when the wind was high,
reaching about 25 m. per sec. (56 miles per hour), and at the same time
cold, falling below zero (freezing) during the night. The barometric
pressure was only 447 mm. (17.6 inches). During the nights, when the
observations were made, the water content of the air fell to between
0.5 and 0.9 grammes (7.7 grains to 13.9 grains) per cubic meter (1.3
cu. yd.) or 2.5 to 4 times less than Slipher had to contend with. The
spectra of the Moon and of Mars were photographed in close succession,
two exposures being made in each case. The band “A” was plainly visible
on several plates. No indication of greater prominence of this band
in the spectrum of Mars could be found. Other rain-bands were also
investigated with the same result. Neither were the characteristic
bands of oxygen stronger in the spectrum of Mars, than in that of the
Moon. Slipher believed that he had discerned a difference, although of
a hair’s breadth, which would indicate the presence of oxygen in the
atmosphere of Mars. The conclusion itself is not improbable, but the
amount of oxygen there is in any case considerably smaller than in the
Earth’s atmosphere.

Several statements by Campbell, as well as Slipher’s observations,
indicate that a difference ought to have appeared between the spectrum
of Mars and that of the Moon if the water content in Mars’ atmosphere
had been the same as in the Earth’s at the time of the observation.
This content, as stated before, was about 3 times smaller on Mount
Whitney than at Flagstaff. At the latter place, the measurements gave
1.75 as the ratio of water vapour on Mars to that on the Earth. The
amount of water vapour with the Sun in zenith on Mars should therefore,
according to Campbell’s observations, only reach 0.4 gramme (6.1
grains) per cubic meter (1.3 cu. yds.) corresponding to a dew-point of
-28° C. (-18.4° F.) or to an actual temperature of -17° C. (+1.4° F.)
allowing also for a desert climate with only 31 per cent. saturation.
This temperature is probably higher than the mean for a summer day as
the observations were made at noon on Mars.

It must now be evident that we should consider Mars as unfit to
harbour living beings. There is possibly a slight amount of oxygen in
the thin air but the extremely low temperature and the scant supply
of water vapour form insurmountable obstacles to the subsistence of
even the simplest forms of life in the equatorial regions on Mars.
The temperature difference between day and night must be enormous on
account of the desert climate. Even if life could develop during the
day, which has nearly the same duration as with us--Lowell fixed it at
24 hours, 37 minutes, 22.6 seconds--and during which the temperature
possibly might rise above the freezing point, it would nevertheless be
destroyed without mercy by the bitter frost at night.

Campbell has offered an explanation of the indications of water vapour
on Mars, apparent on Slipher’s photographs. An analysis of the latter’s
observations shows that the Moon was photographed about four hours
later in the night than was Mars. On all occasions except one, clouds
appeared in the sky. This indicates the presence of moisture in the
air, so that the humidity should change with the temperature which
latter rapidly falls in the course of the night. Campbell, himself,
found during the clear nights, when he made his observations, that
the humidity in the hours of the night up to midnight falls to a
fraction--a half or a third--of its original value an hour or so after
sunset. This rapid temperature drop is probably confined to the strata
immediately above the observation point but the moisture is strongly
concentrated downward so that this change in humidity undoubtedly
should have been taken into account. Or better, observations should
be avoided in the beginning of the night and the Moon and Mars
photographed as soon after each other as possible, precautions taken
by Campbell but not by Slipher. That the latter found less traces of
water on the lunar photos than on the martian ones is, therefore,
probably due to the fact that the former were taken about midnight but
the latter not long after sunset, when the atmosphere contained much
more water vapour. Thus, we learn how a small slip, more obvious to the
meteorologist than to the astronomer, may spoil a labour otherwise done
with extraordinary care.

To Campbell’s critique Very answered by the suggestion that the
meteorological conditions during the Mount Whitney observations should
have been exceptionally unfavourable. The entire south-west of the
United States and the north of Mexico were visited at that time by
cloudy weather and heavy downpours. Very contends that this humidity
should partly have extended to the high strata above Mount Whitney and
therefore rendered the calculation of the moisture content of the air
entirely unreliable.

Simultaneously (August, 1910) new measurements were published of
Slipher’s photo-plates from February, 1908, which Very had examined.
The result was now that the rain-band “A” was 2.5 times more pronounced
in the spectrum of Mars than in that of the Moon. Furthermore, the
oxygen absorption-band “B” was 1.5 times stronger for Mars than for the
Moon. Great quantities of water vapour and oxygen should, therefore,
undoubtedly exist in the atmosphere of Mars.

In the meantime Campbell had not been idle. The difficulty with the
older measurements consisted in the fact that the absorption line
of water vapour in the atmosphere of Mars occupies the identical
place of the line due to vapour in the Earth’s atmosphere. There
exists, however, a method, as already pointed out by Campbell in 1896,
of separating the two, which method is available when Mars either
approaches or departs from the Earth with sufficient velocity. The
latter could be determined both from the known motions of the two
planets and from the displacement of certain spectral lines of the Sun.
These two determinations were in almost perfect agreement; for instance
January 26–27, 1910, astronomical calculations gave a relative velocity
of 19.1 km. (11.86 miles) per second and spectroscopical measurements
19.2 km. (11.93 miles) per second, while on February 3–4 the relative
velocity was 18.1 km. (11.24 miles) a difference of 1 km. (.62 miles)
per second. This trial shows the accuracy of the method. Among the
absorption lines of water vapour and of oxygen there was, however, none
due to the atmosphere of Mars. Campbell assumes that such lines would
certainly have been visible if they had been only one-fifth as strong
as the so-called tellurian lines. The advantage of this method is
evidently that the “martian” and the “tellurian” lines lie close beside
each other on the same plate so that differences in sensitiveness,
exposure, and atmospheric conditions are entirely eliminated.

From these and the following data we may calculate water content and
temperature of the atmosphere on Mars anew: water vapour at the point
of observation was 1.9 grammes (29.3 grains) per cubic meter (1.3 cu.
yds.), zenith-distance of Mars 55° and incident as well as reflected
sun-rays formed an angle of 70° with the surface of Mars; hence, the
amount of moisture at the surface was only 0.12 gramme (1.85 grains)
per cubic meter (1.3 cu. yds.), corresponding to -38° C. (-36.4°
F.) for saturated air and to -27° C. (-16.6° F.) for air of 31 per
cent. saturation. Oxygen content per cubic meter (1.3 cu. yds.) at
the surface of Mars would be only a sixteenth part of corresponding
numerical value on the Earth. This determination is more accurate
than any of the previous ones and reduces the temperature another
10° C. (18° F.) below the lowest value derived earlier in this chapter.
We should remember, however, that, during the trial of September,
1909, the sun stood practically in zenith on Mars, while in January
and February, 1910, we are concerned with a point where sunrise had
occurred about four and a half hours previously. The latter observation
should give a value close to, but slightly above, the mean diurnal
temperature on Mars.

No determination comparable in precision with this one by Campbell
appears to have been made. We must therefore recognize it as conclusive.

We may easily calculate the surface temperature of a planet from the
intensity of the solar radiation received, or insolation, provided
the surrounding vapour shell contains no heat retarding gas. The most
important gases of this kind are water vapour, which, as we just have
seen, is very sparse in the atmosphere of Mars, and carbon dioxide,
of which there probably also, for reasons stated below, is only a
scant supply in the martian gas shell. Such calculations were first
performed by Christiansen of Copenhagen, who assumed 2.5 calories as
the solar constant on Earth, _i. e._, the amount of energy received
through insolation per minute by each square centimeter (.15 sq. in.)
of the Earth’s surface when at right angle to the radiation and on
mean distance from the Sun. On Mars, the radiating energy received
under similar conditions is only about 1.1 calories. The surface of
the planet is heated until it radiates as much energy into space as it
receives from the Sun. In this way we obtain an average temperature
of -37° C. (-34.6° F.) for the entire surface of Mars. The regions,
exposed to the Sun in zenith at noon, might, if heat were not conveyed
therefrom, possibly reach a daily mean temperature of +8° C. (46.4° F.)
and perhaps slightly more at noon. Probably not even the freezing
point is reached, as the heat is rapidly carried away by the freely
circulating air. The above-mentioned mean temperature of -37° C.
(-34.6° F.) seems on the whole to agree well with the observations by
Campbell on Mount Whitney.

Recent accurate determinations of the intensity of the solar radiation
by Abbot, K. Ångström and others, indicate that it has been estimated
about 20 per cent. too high. If we take the solar constant to an even
2.0 calories, which is a trifle high, we reach the conclusion that the
mean temperature on Mars would fall about 50 degrees below freezing.
Equatorial regions might then reach an average of -8° C. (+17.6° F.)
and at noon the temperature might possibly rise slightly above zero
(32° F.). A higher temperature yet might be attained at the pole where
the Sun during the summer remains for months above the horizon or a
high mark of +8° C. (46.4° F.) provided no heat were carried away by
air currents. Such losses naturally must occur, and the temperature
probably hovers around freezing. At the martian poles we might possibly
imagine the existence of some low forms of vegetation (snow-algæ,
etc.) during the height of the short summer.

When we hitherto on the authority of Lowell, Very, and others, have
assumed an average temperature of +10° C. (50° F.) on Mars, we have
done so on the supposition that the atmosphere of the planet contained
great quantities of heat-conserving gases. This assumption appears
to be no more tenable than the belief in the high temperature on
Mars. After all, the temperature is probably about 10° C. (18° F.)
higher than our last calculation would indicate--or about -40° C.
(-40° F.)--because the air on Mars is very clear and admits, therefore,
all sun-rays, retaining also a fraction by virtue of what little water
vapour, carbon dioxide, and other heat-conserving gases there may be
present in the atmosphere. The mean summer temperature at the martian
equator (-27° C. or -16.6° F. acc. to Campbell’s data) would then lie
about 13° C. (23.4° F.) above the mean for the planet. This agrees
closely with conditions on the Earth where the highest mean in July
at the equator is 27° C. (80.6° F.) and the mean for the earth 16° C.
(60.8° F.).

We are consequently obliged to revise in their entirety our ideas about
Mars. The belief that organic life (green vegetation) causes the colour
of the so-called seas on Mars, as assumed by Lowell, or that the red
tints belong to the gorgeous dress in which autumn arrays the plants
before their leaves are shed under the attacks of frost, as intimated
by Flammarion, must nowadays take its place in the shadowy realm of
dreams.

Those who do not believe that the so-called canals are real
waterways, devoted to freight carrying and irrigation, or illusions,
which conception the photographs contradict (for example Fig. 18),
generally consider that they signify cracks or fissures. As in the
crust of the Earth, they generally run in nearly straight lines or
in regularly bent curves (Fig. 17 and 17a). Flammarion mentions that
the renowned physicist Fizeau looked upon the “canals” as cracks in
the ice-coverings of the oceans on Mars. Penard, in 1888, expressed
the more likely opinion that they correspond to the fissures in the
crust of the Earth. Flammarion contends that such fissures do not have
the rectilinear configuration of the “canals.” This is completely in
error, as shown on the map here reproduced (Fig. 16). It is also stated
that they are so inexplicably long, for instance the canal Phison is
2250 English miles (Lowell) or 3620 km. in length. The longest known
earthquake crack along the entire length of which a dislocation has
taken place _at one time_ is 600 km. (373 miles) in extension; the
violent earthquake in California, 1906, originated from this crack.
There is no doubt, moreover, that a great fissure in the Earth follows
the coast of Chile from Arica to the Strait of Magellan in a nearly
north and south direction for a distance of about 32 parallels or 3560
km. (2210 miles). This fissure is almost as long as Phison on Mars.
Such cracks exist along the entire coast of the Pacific Ocean. As yet,
we do not know their position in much detail, because long stretches
run below the sea or through territories not yet occupied by civilized
people. As an example of a small fissure, a picture taken by Sederholm
from Segelskär, east of Hangö in the Baltic, may serve (see Fig. 15).
As the studies of earthquakes are prosecuted with increasing interest
in later years, fissures of all dimensions will undoubtedly soon be
discovered. The solid crust on Mars is, furthermore, somewhat thicker
than that of the Earth as the cooling of that planet has progressed
further. The sections broken off at the bursting of the Martian crust
ought therefore to be much larger both in breadth and in length.
No doubt, the facts that the intensity of gravity on Mars is only
three-eights of its intensity on the Earth and that the curvature of
the Martian surface is twice as sharp as the Earth’s contribute to
this result. Imagine two vaults, one built with higher and broader
wedge-shaped stones than the other and with half the radius and
furthermore loaded only one-third as heavily as the other, and it
will become evident that we can permit a much larger span in the former
than in the latter case without fear of collapse. In other words, it
requires a much more extensive caving or shrinkage of the molten mass
beneath the crust of Mars to cause a rupture than under the terrestrial
crust.

[Illustration:

  Fig. 15. Narrow bay on the left side of Segelskar, east of
           Hango in Finland. The bay owes its existence to the fact
           that the ice has laid bare a territory coursed through by
           fissures. Photo by I. I. Sederholm.
]

[Illustration:

  Fig. 16. Earthquake centres in Calabria and in Sicily. On the
           larger map the damaged localities are indicated, on the
           smaller the most prominent tremor lines. Drawn by I. I.
           Sederholm.
]

As a consequence, the fissures on Mars ought to be longer than the
corresponding formations on the Earth. A thorough study of the large
fissure in Calabria shows that it consists of a veritable network of
smaller straight cracks (as is apparent on Figure 16, which is taken
from a work by the well-known Finnish geologist Sederholm). On this
map the radial cracks (see _Worlds in the Making_, Fig. 16), charted
by Suess, are also shown, and their direction under the sea designated
by dotted lines. The sketch in the upper left corner of Fig. 16 is in
striking similarity to a picture drawn by Schiaparelli in Mercator’s
projection of the planet Mars (see map at the end of book). We notice
on both, the numerous equidistant lines corresponding to parallel
cracks and duplex canals. Not every fissure has its parallel and not
every canal its mate--generally only one of the latter is visible and
sometimes both disappear.

As the radial cracks in the drawing by Suess if extended meet in the
Lipari Islands, so also several canals on Mars run together in a
so-called lake (Lowell called them “groves” or “oases”) which evidently
is a centre of collapse (many appear on Fig. 17). It is plain that all
crossings of the “canals” are not necessarily such centres of collapse.
(See maps Figs. 17 and 17a at end of book).

We shall consequently assume that the canals on Mars correspond to the
geological dislocation fissures on the Earth. Along these fissures
emerge the gases liberated in the cooling process on both planets;
which are similar gases to those which escape through the volcanoes.
These vapours are primarily water, next carbon dioxide and, in
considerably smaller quantities, sulphuretted gases and hydrochloric
acid. They discharge through cracks in regions which, geologically
speaking, not so long ago were the scene of volcanic activity. In the
dislocation-grooves, lakes, and water courses are often formed, as we
may observe in several places in Sweden, for instance near Stockholm.

Assume now a gradual cooling of our earth. Most territories are covered
by stratified, comparatively light rocks. To the dislocation fissures
water gathers from the surrounding strata and occasionally from the
interior, partly washes away the loose material and transforms the
fissures into furrows, generally with flat bottoms. Dissolved salts
are carried to the sea. As cooling proceeds, the ocean commences to
freeze. Each summer the surface melts to a certain extent, as is the
case now in our polar regions. Finally, the entire ocean freezes to the
bottom, the ice is now to be considered as a kind of rock, flexures and
dislocations cease and the ice assumes a smooth surface. In the strong
sunlight during the summer this surface thaws, as do the water-courses
on the mainland, and these continue to carry their salts to the open
surface water. At the approach of winter the latter solidifies again
but not as the water in our inland lakes from the top but from the
bottom, as ordinary sea-water possesses its greatest density below
the freezing point while the opposite is true of fresh water. The
consequence is that the ice foundation grows upward and as the surface
water becomes increasingly shallow it is turned into a concentrated
salt solution. With a further drop in temperature the ice formation is
accompanied by crystallization of the salts.

Something similar takes place on the Martian mainland in its flat river
basins, which correspond to the salt lakes in our deserts. On account
of the bitter cold and the consumption of the water in the process of
disintegration (the carbon dioxide has been largely used up in the
same manner), precipitation has almost ceased on Mars and most of the
water in circulation emerges from the interior of the planet along
the fissures. As it contains hydrochloric acid and carbon dioxide it
extracts from the soil salts, such as the chlorides of sodium (common
salt), of calcium and of magnesium, all present in common sea-water
to which it was brought by the rivers. The compounds of calcium and
magnesium are not precipitated as carbonated salts through the medium
of crustacea as is the case on earth. The strong solar radiation during
the summer partly evaporates the water into the thin air, leaving the
salts behind. On account of the low temperature, this vaporization on
Mars is probably slower than on the Earth. Along the cracks in the
crust, a kind of dry salt-lakes are formed similar to the generally
shallow and occasionally dry lakes common in the deserts of Central
Asia as described by Hedin. We know that Mars possesses a pronounced
desert climate. There finally remains in the lowest sections of the
water courses a concentrated salt solution, which parts with its water
more and more reluctantly, so that the salts which most strongly hold
the water crystallize at the deepest points. If the winter’s cold is
sufficiently severe (below -55° C. or -67° F.) ice is extracted even
from the most concentrated solutions, which mainly contain chloride of
calcium. In spite of such extreme temperatures, evaporation into the
rare atmosphere is not negligible and the ice crystals partly vanish,
to reappear in the coldest regions of the planet, that is, around the
pole which at the time is turned away from the sun. On the ocean, now
frozen solid throughout, a polar-cap of snow and hoar-frost is formed
which finally reaches as far as the 38th parallel on the southern
hemisphere (see Figs. 18 and 19), where winter occurs when Mars is most
removed from the sun, and to the 58th parallel (see Fig. 19) on the
northern hemisphere where winter reigns while Mars is nearest to the
Sun and consequently not quite so cold. Similar conditions obtain on
the Earth although not to such a marked degree.

[Illustration:

  Fig. 18. Photograph of Mars, enlarged and retouched; taken by
           Lampland
]

[Illustration:

  Fig. 19. Appearance of Mars April 8, 1907, as observed by
           Quénisset. Below one pole a dark line is plainly visible.

]

[Illustration:

  Fig. 20. The south pole spot on Mars observed by Jarry-Desloges
           July 10, 1909.
]

[Illustration:

  Fig. 21. The appearance of Mars during an observation by
           Antoniadi in 1909. The whole disk is somewhat hazy; below,
           the details are hidden by clouds of sand.
]

In the vicinity of the snow-white polar cap, whether there be continent
or sea, bodies of water occur with solidly frozen surface covered by
crystals of very hygroscopic salts, such as the chlorides of calcium,
magnesium, and sodium. When the summer warmth returns and the polar
cap is heated, the hoar-frost evaporates, and the now comparatively
humid air spreads over the surrounding territory. We observe also
frequent mist-formations in these places. The ground near the edge
of the polar snow assumes then often a dark hue on account of the
moisture (Fig. 19). Occasionally canals and lakes appear in the polar
cap (see Fig. 20). This is evidently due to hot emanations along the
cracks. The moist air sweeps over the salts, which then absorb water
and dissolve into concentrated solutions. New quantities of water
vapour are supplied from the pole as they distill over toward the
other pole, where winter now exists, and push on toward the equator
which they finally pass. In their course they dissolve the salts
in the depressions along the fissures and particularly at the deep
crossings where the centres of collapse or the so-called “oases” are
located. Lowell has observed that the “canals” in this manner gradually
“liquify” from 78° N. Lat. to the equator in fifty-two days.

The canal theory presents great difficulties to the explanation of this
curious phenomenon. In order to make the water flow it must be assumed
that the surface of Mars is entirely smooth or at least very nearly so
and that the inhabitants convey the water melted at the poles through
pumping stations. The canals vary in width; according to Lowell, their
mean is 16 km. (10 miles), according to Flammarion between 300 and 60
km. (185 and 37 miles) which latter estimate probably is too high. The
same canal differs widely in breadth in successive years and sometime
disappears altogether. When the supply of water vapour is scant, only
the most hygroscopic salts are dissolved, _i. e._, those deposited in
the deepest furrow of the canal, but when the moisture sweeping over
the canal is more abundant the broader portions absorb water, darken
and thus become visible. The same holds true in regard to the inland
lakes (“oases”). As the water vapour diffuses in the air, the canal
becomes liquid along its entire length independent of the altitude of
its various parts.

All agree upon the desert climate of the mainland on Mars. Like most
deserts on the Earth, it is, therefore, probably a table-land, where
one plateau mounts above the other, each one nearly level. By the
action of the wind, the upper layers have been transformed into fine
sand. On the dead planet no further sediments are deposited by the
sea. The only accretions to the planet are meteorites and cosmic dust
which slowly rains down. It contains among other substances iron,
partly metallic and partly in the form of protoxides (which have a
light green colour).[5] The oxygen in the atmosphere of Mars transforms
these compounds to ferro-oxide which has different colours according
to its coarseness, but generally is ochre. The surface of Mars is also
described as possessing this colour. Dross has, therefore, assumed
that the Martian soil is mingled with ferro-oxide. The finest dust,
however, is yellow while larger crystals tend toward violet. We often
observe on Mars that the details are covered by a yellow veil. This is
of course finely powdered ferro-oxide probably mixed with less coloured
sand which the desert wind whirls up over large portions of Mars. Vast
sections of the planet bore such wrappings in the autumn of 1909, as
observed and described by Antoniadi in Paris (see Fig. 21). Similar
observations have previously been made by W. H. Pickering and others.

    [5] In the deepest portions of the oceans on the Earth where
        no sediments from the coasts are deposited, large
        accumulations containing iron compounds have been found
        and the presence of certain minerals therein points to
        their meteoric origin.

As a rule, only the central and the polar regions of the surface of
Mars can be seen. Territories near the equator more than 40 to 50
degrees removed from the point in line with the Sun and the centre of
the planet are generally hidden behind a thin, white veil of mist. As
soon as the Sun leaves the zenith and reaches half-way to the horizon
the moisture of the air is precipitated near the ground. This shows
that the planet does not possess any quantities of heat-conserving
vapours in its gas shell. The mist does not extend to the poles, whose
white caps always appear distinctly, because the Sun cannot greatly
affect the evaporation in regions where the Sun’s altitude is neither
very high nor very variable. The same holds true for other snow-covered
tracts, even if they are not located in the immediate vicinity of the
pole.

When the supply of water vapour is scant, only the most salient canals
come into view. As a rule they do not then appear double, as one of the
mates is always less prominent. Lowell showed, he believed, that it
always is the same canal out of a pair which first comes to sight and
that its position always remains unchanged in contrast to Schiaparelli
who has reached the opposite conclusion. This, of course, is quite
natural.

On account of the small amount of water vapour in the atmosphere of
Mars true clouds are rare. Figure 22 shows such a cloud at the edge
of the planet. The aforementioned mists are often called clouds, for
instance by Pickering.

[Illustration:

  Fig. 22. Cloud at the upper right edge of Mars observed by
           Molesworth, March 7, 1901
]

[Illustration:

  Fig. 23. Mars as observed by Lowell, July 11, 1907. Even the
           dark portions appear coursed through by “canals.”
]

[Illustration:

  Fig. 24. Mars, as observed by E. M. Antoniadi, October 6, 1909.
]

That elevations really may be found on Mars is evident from the fact
that snow or hoar-frost often remains in patches near the pole and
occasionally quite far therefrom, for instance on the large island
Hellas (40° S. Lat.), while it disappears from the surroundings and
sometimes from the pole itself (the south pole). Such a highland
covered by ice exists near the south pole, and is shown near the
upper edge of Fig. 24. In places where snow always remains, a feeble
glacier formation may occur. Most investigators assume that mountains
and plateaus exist on Mars, although of modest altitude (Campbell
believes that he has observed peaks 3000 m. (9800 ft.) high). Lowell
who diligently has looked for mountains at the edge of the illuminated
part of Mars, has reached the conclusion that they, if present,
cannot rise more than 600 to 900 m. (2000 to 3000 ft.) above the
surrounding plains. It were indeed improbable that all inequalities
of the Martian surface should have been removed in the process of
disintegration, which although at work for enormous extensions of time,
has long been extremely feeble and is unassisted by torrents of rain
which might rapidly wash the products into the valleys. At present, it
is mainly the sand carried by the desert wind that slowly reduces the
roughnesses and in this process extensive highlands are hardly touched.
But, without the assumption, in itself very unlikely, of a nearly level
surface on Mars it becomes difficult to comprehend how the canals, if
filled with pure water, can proceed in straight lines without reference
to existing differences in altitude. Like the rivers on the Earth they
ought to bend according to the topography, even if constructed by
engineers.

When the canals freeze at the approach of winter, they invariably
have been observed to disappear in company with the lakes or oases
at their crossings. They are then all covered by the reddish-yellow
dust carried by wind from the surroundings. When a canal is about to
reappear it frequently first comes to sight as a dark streak evidently
the result of moistened ferro-oxide. Occasionally a mist formation
precedes the appearance of the canal. It is plain that the cold, misty
air settles in the valleys, there as here, and gives up its moisture to
the salts on their bottom and the canal is thus brought out as a dark
line. Sometimes the vicinity also assumes a darker shade indicating
the absorption of some moisture. On the sides of the canals the less
hygroscopic salts are deposited. Possibly the green colour of the
canals is partly a contrast-effect due to the red surrounding, possibly
also the result of finely divided matter in the liquid. It is also
conceivable that the cause is the reducing influence on the ferro-oxide
of the sulphuric gases emerging from the fissures; an exceedingly
small quantity accomplishes in this case large results. F. le Coultre
describes the colour as being sometimes a dead black.

Something similar applies to the seas. When these freeze, especially in
shallow places, yellowish-red dust from the continent settles on their
surface and lends it hues between the original dark green and the light
yellowish-red. When the ice subsequently melts this dust sinks in the
water which latter resumes its dark green colour.

Chloride solutions, if concentrated, freeze at the following
temperatures; that of calcium at -55° C. (-67° F.) that of magnesium
at -44° C. (-48.2° F.) and that of sodium at -22° C. (-7.6° F.). If
now, as we previously have seen, the mean temperature of Mars as
a whole is about -40° C. (-40° F.), of the equatorial belt about
-10° C. (+14° F.), and of the pole in the height of summer about 0° C.
(32° F.), it is evident that a liquefaction of the ocean surface and of
the canals, particularly where salts are deposited, very readily may
take place. We should in this connection remember that the ice on Mars
is stationary while on Earth it is in motion. The consequence is that
sand and dust in the course of thousands of years have accumulated on
the bottom of the shallow basins in the polar ice. These seas appear
therefore dark in spite of their exceedingly small depth and the white
salt and ice-crystals remaining undissolved are unable to display
their light colour. Even in the “ocean,” Lowell was persuaded that he
had observed canals (see Fig. 23), and it may be possible that cracks
are in evidence there, particularly in the most shallow sections, as
is the case in the Tyrrhenian sea north of Sicily. It is significant
that Flammarion has reached the conclusion, which at first appears
highly hazardous, that the freezing point of water is lower on Mars
than on Earth. This is entirely correct, if we let water stand for salt
solutions.

It is customary to point to the strictly uniform breadth and the
rectilinear appearance of the canals as clear evidence of their being
artificial, _i. e._, the work of engineers. The Italian astronomer
Cerulli strongly objected to this conception. “In the exceedingly
rare cases when both sides of the canal plainly may be seen,” states
Schiaparelli, “I have observed curves and notches in the borders.”
This occurred with the canals Euphrates and Triton in 1879, and with
the Ganges in 1888. And it would seem obvious that watercourses
produced in old furrows would not, as a rule, be of uniform breadth.
Antoniadi, by his observations in the autumn of 1909 (see Fig. 17a and
24), has confirmed this opinion, as has le Coultre, who found twice
as many irregular canals as rectilinear ones. Antoniadi remarks that
some canals appear to be collections of lakes strung out in a certain
direction while others are narrow lines which bend and twist. “The
complicated network of straight lines is probably illusory.” The spots
on Mars, he continues, are very irregular, and “present by no means
any geometrical form” (on which the belief largely is founded that
they are the product of intelligent beings). “The appearance of the
planet reminds one of that of the Moon (except that the latter is dead,
_i. e._, unchangeable) or of a terrestrial landscape viewed from a
balloon.”--“In a word, the ‘geometry’ of Mars is revealed as a pure
illusion.” Exceedingly instructive is a comparison between the two maps
of Mars drawn by Schiaparelli (1886) and by Antoniadi (1909) reproduced
here and found at the end of the volume. While Schiaparelli as a rule
represents the canals as narrow, straight, or slightly curved bands
of uniform width, these formations on the Antoniadi chart frequently
dissolve into a series of dark spots joined by less obscure sections
(see for example the canals Nectar and Oeroe at the Sunlake). The
same is true about several of the so-called “seas,” particularly the
Tyrrhenian (Mare Tyrrhenum), and the Sunlake (Lacus Solis); also about
the “Ocean-bays” such as the well-known Syrtis major which with the
Sunlake form the most conspicuous objects on the surface of Mars. These
maps are, moreover, of great interest because several canals and other
features present on one are absent on the other and vice versa. In this
way, we obtain a vivid conception of the remarkable changeableness of
the Martian surface as contrasted with the exterior of the Earth. The
latter, if viewed from Mars, would not have presented any noticeable
change in historical time except for the seasonal variation of the snow
fields. This peculiarity of Mars is only explained by the fact that the
geographical features of that planet as a rule are surface formations
of a slight depth and therefore subject to rapid transformations.

Frequently, large white spots suddenly appear, especially near the
lakes, such as the spot at Lake Phœnix near the centre of Fig. 24 which
represents Mars on October 6, 1909, according to Antoniadi. These
white spots disappear as suddenly as they show forth. The white colour
is probably due to a very thin snow or hoar-frost, which is easily
condensed in the vicinity of the lakes but which as readily vanishes at
the approach of a warm draft or of sunshine.

Occasionally, dark spots on Mars are described as dissolving under
strong enlargement into dark and light squares giving the appearance
of a chessboard. This reminds one of the bayirs in Turkestan (see Fig.
9).

The collections of lakes along the cracks on Mars which appear to
us as “canals” are repeatedly filled up by sand and dried out. They
are revived through new depressions along the dislocation fissures,
corresponding to our earthquakes, when vapours of water and other
gases pour forth and condense to lakes in the deepest pockets of the
fissures. Canals are therefore created rather rapidly, sometimes over
night, and vanish occasionally as suddenly. The most remarkable case of
“new” canals was made known through a communication by Lowell. Two new
canals, at the time the most conspicuous on the surface of Mars, were
observed east of “Syrtis magna” on September 30, 1909, from Flagstaff
observatory, when they also were photographed, which precludes an
illusion. (On the other hand there was no sign of the great canal
Amenthes, shown on the map Fig. 17, a short distance to the left,
_i. e._, east of Syrtis in the very section where the new canals were
observed.) Also two new oases through which the new slightly curved
canals passed were observed for the first time, as were also a few
minor canals in the neighbourhood.

In 1913, the double canal Æthiops (see map at Long. 240°; the canal is
there single) was rediscovered from the Lowell observatory after an
absence of fifteen years.

These data make it evident that one or possibly several rather strong
earthquakes took place east of Syrtis major just prior to September
30, 1909 with the two oases as centres of collapse. The fissures now
made visible have probably existed before but filled with sand and have
now reappeared as a result of the condensation of water vapour when it
emerged into the cold Martian air.

This fact, that the most prominent canals in such manner now suddenly
appear and now as rapidly vanish, ought to convince us beyond doubt
that they are not magnificent products of engineering skill, for the
construction of which we should require centuries on the Earth.

The theory that intelligent men exist on Mars is very popular. With
its help everything may be explained, particularly if we attribute an
intelligence vastly superior to our own to these beings, so that we
not always are able to fathom the wisdom with which their canals are
constructed. The crossings of the latter are said to be cities (Lowell)
fifty times greater than London. The trouble with these “explanations”
is that they explain anything, and therefore in fact nothing. If we
would endeavour to understand the phenomena on Mars, we must in the
first place avoid the formerly so popular principle of “purposiveness”
which led even the most prominent scientists into so many amusing
errors. Neither may we base our conceptions, as does Flammarion, on the
assumption of natural forces unknown to us, no matter how much such a
course may appeal to mystics. Only forces with which we are familiar
can be resorted to, if we really are to understand nature. It seems to
me that such method of research might with good results be applied also
to the planet Mars.




CHAPTER VII

MERCURY, THE MOON, AND VENUS


The planet Mercury probably resembles Mars in many respects, but
differs particularly in lacking an atmosphere. The fissures in the
crust of the Earth or of Mars are as a rule rapidly filled and their
contours largely hidden from sight by alluvium or sand carried by
windstorms, so that they reveal themselves only through tremors and
various emanations along their course. The fissures on Mercury on
the other hand must remain as yawning chasms. It is probable that
reducing gases stream out of these cracks as on Earth, and colour
the environment in a darker shade than the other visible part of
the planet’s surface, that is the hemisphere turned toward the Sun.
Not very volatile gases, such as sal-ammoniac, other chlorides and
sulphur, which on the Earth are deposited inside the fissures, may
here spread over large areas and discolour surrounding territory,
particularly where iron compounds are present, and, under the attack of
sulphur, turn black. Lowell has made drawings of the dark spots visible
on Mercury, one of which is reproduced as Fig. 25. These spots lie, as
on Antoniadi’s drawing of the surface of Mars (Fig. 17a), arranged in
lines which are almost straight or of a slight curvature only. This
seems to indicate that the spots belong to areas immediately adjoining
enormous fissures. According to Lowell’s drawing, these cracks are
far more regularly distributed on Mercury than on Mars. Very close to
the centre of the ever-sunny side we see a dark spot, a “lake.” It is
evident that this spot is located in by far the hottest point on the
surface of Mercury. This gives rise to the following conception. The
hottest part of Mercury was naturally the last to solidify. Mercury
evidently ceased to rotate around its own axis, leaving one side
continually exposed to the Sun, while its surface yet consisted of
lava that was fluid, at least where the sunshine was most intense. The
weakest point on the planet was therefore the one just opposite the
Sun. When later collapses occurred the cracks commenced at this weak
point. We see on the figure how not less than six fissures radiate
from this centre. Others were formed where the crust broke off from
adjacent solid portions. These latter fissures have a less rectilinear
appearance than those diverging from the centre of collapse. Along
these faults, reducing gases no doubt issue from the interior of the
planet and give a dark tone to the surface layers, which probably
consist of ferruginous dust falling from space. In the neighbourhood
of the Sun, such dust ought to be more plentiful, concentrated as it
were by the gravitation of the Sun. Mercury lies five times nearer
the Sun than the Earth does, and twelve times nearer than Mars. There
probably also exist on Mercury, as on the Moon, large mountains which
are not subject to the wear of running water and blowing sand. We
cannot, however, observe them from the Earth. Possibly they correspond
to the widely extended spots, which several investigators as Schroeter,
Vogel, and others, have noticed, formations resembling the “seas” on
the Moon. Vogel believed that he had found traces of water vapour in
the atmosphere of Mercury as in that of Mars, a belief in both cases
undoubtedly founded on erroneous observations.

[Illustration:

  Fig. 25. Drawing by Lowell, representing the planet Mercury
           with “canals.”
]

[Illustration:

  Fig. 26. A part of the moon near its south pole. The big crater
           above, in the interior and on the walls of which a large
           number of smaller craters appear, is Clavius. A little
           below and to the right is Longomontanus just at the edge
           of the shadow; almost in the middle of the picture appears
           Tycho with its central cone. The moon diameter corresponds
           to 43.4 cm. Photo by Yerkes Observatory.
]

[Illustration:

  Fig. 27. Mare Serenitatis (below), Mare Tranquillitatis (upper
           left) and vicinity. To the left of Mare Serenitatis the
           great crater Posidonius; 2.8 cm. from the right edge a
           small white spot may be seen. This is the remarkable
           crater Linné, said to have undergone changes. The
           moon diameter corresponds to 35.7 cm. Photo by Yerkes
           Observatory.
]

The part of Mercury which is turned from the Sun must be characterized
by a tremendous cold due to radiation into space. The temperature
stays probably about 200° C. (360° F.) below the freezing point of
water (328° below zero F.). Even the most concentrated solutions we
know of freeze to ice precipitating the salt considerably above this
temperature. Moisture in fluid state can, therefore, not very well
exist on this side. On the sunny hemisphere it must be lacking as well,
due to evaporation over to the cold side. As a result, the desolation
on Mercury must be far greater than that on Mars and surface changes
caused by variations in temperature are almost precluded. On account
of the so-called libration, certain dark portions near the boundary of
the illuminated hemisphere occasionally enter the sunlight. But, during
this interval, all traces of moisture are undoubtedly driven away from
these parts, also never to return.

The Earth’s moon is not entirely as stagnant as Mercury, although on
the whole it closely resembles this planet. The Moon always turns the
same side to the Earth--a small libration exists here also--so that
each part of its surface is illumined by the Sun during one half of the
synodical month (29.53 days). This time, however, is so long that the
moon’s surface in the meantime almost assumes the temperatures due to
continuous sunlight and continuous night.

Some investigators, as W. H. Pickering, are persuaded that portions
of the moon just emerged from the shadow show a lighter colour than
after a short time of illumination. These observations, however, have
not been accepted as correct. According to Pickering, the light colour
should result from a slight formation of snow or hoar-frost during the
long night of 355 hours. If an appreciable trace of vapour existed on
the Moon, it ought to evaporate and form white caps over the poles
where the Sun’s heat is not sufficiently strong to melt them. As no
such signs have ever been observed, the faith in snow on the Moon is
not likely to find many defenders.

The lunar mountains are not attacked by water or sandstorms, nor do
they peel off due to rapid heating by the Sun. They rise, therefore, to
full stature over their surroundings. Their height can be measured by
the length of their shadows. Mädler, in this manner, computed one of
the peaks of the Mountain Newton to rise 7300 m. (24,000 ft.) above the
territory on which its shadow falls. Six peaks reach between 6000 and
7000 m. (19,500 and 24,000 ft.), 21 between 5000 and 6000 m. (16,500
and 19,500 ft.), 82 between 4000 and 5000 m. (13,000 and 16,500 ft.),
and 582 reach 2000 m. (6500 ft.) and more. These figures show the
extraordinary mountainous character of the Moon’s surface compared to
that of the Earth which is thirteen times larger.

In Fig. 26, we see a picture of the portion of the Moon most rich in
volcanoes, with the crater Tycho in the centre and Clavius above.

The numerous volcanoes are particularly characteristic of the Moon.
They vary in magnitude from a diameter of over 200 km. (125 miles),
for example the colossal Clavius with its companion craters, down to
dimensions just visible with the aid of a telescope. The largest exceed
our biggest many times in width and differ essentially from them,
inasmuch as their bottoms are flat, occasionally provided with smaller
volcanic cones--see the crater Longomontanus to the right of Tycho on
Fig. 26--and surrounded by a high (inwardly often very steep, outwardly
more sloping) wall as on Clavius, Longomontanus, and Tycho. The
largest, as for example Clavius, may be compared to a province such as
Bohemia, surrounded as it is on all sides by mountains. The elevated
ring, as well as the interior of Clavius, is adorned with numerous
large and small craters. The smallest of these resemble hemispherical
excavations in the crust of the Moon, or they may be small volcanic
cones which break through the walls. Sometimes they are strung out like
pearls along rents in the ground.

All these volcanoes have undoubtedly given passage from the interior
to the surface of the Moon for enormous volumes of gases previously
enclosed in the lunar magma. Nor is it less certain that these gases
have consisted largely of water vapour. If this had been condensed to
water, oceans and rivers would have been formed, and on the bottom
of the seas would have been deposited sediments carried down from
the mountains. Such, however, is not the case. The so-called “seas”
on the Moon are indeed on a lower level than their surroundings, but
their surface is even (see Fig. 27 with Mare Serenitatis below and
Mare Tranquillitatis above to the left; see also Fig. 29 with Mare
Imbrium below; it is bounded on the right by the “Carpathians”). The
“seas” consist of volcanic rocks, and are not at all covered with loose
sediments which if present ought to reflect light better than the
volcanic vitreous rocks. But the lunar “seas” are much darker than the
environments. This shows that seas proper, or bodies of water, have
probably never existed on the Moon. Even before the surface had changed
from its molten condition the water vapour had departed from the
atmosphere, and the new quantities which the volcanoes emitted from the
depths below disappeared so rapidly that lakes were never formed. The
history of other atmospheric gases on the Moon was no doubt similar.
All evidence, therefore, points to the conclusion that life never
inhabited its rough surface. Fig. 27 shows that the “sea-bottoms” are
not free from volcanoes. They also abound in folds, corresponding to
mountain-chains on the Earth. These folds indicate old breaks in the
crust while it was yet very thin. To the right, in Mare Serenitatis,
appear a few white spots which W. H. Pickering ascribed to snow. The
largest is the much discussed “crater” (?) Linné. Mare Serenitatis is
surrounded by a ring of volcanoes.

A noted astronomer, Cerulli, observed, when he directed a glass of
moderate power, such as opera glasses, toward the Moon that the spots
seemingly arranged themselves in rows forming intersecting lines
similar to the canal-system on Mars. As the regularity disappeared
with greater enlargement, Cerulli believed that the canal-system on
Mars also would dissolve into small spots if a sufficiently powerful
telescope were used. His idea, which partly has been verified, was more
recently adopted by the Englishman, Maunder, who denies the existence
of canals on Mars. Photography, however, has proved their reality (Fig.
18).

If we disregard the illusory reticulation, there are nevertheless
on the surface of the Moon numerous designs of a nearly rectilinear
outline. There are to begin with the sinuses, extended trenches,
often dotted along their sides with minor volcanoes. Fig. 27 shows,
in the upper right corner, two such sinuses, the right one with a
small volcano, Hyginus, in the middle. There are, further, five
such volcanoes in its left arm, not visible on the photograph, and
two in the right arm. The second, “Sinus Ariadaeus,” commences to
the left with the Volcano Ariadaeus, not visible on the figure. The
explanation of the origin of these sinuses is probably to be found in
the different contraction of the Moon’s surface layer and of the hotter
substrata immediately after the solid crust was formed. In a way, they
correspond therefore to cracks in the glazing on porcelain. Like the
two sinuses just mentioned they frequently commence and end with small
craters which formed weak spots in the crust that facilitated the
original break. Later on, volcanoes broke through along the sinuses
themselves. In several regions of the Moon, and particularly in the
equatorial belt, observers have claimed discoveries of new sinuses
and occasionally of minor craters, “which could not possibly have
escaped notice if they had existed before.” At present, the almost
unanimous verdict is that such changes are very improbable, and that
the visibility of the “new” objects largely depends on favourable
sidelight, so that they might well have been overlooked if the
region in question previously was examined under less advantageous
illumination.

The most peculiar formations on the Moon are the so-called “bright
streaks” which as a rule issue in almost straight lines from some of
the larger craters, particularly Tycho and Copernicus. Those around
Tycho (see Fig. 28) do not seem to be either raised above nor depressed
below the surroundings to any degree worth mentioning. For this reason
they are not visible under oblique illumination as in Fig. 25. They
proceed in straight lines independent of elevations. This quality
is in striking conformity with the characteristics of fissures on
earth as for instance those that traverse the Tyrrhenian Sea and the
Calabrian mountains. They also resemble the canals on Mars in this
respect. Nasmyth and Carpenter caused a glass ball, containing water
under pressure, to break at one point and obtained a system of beams
radiating from this point and vividly reminding of the streaks around
the lunar craters. The same effect appears if a homogeneous plate, of
glass for instance, is broken by a blow in one point. No doubt these
streak centres were once centres of collapse although they sometimes
now are found at a considerable elevation, like Tycho. This may be
the result of a later secular lifting of the rocky substrata like the
slow rise of the Scandinavian peninsula. The streaks around Copernicus
(see Fig. 29) are very different from those around Tycho. They are not
rectilinear and consist next to the crater of distinct mountain chains
plainly visible under oblique illumination. They penetrate into Mare
Imbrium (Fig. 29 below) crossing the mighty “Carpathian” mountains.
Frequently, they are provided with minor volcanoes, as in the streak
directed almost straight downward on the figure, _i. e._, to the north.
They are obviously volcanic fissures like those on the Earth.

[Illustration:

  Fig. 28. Tycho in full illumination with surrounding magnificent
           system of streaks. In the lower right corner Copernicus
           with a less regular system appears. Between them Mare
           Nubium, in the upper right Mare Humonum, with the
           great crater Gassendi below. The moon diameter
           corresponds to 16.7 cm. Photo by Yerkes Observatory.
           Compare Figs. 25 and 28, showing parts of the same
           territory under side light.
]

The streaks, in many cases, would not be visible at all were it not
for their different colour, which is considerably lighter than that
of the surroundings. The only explanation offered for this fact is
the assumption that the original cracks were filled by some light
matter forced out from the interior of the Moon, that is by the
lunar magma. This magma was not very viscous, as it has spread out
considerably beyond the edges of the cracks proper. These presumably,
like those on Earth, were of a rather moderate width, not enough to
be distinguishable at the Moon’s distance. Similar light-flowing
emanations from long fissures are known also on our planet, for
instance, from the Laki eruption on Iceland in 1783. The colour
may be light simply by comparison with the previously solidified
crust, which, optically, as regards reflexion of light, has proved
very similar to obsidian or, even more like another volcanic mineral
product, vitrophyre. It is also possible, however, that gas bubbles
were liberated as the lava solidified and gave the surface a milkwhite
appearance--gravity on the Moon is only one sixth of that on the Earth
so that the bubbles would rise and evaporate extremely slowly from
the magma. Due to the very low atmospheric pressure on the Moon, the
bubbles would also occupy a larger volume than in a corresponding case
on Earth and become more conspicuous in proportion. They probably
partly remained on the surface of the outpoured lava as a thin scum,
which hardened in that state. Since then, it has suffered no more
change than all other formations on the Moon, while on the Earth it
would soon have been scoured away by sand and water.

[Illustration:

  Fig. 29. The great lunar crater Copernicus surrounded with streaks.
           Below the Carpathian mountain range and at bottom part of
           Mare Imbrium. The moon diameter corresponds to 55 cm.
           Photo by Yerkes Observatory.
]

Before we leave the Moon it may be well to say a few words about its
colour. Mädler states in agreement with several other observers that
Mare Serenitatis, a “sea” on the Moon’s north side (25° latitude) just
to the right of the centre meridian (see Fig. 27) is remarkable for
its beautiful pure green colour, while Mare Crisium about 16° Lat. N.
near the right edge of the Moon is of a dark grey-green hue. In Mare
Humorum (about 22° Lat. S., not far from the edge of the Moon, see Fig.
28) grey and dark green shades alternate and in Mare Frigoris, just
inside the lunar north pole, the colour is a dingy yellowish-green.
In other words, the characteristic colour of the great lava seas is
apparently green. This agrees closely with conditions on the Earth
where similar formations are coloured green by silicates of ferrous
protoxides, certain species of which are called green-stones. Franz,
however, questions the observations of Mädler and professes the belief
that very light craters appear bluish and assumes this to be a contrast
effect to the general yellow hue of the Moon. Langley investigated the
lunar radiation with the spectroscope and found that the ratio of
blue to yellow was smaller in the moonlight than in the sunlight, for
which reason the general colour of the moon resembles that of yellow
sandstone.

A very interesting observation was made at the Lowell observatory when
investigating the spectrum of the sparse light reflected from the Earth
to the portions of the Moon not exposed to the sunlight. It proved to
be of a far more blue tinge than sunlight reflected from the Moon.
Our conclusion must be that the Earth shines with a blue lustre. This
is perfectly natural, as the diffused light which reaches us after
having been scattered by particles suspended in the air (and by gas
molecules as well) is a deep blue and there exists no reason why that
part of the light which is thrown outward into space should be of a
different colour. The Earth, therefore, is blue in contradistinction
to Mars which is red, on account of its desert surface, and Venus
which is bright white. The cloudy portions around the equator and the
poles should appear light blue from without and should be separated
by dark blue bands over the so-called horse latitudes, under which the
cloudless desert regions are located on either side of the equator.
(Compare the title page illustration.)

Compared to Mars the Moon offers a scene of far greater desolation.
On Mars, we observe at least some considerable changes such as the
disappearance of the white pole-caps at midsummer when at the same time
a dark ring appears to surround them; then the “lakes” and the “canals”
come into view, beginning close to the ring mentioned, later on
nearer the equator, and finally on its other side, while the opposite
pole-cap puts on its winter hue. Again, we have the sudden appearance,
and equally hasty disappearance, of white spots, particularly in the
neighbourhood of the lakes, and the sand storms which hide the surface
of Mars and often fill its canals. The abruptness of the changes
indicate that they are confined to a very thin surface layer. The
formation, on the other hand, of canals, for many years unobserved,
must be ascribed to a volcanic activity which, while feeble, yet must
be seated in the deeper portions of the planet. In addition, a stunted
vegetation of low forms is not unthinkable in the polar regions.

As against this, the Moon is undoubtedly a stellar body entirely
insusceptible of surface change. Near its centre, it is probably not
completely solidified and an extremely slow growth of the firm crust
is therefore likely. Gases are no doubt set free during this process,
but they are unable to penetrate the enclosing thick armour and remain
therefore as bubbles in the hardening magma.

As a matter of fact, no changes have, with certainty, been detected on
the Moon’s surface. It is true that the great Wm. Herschel, known as
an excellent observer, believed that he discovered, in 1873, mountains
which had not existed before that time, and Schröter, who diligently
studied the lunar surface, was of the opinion that he too had discerned
numerous changes. These discoveries, however, were doubted by careful
critics, and after the publication of Mädler’s great work about the
Moon (1837) the complete stagnation on that body was taken for granted.
Nevertheless, there are several astronomers, such as Schmidt in Athens
(1866) and lately W. H. Pickering in Cambridge, Mass., who think that
they have discerned considerable modifications. The former held that
the crater Linné (Fig. 27) had vanished since the publication of
Mädler’s work. In 1867, Mädler, himself, proclaimed that it had the
same appearance as before. Pickering, again, reports periodic changes
of “snow” and “vegetation.” (Compare Fig. 27 taken from Pickering’s
Moon-atlas.) Closer analysis, however, indicates that the phenomena
are probably only apparent and depend on the angle of illumination at
each particular time of observation. For some time, rather more than
a quarter of a century, photography has been pressed into the service
of lunar investigation with far more objective results than would be
possible through direct ocular inspection alone. During this period,
which, it must be admitted, is not very long, no distinct signs of
changes have been recorded by the photographic plates.

The great difference between Mars and the Moon depends upon the
existence of a real atmosphere on the former. The oxygen will probably
vanish from Mars also, being used up in the course of disintegration.
But nitrogen, argon, and the other permanent gases will always remain,
as will the water vapour from the bodies of water ever present,
particularly around the south pole. It is true that this water vapour
also will diminish with sinking temperature and when the latter finally
has reached the freezing point of the salt solutions on Mars, the
canals and the lakes will cease to thaw out or liquify under the vapour
distilled over from the warm to the cold pole. But sand storms and thin
mist formations will always appear and cause colour changes on the
desolate planet.

If we wish to picture to us the future fate of our Earth when it
gradually enters the reign of darkness and cold in consequence of the
enfeebling of the Sun, we must seek our illustration on Mars and not
on the Moon. Slowly are the oceans going to freeze, finally down to
their bottom, the abundance of the rainfalls will diminish, only light
snow will now and then bring change to a surface evermore transformed
into a sand desert as far as the continents reach. Rents in the rocky
substrata of the latter will appear as dark lines, caused by the gases
rising from the interior. When the temperature at the equator has
fallen below the freezing point, the polar regions will remain the only
parts where a light covering of frost will melt in the height of the
summer season and where the last feeble organisms will eke out their
hard existence, resorting to a prolonged winter’s sleep of their seeds
and spores. Finally, the last remnant of life will also disappear and
sandstorms alone, save for the gasps of gas emanation from fissures
in the rocky ground, will bring relief to the monotonous desolation.
Falling meteoric dust, which now exists in original state only on
the bottom of the oceans, will gradually cover the entire surface of
the Earth with a mantle coloured brick-red through the influence of
atmospheric oxygen. When the oxygen itself is used up, the meteoric
dust will retain its original greyish-green hue and lend it to the
funeral pall of the Earth.

Very different conditions obtain on our neighbour planet, which is
closer both to the Sun and to ourselves, the radiant Venus, an object
of interested human attention already in ancient times. The average
temperature there is calculated to about 47° C. (116.6° F.) assuming
the sun constant to two calories per cubic centimeter (.061 cu. in.)
per minute. The humidity is probably about six times the average of
that on the Earth, or three times that in Congo where the average
temperature is 26° C. (78.8° F). The atmosphere of Venus holds about
as much water vapour 5 km. (3.1 miles) _above_ the surface as does the
atmosphere of the Earth _at_ the surface. We must therefore conclude
that everything on Venus is dripping wet. The rainstorms on the other
hand do not necessarily bring greater precipitation than with us. The
cloud-formation is enormous and dense rainclouds travel as high up as
10 km. (6.2 miles). The heat from the Sun does not attack the ground
but the dense clouds, causing a powerful external circulation of air
which carries the vapour to higher strata where it condenses into
new clouds. Thus, an effective barrier is formed against horizontal
air currents in the great expanses below. At the surface of Venus,
therefore, there exists a complete absence of wind both vertically, as
the Sun’s radiation is absorbed by the ever present clouds above, and
horizontally due to friction. Disintegration takes place with enormous
rapidity, probably about eight times as fast as on the Earth, and the
violent rains carry the products speedily downhill where they fill the
valleys and the oceans in front of all river mouths.

A very great part of the surface of Venus is no doubt covered with
swamps, corresponding to those on the Earth in which the coal deposits
were formed, except that they are about 30° C. (54° F.) warmer. No
dust is lifted high into the air to lend it a distinct colour; Only
the dazzling white reflex from the clouds reaches the outside space
and gives the planet its remarkable, brilliantly white, lustre. The
powerful air currents in the highest strata of the atmosphere equalize
the temperature difference between poles and equator almost completely
so that a uniform climate exists all over the planet analogous to
conditions on the Earth during its hottest periods.

The temperature on Venus is not so high as to prevent a luxuriant
vegetation. The constantly uniform climatic conditions which exist
everywhere result in an entire absence of adaptation to changing
exterior conditions. Only low forms of life are therefore represented,
mostly no doubt belonging to the vegetable kingdom; and the organisms
are nearly of the same kind all over the planet. The vegetative
processes are greatly accelerated by the high temperature. Therefore,
the lifetime of the organisms is probably short. Their dead bodies,
decaying rapidly, if lying in the open air, fill it with stifling
gases; if embedded in the slime carried down by the rivers, they
speedily turn into small lumps of coal, which, later, under the
pressure of new layers combined with high temperature, become particles
of graphite. Fossils proper are not formed as was also the case in the
early periods of the Earth.

The temperature at the poles of Venus is probably somewhat lower,
perhaps about 10° C. (18° F.) than the average temperature on the
planet. The organisms there should have developed into higher forms
than elsewhere, and progress and culture, if we may so express it,
will gradually spread from the poles toward the equator. Later, the
temperature will sink, the dense clouds and the gloom disperse, and
some time, perhaps not before life on the Earth has reverted to its
simpler forms or has even become extinct, a flora and a fauna will
appear, similar in kind to those that now delight our human eye, and
Venus will then indeed be the “Heavenly Queen” of Babylonian fame, not
because of her radiant lustre alone, but as the dwelling place of the
highest beings in our solar system.

The ancients believed that the fates of men could be read in the
stars and this faith persisted with the power of a religion until
a few centuries ago. It was shared by the foremost astronomers,
pre-eminently by Tycho Brahe, who endeavoured to support it through
his investigations. Traces are yet to be found in popular conceptions.
These ideas have been verified today in a certain sense although with a
wholly different meaning than held by our forefathers. The planets do
tell us the conditions that existed on the Earth at the first dawn of
life and we can also draw from them a prediction of the fate that once,
after milliards of years perhaps, will befall the latter descendants of
present generations.

In one respect the dreams of our ancestors have not proved true,
namely, with reference to the habitability of the other globes in our
solar system. According to the great Kant, conditions on the wandering
stars outside of the Earth’s orbit were so favourable to life that
their inhabitants ought to have reached a far higher development than
beings on the Earth. The last remnant of this conception lives in the
speculations about the marvellously proficient engineers who built the
magnificent system of giant canals on Mars. A thorough critique has
demonstrated that any other planet in our solar system hardly can offer
an abode for higher beings, except this very Earth, which therefore
justly may be called “the best of worlds” among those that we know. And
yet, it was undoubtedly a great truth that Giordano Bruno gave his life
for, because it is highly probable, nay almost certain, that around
the countless suns which dot the firmament spin dark bodies, although
unfortunately our most powerful lenses do not reveal them. A number
of these unseen stellar bodies shelter living beings, which even might
have climbed to a higher point on the ladder of evolution than have the
inhabitants of the Earth.

[Illustration:

  Fig. 17. Map of the planet Mars in Mercator’s projection
           according to drawing by Schiaparelli. A comparison
           with the drawing by Antoniadi (Fig. 17a) suggests that
           Schiaparelli has made a somewhat diagrammatic picture,
           which sets forth a very great number of strictly straight
           “canals.”
]

[Illustration: Fig. 17a. Map of the planet Mars in Mercator’s
projection, drawn in 1909 by E. M. Antoniadi.

  Explanations:  Meredies = south; Oriens = east; Occidens = west;
                 Septentrio = north; Nix = snow.
  Abbreviations: M = Mare, sea; S = Sinus, bay; Fr = Fretum, channel;
                 L = Lacus, lake; Fl = Flumen, river;
                 R = Regio, region; I = Insula, island;
                 Pr = Promontorium, cape.
]




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The Essence of Astronomy

Things Every One Should Know About the Sun, Moon and Stars

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irresistible to art and nature lovers, and not in vain. Withal the most
intense facts of astronomical knowledge and discovery are given beside
the tapestried romance which earlier peoples wove with star-stuff as
the medium of their art. It is a book delightful to the eye, scholarly
in conception and treatment, and charming to the inner sense.”--From
_Scientific American._

“Here all the myths, legends, and facts relating to all our northern
constellations are set forth fully as well as with scientific
exactness, are illustrated with clear maps and diagrams, and are
brought into close association with their classic origins by
beautifully reproduced photographs of works of art and architecture.”

                    _The Outlook._

  New York      G. P. Putnam’s Sons      London




Transcriber’s Notes


Punctuation and spelling were made consistent when a predominant
preference was found in this book; otherwise they were not changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.

The spelling and accent marks of non-English words have not been
changed.

Ambiguous hyphens at the ends of lines were retained; occurrences of
inconsistent hyphenation have not been changed.

Illustrations have been moved, when necessary, to fall between
paragraphs and outside of quoted text. In some cases, this places them
on different pages than the ones given in the Table of Illustrations.
In versions of this eBook that support “links”, the page numbers in
that table link to the correct illustrations.

Wide tables on pages 113 and 146 have been split to fit within
recommended width limits. The left-hand identifier columns have been
duplicated in both halves of the split tables.





End of Project Gutenberg's The Destinies of the Stars, by Svante Arrhenius