LIGHTNING, THUNDER

  AND

  LIGHTNING CONDUCTORS.

  _WITH AN APPENDIX ON THE RECENT CONTROVERSY
  ON LIGHTNING CONDUCTORS._

  BY

  GERALD MOLLOY, D. D., D. Sc.

  _ILLUSTRATED._

  [Illustration]

  NEW YORK:

  THE HUMBOLDT PUBLISHING CO.,

  28 LAFAYETTE PLACE.




LIGHTNING, THUNDER, AND LIGHTNING CONDUCTORS.




CONTENTS.


  LECTURE I.                                                  Pages 5-26

  LIGHTNING AND THUNDER.

  Identity of Lightning and Electricity--Franklin’s Experiment--Fatal
  Experiment of Richman--Immediate Cause of Lightning--Illustration from
  Electric Spark--What a Flash of Lightning Is--Duration of a Flash of
  Lightning--Experiments of Professor Rood--Wheatstone’s
  Experiments--Experiment with Rotating Disc--Brightness of a Flash of
  Lightning--Various Forms of Lightning--Forked Lightning, Sheet
  Lightning, Globe Lightning--St. Elmo’s Fire--Experimental
  Illustration--Origin of Lightning--Length of a Flash of
  Lightning--Physical Cause of Thunder--Rolling of Thunder--Succession
  of Peals--Variation of Intensity--Distance of a Flash of Lightning


  LECTURE II.                                                Pages 26-53

  LIGHTNING CONDUCTORS.

  Destructive Effects of Lightning--Destruction of
  Buildings--Destruction of Ships at Sea--Destruction of Powder
  Magazines--Experimental Illustrations--Destruction of Life by
  Lightning--The Return Shock--Franklin’s Lightning Rods--Introduction
  of Lightning Rods into England--The Battle of Balls and
  Points--Functions of a Lightning Conductor--Conditions of a Lightning
  Conductor--Mischief Done by Bad Conductors--Evil Effects of a Bad
  Earth Contact--Danger from Rival Conductors--Insulation of Lightning
  Conductors--Personal Safety in a Thunder Storm--Practical
  Rules--Security Afforded by Lightning Rods


  APPENDIX.                                                  Pages 55-62

  RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.

  Theory of Lightning Conductors Challenged--Lectures of Professor
  Lodge--Short Account of his Views and Arguments--Effect of
  Self-Induction on a Lightning Rod--Experiment on the Discharge of a
  Leyden Jar--Outer Shell only of a Lightning Rod Acts as a
  Conductor--Discussion at the Meeting of the British Association,
  September, 1888--Statement by Mr. Preece--Lord Rayleigh and Sir
  William Thomson--Professor Rowland and Professor Forbes--M. de
  Fonvielle, Sir James Douglass, and Mr. Symons--Reply of Professor
  Lodge--Concluding Remarks of Professor Fitzgerald, President
  of the Section--Summary Showing the Present State of the Question




LIST OF ILLUSTRATIONS.


                                                                    PAGE

  THE ELECTRIC SPARK: A TYPE OF A FLASH OF LIGHTNING,                  8

  CARDBOARD DISC WITH BLACK AND WHITE SECTORS; AS SEEN WHEN AT REST,  12

  SAME DISC; AS SEEN WHEN IN RAPID ROTATION,                          12

  THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE,                  17

  ORIGIN OF SUCCESSIVE PEALS OF THUNDER,                              22

  VARIATIONS OF INTENSITY IN A PEAL OF THUNDER,                       24

  DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES,                 27

  GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY,             32

  GUN COTTON SET ON FIRE BY ELECTRIC SPARK,                           33

  VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK,                 34

  THE RETURN SHOCK ILLUSTRATED,                                       35

  PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR,                    48

  INDUCTION EFFECT OF LEYDEN JAR DISCHARGE,                           56




LECTURE I.

LIGHTNING AND THUNDER.


The electricity produced by an ordinary electric machine exhibits,
under certain conditions, phenomena which bear a striking resemblance
to the phenomena attendant on lightning. In both cases there is a flash
of light; in both there is a report, which, in the case of lightning,
we call thunder; and, in both cases, intense heat is developed, which
is capable of setting fire to combustible bodies. Further, the spark
from an electric machine travels through space with extraordinary
rapidity, and so does a flash of lightning; the spark follows a zig-zag
course, and so does a flash of lightning; the spark moves silently and
harmlessly through metal rods and stout wires, while it forces its
way, with destructive effect, through bad conductors, and it is so,
too, with a flash of lightning. Lastly, the electricity of a machine
is capable of giving a severe shock to the human body; and we know
that lightning gives a shock so severe as usually to cause immediate
death. For these reasons it was long conjectured by scientific men
that lightning is, in its nature, identical with electricity; and that
it differs from the electricity of our machines only in this, that it
exists in a more powerful and destructive form.


=Identity of Lightning and Electricity.=--But it was reserved for
the celebrated Benjamin Franklin to demonstrate the truth of this
conjecture by direct experiment. He first conceived the idea of drawing
electricity from a thundercloud in the same way as it is drawn from
the conductor of an electric machine. For this purpose he proposed
to place a kind of sentry-box on the summit of a lofty tower, and to
erect, on the sentry-box, a metal rod, projecting twenty or thirty
feet upward into the air, pointed at the end, and having no electrical
communication with the earth. He predicted that when a thundercloud
would pass over the tower, the metal rod would become charged with
electricity, and that an observer, stationed in the sentry-box, might
draw from it, at pleasure, a succession of electric sparks.

With the magnanimity of a really great man, Franklin published this
project to the world; being more solicitous to extend the domain of
science by new discoveries, than to secure for himself the glory
of having made them. The project was set forth in a letter to Mr.
Collinson, of London, which bears date July 29, 1750, and which, in the
course of a year or two, was translated into the principal languages
of Europe. Two years later the experiment suggested by Franklin was
made by Monsieur Dalibard, a wealthy man of science, at his villa near
Marly-la-Ville, a few miles from Paris. In the middle of an elevated
plain Monsieur Dalibard erected an iron rod, forty feet in length, one
inch in diameter, and ending above in a sharp steel point. The iron rod
rested on an insulating support, and was kept in position by means of
silk cords.

In the absence of Monsieur Dalibard, who was called by business to
Paris, this apparatus was watched by an old dragoon, named Coiffier;
and on the afternoon of the tenth of May, 1752, he drew sparks from the
lower end of the rod at the time that a thundercloud was passing over
the neighborhood. Conscious of the importance that would be attached to
this phenomenon, the old dragoon summoned, in all haste, the prior of
Marly to come and witness it. The prior came without delay, and he was
followed by some of the principal inhabitants of the village. In the
presence of the little group, thus gathered together, the experiment
was repeated--electric sparks were again drawn, in rapid succession,
from the iron rod; the prediction of Franklin was fulfilled to the
letter; and the identity of lightning and electricity was, for the
first time, demonstrated to the world.


=Franklin’s Experiment.=--Meanwhile Franklin had been waiting, with
impatience, for the completion of the tower of Christchurch, in
Philadelphia, on which he intended to make the experiment himself.
He even collected money, it is said, to hasten on the building.
But, notwithstanding his exertions, the progress of the tower was
slow; and his active mind, which could ill brook delay, hit upon
another expedient, remarkable alike for its simplicity and for its
complete success. He constructed a boy’s kite, using, however, a silk
pockethandkerchief, instead of paper, that it might not be damaged by
rain. To the top of the kite he attached a pointed iron wire about a
foot long, and he provided a roll of hempen twine, which he knew to be
a conductor of electricity, for flying it. This was the apparatus with
which he proposed to explore the nature of a thundercloud.

The thundercloud came late in the afternoon of the fourth of July,
1752, and Franklin sallied out with his kite, accompanied by his son,
and taking with him a common door-key and a Leyden jar. The kite
was soon high in air, and the philosopher awaited the result of his
experiment, standing, with his son, under the lee of a cowshed, partly
to protect himself from the rain that was coming, and partly, it is
said, to shield himself from the ridicule of passers-by, who, having
no sympathy with his philosophical speculations, might be inclined to
regard him as a lunatic. To guard against the danger of receiving a
flash of lightning through his body, he held the kite by means of a
silk ribbon, which was tied to the door-key, the door-key being itself
attached to the lower end of the hempen string.

A flash of lightning soon came from the cloud, and a second, and a
third; but no sign of electricity could be observed in the kite, or
the hempen cord, or the key. Franklin was almost beginning to despair
of success, when suddenly he noticed that the little fibres of the
cord began to bristle up, just as they would if it were placed near an
electric machine in action. He presented the door-key to the knob of
the Leyden jar, and a spark passed between them. Presently a shower
began to fall; the cord, wetted by the rain, became a better conductor
than it had been before, and sparks came more freely. With these sparks
he now charged the Leyden jar, and found, to his intense delight, that
he could exhibit all the phenomena of electricity by means of the
lightning he had drawn from the clouds.

In the following year a similar experiment, with even more striking
results, was carried out, in France, by de Romas. Though it is said he
had no knowledge of what Franklin had done in America, he, too, used
a kite; and, with a view of making the string a better conductor, he
interlaced with it a thin copper wire. Then, flying his kite in the
ordinary way, when it had risen to a height of about 550 feet, he drew
sparks from it which, we are told, were upwards of nine feet long, and
emitted a sound like the report of a pistol.


=Fatal Experiment of Richman.=--There can be no doubt that experiments
of this kind, made with the electricity of a thundercloud, were
extremely dangerous; and this was soon proved by a fatal accident.
Professor Richman, of St. Petersburgh, had erected on the roof of his
house a pointed iron rod, the lower end of which passed into a glass
vessel, intended, as we are informed, to measure the strength of the
charge which he expected to receive from the clouds. On the sixth of
August, 1753, observing the approach of a thunderstorm, he hastened
to his apparatus; and as he stood near it, with his head bent down,
to watch the effect, a flash of lightning passed through his body and
killed him on the spot. This catastrophe served to fix public attention
on the danger of such experiments, and gave occasion to the saying of
Voltaire: “There are some great lords whom we should always approach
with extreme precaution, and lightning is one of them.”[1] From this
time the practice of making experiments directly with the lightning of
the clouds seems to have been, by common consent, abandoned.


=Immediate Cause of Lightning.=--And now, having set before you some
of the most memorable experiments by which the identity of lightning
and electricity has been demonstrated, I will try to give you a clear
conception regarding the immediate cause of lightning, so far as the
subject is understood at the present day by scientific men. You know
that there are two kinds of electricity, which are called _positive_
and _negative_; and that each of them repels electricity of the same
kind as itself, while it attracts electricity of the opposite kind.
Now, every thundercloud is charged with electricity of one kind or
the other, positive or negative; and, as it hovers over the earth, it
develops, by what is called _induction_, or influence, electricity
of the opposite kind in that part of the earth which is immediately
under it. Thus we have two bodies--the cloud and the earth--charged
with opposite kinds of electricity, and separated by a stratum of the
atmosphere. The two opposite electricities powerfully attract each
other; but for a time they are prevented from rushing together by the
intervening stratum of air, which is a non-conductor of electricity,
and acts as a barrier between them. As the electricity, however,
continues to accumulate, the attraction becomes stronger and stronger,
until at length it is able to overcome the resistance of this barrier;
a violent disruptive discharge then takes place between the cloud
and the earth, and the flash of lightning is the consequence of the
discharge.

[Illustration: THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.]

The whole phenomenon may be illustrated, on a small scale, by means
of this electric machine of Carré’s which you see before you. When
my assistant turns the handle of the machine negative electricity is
developed in that large brass cylinder, which in our experiment will
represent the thundercloud. At a distance of five or six inches from
the cylinder I hold a brass ball, which is in electrical communication
with the earth through my body. The electrified brass cylinder acts
by induction, or influence on the brass ball, and develops in it, as
well as in my body, a charge of positive electricity. Now, the positive
electricity of the ball and the negative electricity of the cylinder
are mutually attracting each other, but the intervening stratum of air
offers a resistance which prevents a discharge from taking place. My
assistant, however, continues to work the machine; the two opposite
electricities rapidly accumulate on the cylinder and the ball; at
length their mutual attraction is strong enough to overcome the
resistance interposed between them; a disruptive discharge follows,
and at the same moment a spark is seen to pass, accompanied by a sharp
snapping report.

This spark is a miniature flash of lightning; and the snapping report
is a diminutive peal of thunder. Furthermore, at the moment the spark
passes you may observe a slight convulsive movement in my hand and
wrist. This convulsive movement represents, on a small scale, the
violent shock, generally fatal to life, which is produced by a flash of
lightning when it passes through the body.

I can continue to take sparks from the conductor as long as the machine
is worked; and it is interesting to observe that these sparks follow
an irregular zig-zag course, just as lightning does. The reason is the
same in both cases: a discharge between two electrified bodies takes
place along the line of least resistance; and, owing to the varying
condition of the atmosphere, as well as of the minute particles of
matter floating in it, the line of least resistance is almost always a
zig-zag line.


=What a Flash of Lightning is.=--Lightning, then, may be conceived as
an electrical discharge, sudden and violent in its character, which
takes place, through the atmosphere, between two bodies highly charged
with opposite kinds of electricity. Sometimes this electrical discharge
passes, as I have said, between a cloud and the earth; sometimes it
passes between one cloud and another; sometimes, on a smaller scale,
it takes place, between the great mass of a cloud and its outlying
fragments.

But, if you ask me in what the discharge itself consists, I am utterly
unable to tell you. It is usual to speak and write on this subject as
if electricity were a material substance, a very subtle fluid, and as
if, at the moment the discharge takes place, this fluid passes like a
rapid stream, from the body that is positively electrified to the body
that is negatively electrified. But we must always remember that this
is only a conventional mode of expression, intended chiefly to assist
our conceptions, and to help us to talk about the phenomena. It does
not even profess to represent the objective truth. All that we know
for certain is this: that immediately before the discharge the two
bodies are highly electrified with opposite kinds of electricity; and,
that immediately after the discharge, they are found to have returned
to their ordinary condition, or, at least, to have become less highly
electrified than they were before.

The flash of light that accompanies an electric discharge is often
supposed to be the electricity itself, passing from one body to
the other. But it is not; it is simply an effect produced by the
discharge. Heat is generated by the expenditure of electrical energy,
in overcoming the resistance offered by the atmosphere; and this heat
is so intense, that it produces a brilliant incandescence along the
path of the discharge. When a spark appears, for example, between the
conductor of the machine and this brass ball, it can be shown, by very
satisfactory evidence, that minute particles of these solid bodies are
first converted into vapor, and then made to glow with intense heat.
The gases, too, of which the air is composed, and the solid particles
floating in the air, are likewise raised to incandescence. So, too,
with lightning; the flash of light is due to the intense heat generated
by the electrical discharge, and owes its character to the composition
and the density of the atmosphere through which the discharge passes.


=Duration of a Flash of Lightning.=--How long does a flash of lightning
last? You are aware, I dare say, that when an impression of light is
made on the eye, the impression remains for a sensible interval of
time, not less than the tenth of a second, after the source of light
has been extinguished or removed. Hence we continue, in fact, to see
the light, for at least the tenth of a second, after the light has
ceased. Now, if you reflect how brief is the moment for which a flash
of lightning is visible, and if you deduct the tenth of a second from
that brief moment, you will see, at once, that the period of its actual
duration must be very short indeed.

The exact duration of a flash of lightning is a question on which no
settled opinion has yet been accepted generally by scientific men.
Indeed, the most widely different statements have been made on the
subject, quite recently, by the highest authorities, each speaking
apparently with unhesitating confidence. Thus, for example, Professor
Mascart describes an experiment, which he says was made by Wheatstone,
and which showed that a flash of lightning lasts for less than
_one_-thousandth of a second;[2] Professor Everett describes the same
experiment, without saying by whom it was made, and gives, as the
result, that “the duration of the illumination produced by lightning
is certainly less than the _ten_-thousandth of a second;”[3] Professor
Tyndall, in his own picturesque way, tells us that “a flash of
lightning cleaves a cloud, appearing and disappearing in less than the
_hundred_-thousandth of a second;”[4] and according to Professor Tait,
of Edinburgh, “Wheatstone has shown that lightning certainly lasts less
than the _millionth_ of a second.”[5]


=Experiments of Professor Rood.=--I cannot say which of these
statements is best supported by actual observation; for none of the
writers I have quoted gives any reference to the original memoir from
which his statement is derived. As far as my own reading goes, I have
only come across one original record of experiments, made directly on
the flash of lightning itself, with a view to determine the period of
its duration. These experiments were carried out by Professor Ogden
Rood, of Columbia College, New York, between the years 1870 and 1873,
and are recorded in the _American Journal of Science and Arts_.[6]

For the description of his apparatus, and for the details of his
observations, I must refer you to the memoir itself; but I may tell you
briefly that the results at which he arrived, if they be accepted, must
lead to a considerable modification of the views previously entertained
on the subject. In the first place, he satisfied himself that what
appears to the eye a single flash of lightning is usually, if not
always, multiple in its character; consisting, in fact, of a succession
of distinct flashes, which follow one another with such rapidity as
to make a continuous impression on the retina. Next, he proceeded to
measure approximately the duration of these several component flashes;
and he found that it varied over a wide range, amounting sometimes to
fully the twentieth of a second, and being sometimes less than the
sixteen-hundredth of a second.


=Wheatstone’s Experiments.=--These results are extremely interesting;
but we can hardly regard them as finally established, until they have
been confirmed by other observers. I may remark, however, that they
fit in very well with the experiments made by Professor Wheatstone,
many years ago, on the duration of the electric spark, which, as I told
you, is a miniature flash of lightning. In these classical experiments,
which leave nothing to be desired in point of accuracy, Professor
Wheatstone showed that a spark taken directly from a Leyden jar, or a
spark taken from the conductor of a powerful electric machine, that is,
just such a spark as you have seen here to-day, lasts for less than the
millionth of a second.

But he also showed that the duration of the spark is greatly increased,
when a resisting wire is introduced into the path of the discharge.
Thus, for example, when the discharge from a Leyden jar was made to
pass through half a mile of copper wire, with breaks at intervals, the
sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀
of a second.[7] Hence we should naturally expect that the period of
illumination would be still further increased, in the case of a flash
of lightning, where the resistance interposed is enormously greater
than in either of the experiments made by Wheatstone.[8]


=Experiment of the Rotating Disk.=--It would be tedious, on an occasion
like the present, to enter into an account of Wheatstone’s beautiful
and ingenious method of investigation, by which the above facts have
been established; but I will show you a much more simple experiment
which brings home very forcibly to the mind how exceedingly short
must be the duration of the electric spark. Here is a circular disk
of cardboard, the outer part of which, as you see, is divided into
sectors, black and white alternately, while the space about the centre
is entirely white. The disk is mounted on a stand, by means of which
I can make it rotate with great velocity. When it is put in rotation,
the effect on the eye is very striking--the central space remains white
as before, but in the outer rim the distinction of black and white
absolutely disappears and gives place to a uniform gray. This color is
due to the blending together of black and white in equal proportions;
the blending being effected, not on the cardboard disk, but on the
retina of the eye.

[Illustration: CARDBOARD DISK AS SEEN WHEN AT REST.]

[Illustration: SAME DISK AS SEEN WHEN IN RAPID ROTATION.]

I mentioned just now that an impression made on the retina lasts for
the tenth of a second after the cause of it has been removed. Now, when
this disk is in rotation, the sectors follow one another so rapidly
that the particular part of space occupied at any moment by a white
sector will be occupied by a black sector within a time much less than
the tenth of a second. It follows that the impression made by each
white sector remains on the retina until the following black sector
comes into the same position; and, in like manner, the impression made
by each black sector remains until the following white sector takes up
the position of the black. Therefore, the impression made by the whole
outer rim is the impression of black and white combined--that is, the
impression of gray.

So far, I dare say, the phenomenon is already familiar to you all.
But I propose now to show you the revolving disk illuminated by
the electric spark; and you will observe that, at the moment of
illumination, the black and white sectors come out as clearly and
distinctly as if the disk were standing still.

For the success of this experiment it is desirable, not only to have
a brilliant spark in order to secure a good illumination of the disk,
but also to have a succession of such sparks, that you may see the
phenomenon frequently repeated, and thus be able to observe it at your
leisure. To attain these two objects, I have made the arrangement which
is here before you.

In front of the disk is a large and very powerful Leyden jar. The
rod connected with the inner coating rises well above the mouth of
the jar, and ends in a brass ball nearly opposite the centre of the
disk. Connected with the outer coating of the jar is another rod
which likewise ends in a brass ball, and which is so adjusted that
the distance between the two balls is about an inch. The two rods are
connected respectively with the two conductors of a Holtz machine, so
that, when the machine is worked, the jar is first quickly charged,
and then it discharges itself, with a brilliant spark, between the
two brass balls. Thus, by continuing to work the machine, we can get,
as long as we choose, a succession of sparks following one another at
short and regular intervals right in front of the disk.

Everything being now ready, and the room partially darkened, the
disk is put in rapid rotation; and you can see, by the twilight that
remains, the outer rim a uniform gray, and the central space white.
But when my assistant begins to turn the Holtz machine, and brilliant
sparks leap out at intervals, the revolving disk, illuminated for a
moment at each discharge, seems to be standing still, and shows the
black and white sectors distinctly visible.

The reason of this is clear: So brief is the moment for which the spark
endures, that the disk, though in rapid motion, makes no sensible
advance during that small fraction of time; therefore, in the image on
the retina, the impression made by the white sectors remains distinct
from the impression made by the black, and the eye sees the disk as it
really is.

I may notice, in passing, a very interesting consideration, suggested
by this experiment. A cannon ball is now commonly discharged with a
velocity of about 1,600 feet a second. Moving with this velocity it
is, as you know, under ordinary circumstances, altogether invisible to
the eye. But suppose it were illuminated, in the darkness of night,
by this electric spark, which lasts, we will say, for the millionth
of a second. During the moment of illumination, the cannon ball moves
through the millionth part of 1,600 feet, which is a little less than
the fiftieth of an inch. Practically, we may say that the cannon ball
does not sensibly change its place while the spark lasts. Further, the
impression it makes on the eye, from the position it occupies at the
moment of illumination, remains on the retina for at least the tenth
of a second. Therefore, if we are looking toward that particular part
of space where the cannon ball happens to be at the moment the spark
passes, we must see the cannon ball hanging motionless in the air,
though we know it is traveling at the rate of 1,600 feet a second, or
about 1,000 miles an hour.


=Brightness of a Flash of Lightning.=--I should like to say one word
about the brightness of a flash of lightning. Somewhat more than
thirty years ago, Professor Swan, of Edinburgh, showed that the eye
requires a sensible time--about the tenth of a second--to perceive
the full brightness of a luminous object. Further, he proved, by a
series of interesting experiments, that when a flash of light lasts
for less than the tenth of a second, its apparent brilliancy to the
eye is proportional to the time of its duration.[9] Now consider the
consequence of these facts in reference to the brightness of our
electric spark. If the spark lasted for the tenth of a second, we
should perceive its full brightness; if it lasted for the tenth part of
that time, we should see only the tenth part of its brightness; if it
lasted for the hundredth part, we should see only the hundredth part
of its brightness; and so on. But we know, in point of fact, that it
lasts for less than the millionth of a second, that is, less than the
hundred-thousandth part of the tenth of a second. Therefore we see only
the hundred-thousandth part of its real brightness.

Here is a startling conclusion, and one, I may say, fully justified
by scientific evidence. That electric spark, brilliant as it appears
to us, is really a hundred thousand times as bright as it seems to
be. We cannot speak with the same precision of a flash of lightning;
because its duration has not yet been so exactly determined. But if we
suppose that a flash of lightning, in a particular case, lasts for the
thousandth of a second, it would follow, from the above experiments,
that the flash is a hundred times as bright, in fact, as it appears to
the eye.


=Various Forms of Lightning.=--The lightning of which I have spoken
hitherto is commonly called _forked_ lightning; a name which seems
to have been derived from the zig-zag line of light it presents to
the eye. But there are other forms under which the electricity of the
clouds often makes itself manifest; and to these I would now invite
your attention for a few moments. The most common of them all, at least
in this country, is that which is familiarly known by the name of
_sheet_ lightning. This is, probably, nothing else than the lighting up
of the atmosphere, or of the clouds, by forked lightning, which is not
itself directly visible.

Generally speaking, after a flash of sheet lightning, we hear the
rolling of distant thunder. But it sometimes happens, especially in
summer time, that the atmosphere is again and again lit up by a sudden
glow of light, and yet no thunder is heard. This phenomenon is commonly
called _summer_ lightning, or _heat_ lightning. It is probably due,
in many cases, to electrical discharges in the higher regions of the
atmosphere, where the air is greatly rarified; and, in these cases, it
would seem to resemble the discharges obtained by means of an induction
coil in glass tubes containing rarified gases. But there is little
doubt that in many cases, too, summer lightning, like ordinary sheet
lightning, is due to forked lightning, which is so remote that we can
neither see the flash itself directly, nor hear the rolling of the
thunder.

Perhaps the most distinct and satisfactory evidence on this subject,
derived from actual observation, is contained in the following letter
of Professor Tyndall, written in May, 1883: “Looking to the south
and south-east from the Bel Alp, the play of silent lightning among
the clouds and mountains is sometimes very wonderful. It may be seen
palpitating for hours, with a barely appreciable interval between
the thrills. Most of those who see it regard it as lightning without
thunder--Blitz ohne Donner, Wetterleuchten, I have heard it named by
German visitors. The Monte Generoso, overlooking the Lake of Lugano, is
about fifty miles from the Bel Alp, as the crow flies. The two points
are connected by telegraph; and frequently when the Wetterleuchten,
as seen from the Bel Alp, was in full play, I have telegraphed to the
proprietor of the Monte Generoso Hotel and learned, in every instance,
that our silent lightning co-existed in time with a thunderstorm more
or less terrific in upper Italy.”[10]

Another form of lightning, described by many writers, is called _globe_
lightning. It is said to appear as a ball of fire, about the size of
a child’s head, or even larger, which moves for a time slowly about,
and then, after the lapse of several seconds, explodes with a terrific
noise, sending forth flashes of fire in all directions, which burn
whatever they strike. Many accounts are on record of such phenomena;
but they are derived, for the most part, from the evidence of persons
who were not specially competent to observe, and to describe with
precision, the facts that fell under their observation. Hence these
accounts, while they are accepted by some, are rejected by others;
and it seems to me, in the present state of the question, that the
existence of globe lightning can hardly be regarded as a demonstrated
fact. At all events, if phenomena of this kind have really occurred, I
can only say that nothing we know about electricity, at present, will
enable us to account for them.[11]


=St. Elmo’s Fire.=--A much more authentic and, at the same time, very
interesting form, under which the electricity of the clouds sometimes
manifests its presence, is known by the name of St. Elmo’s fire. This
phenomenon at one time presents the appearance of a star, shining at
the points of the lances or bayonets of a company of soldiers; at
another, it takes the form of a tuft of bluish light, which seems to
stream away from the masts and spars of a ship at sea, or from the
pointed spire of a church. It was well known to the ancients. Cæsar,
in his Commentaries, tells us that, after a stormy night, the iron
points of the javelins of the fifth legion seemed to be on fire; and
Pliny says that he saw lights, like stars, shining on the lances of
the soldiers, keeping watch by night upon the ramparts. When two
such lights appeared at once, on the masts of a ship, they were
called Castor and Pollux, and were regarded by sailors as a sign of a
prosperous voyage. When only one appeared, it was called Helen, and was
taken as an unfavorable omen.

In modern times St. Elmo’s fire has been witnessed by a host of
observers, and all its various phases have been repeatedly described.
In the memoirs of Forbin we read that, when he was sailing once, in
1696, among the Balearic Islands, a sudden storm came on during the
night, accompanied by lightning and thunder. “We saw on the vessel,” he
says, “more than thirty St. Elmo’s fires. Among the rest there was one
on the vane of the mainmast more than a foot and a half high. I sent a
man up to fetch it down. When he was aloft he cried out that it made
a noise like wetted gunpowder set on fire. I told him to take off the
vane and come down; but, scarcely had he removed it from its place,
when the fire left it and reappeared at the end of the mast, so that
it was impossible to take it away. It remained for a long time, and
gradually went out.”

On the 14th of January, 1824, Monsieur Maxadorf happened to look at a
load of straw in the middle of a field just under a dense black cloud.
The straw seemed literally on fire--a streak of light went forth from
every blade; even the driver’s whip shone with a pale-blue flame. As
the black cloud passed away, the light gradually disappeared, after
having lasted about ten minutes. Again, it is related that on the 8th
of May, 1831, in Algiers, as the French artillery officers were walking
out after sunset without their caps, each one saw a tuft of blue light
on his neighbor’s head; and, when they stretched out their hands, a
tuft of light was seen at the end of every finger. Not infrequently a
traveler in the Alps sees the same luminous tuft on the point of his
alpenstock. And quite recently, during a thunderstorm, a whole forest
was observed to become luminous just before each flash of lightning,
and to become dark again at the moment of the discharge.[12]

This phenomenon may be easily explained. It consists in a gradual and
comparatively silent electrical discharge between the earth and the
cloud; and generally, but not always, it has the effect of preventing
such an accumulation of electricity as would be necessary to produce
a flash of lightning. I can illustrate this kind of discharge with
the aid of our machine. If I hold a pointed metal rod toward the
large conductor, you can see, when the machine is worked and the room
darkened, how the point of the rod becomes luminous and shines like a
faint blue star. I substitute for the pointed rod the blunt handles of
a pair of pliers, and a tuft of blue light is at once developed at the
end of each handle, and seems to stream away with a hissing noise. I
now put aside the pliers, and open out my hand under the conductor--and
observe how I can set up, at pleasure, a luminous tuft at the tips
of my fingers. Now and then a spark passes, giving me a smart shock,
and showing how the electricity may sometimes accumulate so fast that
it cannot be sufficiently discharged by the luminous tuft. Lastly, I
present a small bushy branch of a tree to the conductor, and all its
leaves and twigs are aglow with bluish light, which ceases for a moment
when a spark escapes, to be again renewed when electricity is again
developed by the working of the machine.

[Illustration: THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE.]

Now, if you put a thundercloud in the place of that conductor, you can
easily realize how, through its influence, the lance and bayonet of
the soldier, the alpenstock of the traveler, the pointed spire of a
church, the masts of a ship at sea, the trees of a forest, can all be
made to glow with a silent electrical discharge which may or may not,
according to circumstances, culminate at intervals in a genuine flash
of lightning.


=Origin of Lightning.=--When we seek to account for the origin of
lightning, we are confronted at once with two questions of great
interest and importance--first, What are the sources from which the
electricity of the thundercloud is derived? and, secondly, How does
this electricity come to be developed in a form which so far transcends
in power the electricity of our machines? These questions have long
engaged the attention of scientific men, but I cannot say that they
have yet received a perfectly satisfactory solution. Nevertheless,
some facts of great scientific value have been established, and some
speculations have been put forward, which are well deserving of
consideration.

In the first place, it is quite certain that the atmosphere which
surrounds our globe is almost always in a state of electrification.
Further, the electrical condition of the atmosphere would seem to be
as variable as the wind. It changes with the change of season; it
changes from day to day; it changes from hour to hour. The charge of
electricity is sometimes positive, sometimes negative; sometimes it
is strong, sometimes feeble; and the transition from one condition to
another is sometimes slow and gradual, sometimes sudden and violent.

As a general rule, in fine, clear weather, the electricity of the
atmosphere is positive, and not very strongly developed. In wet weather
the charge may be either positive or negative, and is generally
strong, especially when there are sudden heavy showers. In fog it is
also strong, and almost always positive. In a snowstorm it is very
strong, and most frequently positive. Finally, in a thunderstorm it is
extremely strong, and generally negative; but it is subject to a sudden
change of sign, when a flash of lightning passes or when rain begins to
fall.

So far I have simply stated facts, which have been ascertained
by careful observations, made at different stations by competent
observers, and extending over a period of many years. But as regards
the process by which the electricity of the atmosphere is developed, we
have, up to the present time, no certain knowledge. It has been said
that electricity may be generated in the atmosphere by the friction of
the air itself, and of the minute particles floating in it, against
the surface of the earth, against trees and buildings, against rocks,
cliffs, and mountains. But this opinion, however probable it may be,
has not yet been confirmed by any direct experimental investigation.

The second theory is that the electricity of the atmosphere is due, in
great part at least, to the evaporation of salt water. Many years ago,
Pouillet, a French philosopher, made a series of experiments in the
laboratory, which seemed to show that evaporation is generally attended
with the development of electricity; and, in particular, he satisfied
himself that the vapor which passes off from the surface of salt water
is always positively electrified. Now, the atmosphere is everywhere
charged, more or less, with vapor which comes, almost entirely, from
the salt water of the ocean. Hence Pouillet inferred that the chief
source of atmospheric electricity is the evaporation of sea water.
This explanation would certainly go far to account for the presence
of electricity in the atmosphere, if the fact on which it rests were
established beyond dispute. But there is some reason to doubt whether
the development of electricity, in the experiments of Pouillet, was due
simply to the process of evaporation, and not rather to other causes,
the influence of which he did not sufficiently take into account.

A conjecture has recently been started that electricity may be
generated by the mere impact of minute particles of water vapor against
minute particles of air.[13] If this conjecture could be established as
a fact, it would be amply sufficient to account for all the electricity
of the atmosphere. From the very nature of a gas, the molecules of
which it is composed are forever flying about with incredible velocity;
and therefore the particles of water vapor and the particles of air,
which exist together in the atmosphere, must be incessantly coming
into collision. Hence, however small may be the charge of electricity
developed at each individual impact, the total amount generated over
any considerable area, in a single day, must be very great indeed.
It is evident, however, that this method of explaining the origin of
atmospheric electricity can only be regarded as, at best, a probable
hypothesis, until the assumption on which it rests is supported by the
evidence of observation or experiment.


=Length of a Flash of Lightning.=--It would seem, then, that we are
not yet in a position to indicate with certainty the sources from
which the electricity of the atmosphere is derived. But whatever
these sources may be, there can be little doubt that the electricity
of the atmosphere is intimately associated with the minute particles
of water vapor of which the thundercloud is eventually built up.
This consideration is of great importance when we come to consider
the special properties of lightning, as compared with other forms of
electricity. The most striking characteristic of lightning is the
wonderful power it possesses of forcing its way through the resisting
medium of the air. In this respect it incomparably surpasses all forms
of electricity that have hitherto been produced by artificial means.
The spark of an ordinary electric machine can leap across a space of
three or four inches; the machine we have employed in our experiments
to-day can give, under favorable circumstances, a spark of nine or ten
inches; the longest electric spark ever yet produced artificially is
probably the spark of Mr. Spottiswoode’s gigantic induction coil; and
it does not exceed three feet six inches. But the length of a flash of
lightning is not to be measured in inches, or in feet or in yards; it
varies from one or two miles, for ordinary flashes, to eight or ten
miles in exceptional cases.

This power of discharging itself violently through a resisting
medium, in which the thundercloud so far transcends the conductor of
an electric machine, is due to the property commonly known among
scientific men as electrical _potential_. The greater the distance to
which an electrified body can shoot its flashes through the air, the
higher must be its potential. Hence the potential of a thundercloud
must be exceedingly high, since its flashes can pierce the air to a
distance of several miles. And what I want to point out is, that we
are able to account for this exceedingly high potential, if we may
only assume that the minute particles of water vapor in the atmosphere
have, from any cause, received ever so small a charge of electricity.
The number of such particles that go to make up an ordinary drop of
rain are to be counted by millions of millions; and it is capable of
scientific proof that, as each new particle is added, in the building
up of the drop, a rise of potential is necessarily produced. It is
clear, therefore, that there is practically no limit to the potential
that may be developed by the simple agglomeration of very small cloud
particles, each carrying a very small charge of electricity.[14]

This explanation, which traces the exceedingly high potential of
lightning to the building up of rain drops in the thundercloud,
suggests a reason why it so often happens that immediately after a
flash of lightning “the big rain comes dancing to the earth.” The
potential has been steadily rising as the drops have been getting
larger and larger, until at length the potential has become so high
that the thundercloud is able to discharge itself, and almost at the
same moment the drops have become so large that they can no longer be
held aloft against the attracting force of gravity.


=Physical Cause of Thunder.=--Let us now proceed to consider the
phenomenon of thunder, which is so intimately associated with
lightning, and which, though perfectly harmless in itself, and though
never heard until the real danger is past, often excites more terror
in the mind than the lightning flash itself. The sound of thunder,
like that of the electric spark, is due to a disturbance caused in
the air by the electric discharge. The air is first expanded by the
intense heat that is developed along the line of discharge, and then it
rushes back again to fill up the partial vacuum which its expansion has
produced. This sudden movement gives rise to a series of sound waves,
which reach the ear in the form of thunder. But there are certain
peculiar characteristics of thunder which are deserving of special
consideration.


=Rolling of Thunder.=--They may be classified, I think, under two
heads. First, the sound of thunder is not an instantaneous report
like the sound of the electric spark--it is a prolonged peal lasting,
sometimes, for several seconds. Secondly, each flash of lightning gives
rise, not to one peal only, but to a succession of peals following
one another at irregular intervals. These two phenomena, taken
together, produce that peculiar effect on the ear which is commonly
described as the _rolling_ of thunder; and both of them, I think, may
be sufficiently accounted for in accordance with the well-established
properties of sound.

To understand why the sound of thunder reaches the ear as a prolonged
peal, we have only to remember that sound takes time to travel. Since a
flash of lightning is practically instantaneous, we may assume that the
sound is produced at the same moment all along the line of discharge.
But the sound waves, setting out at the same moment from all points
along the line of discharge, must reach the ear in successive instants
of time, arriving first from that point which is nearest to the
observer, and last from that point which is most distant. Suppose, for
example, that the nearest point of the flash is a mile distant from the
observer, and the farthest point two miles--the sound will take about
five seconds to come from the nearest point, and about ten seconds to
come from the farthest point; and moreover, in each successive instant
from the time the first sound reaches the ear, sound will continue
to arrive from the successive points between. Therefore the thunder,
though instantaneous in its origin, will reach the ear as a prolonged
peal extending over a period of five seconds.


=Succession of Peals.=--The succession of peals produced by a single
flash of lightning is due to several causes, each one of which may
contribute more or less, according to circumstances, toward the general
effect. First, if we accept the results arrived at by Professor Ogden
Rood, of Columbia College, what appears to the eye as a single flash
of lightning, consists, in fact, as a general rule, of a succession
of flashes, each one of which must naturally produce its own peal of
thunder; and although the several flashes, if they follow one another
at intervals of the tenth of a second, will make one continuous
impression on the eye, the several peals of thunder, under the same
conditions, will impress the ear as so many distinct peals.

The next cause that I would mention is the zigzag path of the lightning
discharge. To make clear to you the influence of this circumstance, I
must ask your attention for a moment to the diagram on next page. Let
the broken line represent the path of a flash of lightning, and let O
represent the position of an observer. The sound will reach him first
from the point A, which is nearest to him, and then it will continue to
arrive in successive instants from the successive points along the line
A N and along the line A M, thus producing the effect of a continuous
peal. Meanwhile the sound waves have been traveling from the point B,
and in due time will reach the observer at O. Coming as they do in a
different direction from the former, they will strike the ear as the
beginning of a new peal which, in its turn, will be prolonged by the
sound waves arriving, in successive instants, from the successive
points along the line B M and B H. A little later, the sound will
arrive from the more distant point C, and a third peal will begin. And
so there will be several distinct peals proceeding, so to speak, from
several distinct points in the path of the lightning flash.

[Illustration: ORIGIN OF SUCCESSIVE PEALS OF THUNDER.]

A third cause to which the succession of peals may be referred is to
be found in the minor electrical discharges that must often take place
within the thundercloud itself. A thundercloud is not a continuous mass
like the metal cylinder of this electric machine--it has many outlying
fragments, more or less imperfectly connected with the principal body.
Moreover, the material of which the cloud is composed is only a very
imperfect conductor as compared with our brass cylinder. For these
two reasons it must often happen, about the time a flash of lightning
passes, that different parts of the cloud will be in such different
electrical conditions as to give rise to electrical discharges within
the cloud itself. Each of these discharges produces its own peal of
thunder; and thus we may have a number of minor peals, sometimes
preceding and sometimes following the great crash which is due to the
principal discharge.

Lastly, the influence of echo has often a considerable share in
multiplying the number of peals of thunder. The waves of sound, going
forth in all directions, are reflected from the surfaces of mountains,
forests, clouds, and buildings, and coming back from different
quarters, and with varying intensity, reach the ear like the roar of
distant artillery. The striking effect of these reverberations in a
mountain district has been described by a great poet in words which, I
daresay, are familiar to most of you:

                                “Far along,
  From peak to peak, the rattling crags among,
    Leaps the live thunder! Not from one lone cloud,
  But every mountain now has found a tongue,
    And Jura answers from her misty shroud
    Back to the joyous Alps, that call to her aloud!”


=Variations of Intensity in Thunder.=--From what has been said, it
is easy to understand how the general roar of thunder is subject to
great changes of intensity, during the time it lasts, according to
the number of peals that may be arriving at the ear of an observer in
each particular moment. But every one must have observed that even an
individual peal of thunder often undergoes similar changes, swelling
out at one moment with great power, and the next moment rapidly
dying away. To account for this phenomenon, I would observe, first,
that there is no reason to suppose that the disturbance caused by
lightning is of exactly the same magnitude at every point of its path.
On the contrary, it would seem very probable that the amount of this
disturbance is, in some way, dependent on the resistance which the
discharge encounters. Hence the intensity of the sound waves sent forth
by a flash of lightning is probably very different at different parts
of its course; and each individual peal will swell out on the ear or
die away, according to the greater or less intensity of the sound waves
that reach the ear in each successive moment of time.

But there is another influence at work which must produce variations
in the loudness of a peal of thunder, even though the sound waves, set
in motion by the lightning, were everywhere of equal intensity. This
influence depends on the position of the observer in relation to the
path of the lightning flash. At one part of its course the lightning
may follow a path which remains for a certain length at nearly the
same distance from the observer; then all the sound produced along
this length will reach the observer nearly at the same moment, and
will burst upon the ear with great intensity. At another part, the
lightning may for an equal length go right away from the observer; and
it is evident that the sound produced along this length will reach the
observer in successive instants, and consequently produce an effect
comparatively feeble.

With a view to investigate this interesting question a little more
closely, let me suppose the position of the observer taken as a
centre, and a number of concentric circles drawn, cutting the path of
the lightning flash, and separated from one another by a distance of
110 feet, measured along the direction of the radius. It is evident
that all the sound produced between any two consecutive circles will
reach the ear within a period which must be measured by the time that
sound takes to travel 110 feet, that is, within the tenth of a second.
Hence, in order to determine the quantity of sound that reaches the
ear in successive periods of one-tenth of a second, we have only to
observe how much is produced between each two consecutive circles.
But on the supposition that the sound waves, set in motion by the
flash of lightning, are of equal intensity at every point of its path,
it is clear that the quantity of sound developed between each two
consecutive circles will be simply proportional to the length of the
path enclosed between them.

With these principles established, let us now follow the course of a
peal of thunder, in the diagram before us. This broken line, drawn
almost at random, represents the path of a flash of lightning; the
observer is supposed to be placed at O, which is the centre of the
concentric circles; these circles are separated from one another by a
distance of 110 feet, measured in the direction of the radius; and we
want to consider how any one peal of thunder may vary in loudness in
the successive periods of one-tenth of a second.

[Illustration: VARIATIONS OF INTENSITY IN A PEAL OF THUNDER.]

Let us take, for example, the peal which begins when the sound waves
reach the ear from the point A. In the first unit of time the sound
that reaches the ear is the sound produced along the lines A B and A
C; in the second unit, the sound produced along the lines B D and C E;
in the third unit, the sound produced along D F and E G. So far the
peal has been fairly uniform in its intensity; though there has been a
slight falling off in the second and third units of time, as compared
with the first. But in the fourth unit there is a considerable falling
away of the sound; for the line F K is only about one-third as long
as D F and E G taken together; therefore the quantity of sound that
reaches the ear in the fourth unit of time is only one-third of that
which reaches it in each of the three preceding units; and consequently
the sound is only one-third as loud. In the fifth unit, however, the
peal must rise to a sudden crash; for the portion of the lightning path
inclosed between the fifth and sixth circles is about six times as
great as that between the fourth and fifth; therefore the intensity of
the sound will be suddenly increased about six-fold. After this sudden
crash, the sound as suddenly dies away in the sixth unit of time; it
continues feeble as the path of the lightning goes nearly straight
away from the observer; it swells again slightly in the ninth unit of
time; and then continues without much variation to the end. This is
only a single illustration, but it seems quite sufficient to show that
the changes of intensity in a peal of thunder must be largely due to
the position of the spectator in relation to the several parts of the
lightning flash.


=Distance of a Flash of Lightning.=--I need hardly remind you that, by
observing the interval that elapses between the flash of lightning and
the peal of thunder that follows it, we may estimate approximately the
distance of the nearest point of the discharge. Light travels with such
amazing velocity that we may assume, without any sensible error, that
we see the flash of lightning at the very moment in which the discharge
takes place. But sound, as we have seen, takes a sensible time to
travel even short distances; and therefore a measurable interval almost
always elapses between the moment in which the flash is seen and the
moment in which the peal of thunder first reaches the ear. And the
distance through which sound travels in this interval will be the
distance of the nearest point through which the discharge has passed.
Now, the velocity of sound in air varies slightly with the temperature;
but, at the ordinary temperature of our climate, we shall not be far
astray if we allow 1,100 feet for every second, or about one mile for
every five seconds.

You will observe also that, by repeating this observation, we can
determine whether the thundercloud is coming toward us, or going away
from us. So long as the interval between each successive flash and the
corresponding peal of thunder, continues to get shorter and shorter,
the thundercloud is approaching; when the interval begins to increase,
the thundercloud is receding from us, and the danger is passed.

The crash of thunder is terrific when the lightning is close at hand;
but it is a curious fact, that the sound does not seem to travel as
far as the report of an ordinary cannon. We have no authentic record
of thunder having been heard at a greater distance than from twelve to
fifteen miles, whereas the report of a single cannon has been heard
at five times that distance; and the roar of artillery, in battle,
at a greater distance still. On the occasion of the Queen’s visit to
Cherbourg, in August, 1858, the salute fired in honor of her arrival
was heard at Bonchurch, in the Isle of Wight, a distance of sixty
miles. It was also heard at Lyme Regis, in Dorsetshire, which is
eighty-five miles from Cherbourg, as the crow flies; and we are told
that, not only was it audible in its general effect, but the report of
individual guns was distinctly recognized. The artillery of Waterloo is
said to have been heard at the town of Creil, in France, 115 miles from
the field of battle; and the cannonading at the siege of Valenciennes,
in 1793, was heard, from day to day, at Deal, on the coast of England,
a distance of 120 miles.[15]

So far, I have endeavored to set forth some general ideas on the nature
and origin of lightning, and of the thunder that accompanies it. In
my next Lecture I propose to give a short account of the destructive
effects of lightning, and to consider how these effects may best be
averted by means of lightning conductors.


NOTE TO PAGE 20.

ON THE HIGH POTENTIAL OF A FLASH OF LIGHTNING.

The potential of an electrified sphere is equal to the quantity of
electricity with which the sphere is charged, divided by the radius
of the sphere. Now the minute cloud particles, which go to make up
a drop of rain, may be taken to be very small spheres; and if _v_
represent the potential of each one, _q_ the quantity of electricity
with which it is charged, and _r_ the radius of the sphere, we have _v_
= _q_/_r_. Suppose 1,000 of these cloud particles to unite into one;
the quantity of electricity in the drop, thus formed, will be 1,000_q_;
and the radius, which increases in the ratio of the cube root of the
volume, will be 10_r_. Therefore the potential of the new sphere will
be 1000_q_/10_r_, or 100_q/r_; that is to say, it will be 100 times as
great as the potential of each of the cloud particles which compose
it. When a million of cloud particles are blended into a single drop,
the same process will show that the potential has been increased ten
thousandfold; and when a drop is produced by the agglomeration of a
million of millions of cloud particles, the potential of the drop
will be a hundred million times as great as that of the individual
particles.[16]


FOOTNOTES:

[1] “Il y a des grands seigneurs dont il ne faut approcher qu’avec
d’extrêmes précautions. Le tonnerre est de ce nombre.”--Dict. Philos.
art. Foudre.

[2] Electricité Statique, ii., 561.

[3] Deschanel’s Natural Philosophy, Sixth Edition, p. 641.

[4] Fragments of Science, Fifth Edition, p. 311.

[5] Lecture on Thunderstorms, Nature, vol. xxii., p. 341.

[6] Third Series, vol. v., p. 161.

[7] Phil. Trans. Royal Society, 1834, vol. cxxv., pp. 583-591.

[8] In experiments with a Leyden jar, Feddersen has shown that the
duration of the discharge is increased, not only by increasing the
striking distance, but also by increasing the size of the jar. Now, a
flash of lightning may be regarded as the discharge of a Leyden jar
of immense size, with an enormous striking distance; and therefore we
should expect that the duration of the discharge should be greatly
prolonged. See _American Journal of Science and Arts_, Third Series,
vol. i., p. 15.

[9] See original paper by Swan, Trans. Royal Society, Edinburgh, 1849,
vol. xvi., pp. 581-603; also, a second paper, _ib._ 1861, vol. xxii.,
pp. 33-39.

[10] Nature, vol. xxviii., p. 54.

[11] See, however, an attempt to account for this phenomenon in De
Larive’s Treatise on Electricity, London, 1853-8, vol. iii., pp. 199,
200; and another, quite recently, by Mr. Spottiswoode, in a Lecture on
the Electrical Discharge, delivered before the British Association at
York, in September, 1881, and published by Longmans, London, p. 42.
See also, for recent evidence regarding the phenomenon itself, Scott’s
Elementary Meteorology, pp. 175-8.

[12] See Jamin, “Cours de Physique,” i., 480-1; Tomlinson, “The
Thunderstorm,” Third Edition, pp. 95-103; “Thunderstorms,” a Lecture by
Professor Tait, Nature, vol. xxii., p. 356.

[13] Professor Tait, On Thunderstorms, Nature, vol. xxii., pp. 436-7.

[14] See note at the end of this Lecture, p. 26.

[15] See Tomlinson, The Thunderstorm, pp. 87-9.

[16] See Tait on Thunderstorms, Nature, vol. xxii., p. 436.




LECTURE II.

LIGHTNING CONDUCTORS.


The effects of lightning, on the bodies that it strikes, are analogous
to those which may be produced by the discharge of our electric
machines and Leyden jar batteries. When the discharge of a battery
traverses a metal conductor of sufficient dimensions to allow it an
easy passage, it makes its way along silently and harmlessly. But if
the conductor be so thin as to offer considerable resistance, then the
conductor itself is raised to intense heat, and may be melted, or even
converted into vapor, by the discharge.

On opposite page is shown a board on which a number of very thin wires
have been stretched, over white paper, between brass balls. The wires
are so thin that the full charge of the battery before you, which
consists of nine large Leyden jars, is quite sufficient to convert them
in an instant into vapor. I have already, on former occasions, sent the
charge through two of these wires, and nothing remains of them now
but the traces of their vapor, which mark the path of the electric
discharge from ball to ball. At the present moment the battery stands
ready charged, and I am going to discharge it through a third wire, by
means of this insulated rod which I hold in my hand. The discharge has
passed; you saw a flash, and a little smoke; and now, if you look at
the paper, you will find that the wire is gone, but that it has left
behind the track of its incandescent vapor, marking the path of the
discharge.

[Illustration: DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES.]


=Destruction of Buildings by Lightning.=--We learn from this experiment
that the electricity stored up in our battery passes, without visible
effect, through the stout wire of a discharging rod, but that it
instantly converts into vapor the thin wire stretched across the spark
board. And so it is with a flash of lightning. It passes harmlessly, as
every one knows, through a stout metal rod, but when it comes across
bell wires or telegraph wires, it melts them, or converts them into
vapor. On the sixteenth of July, 1759, a flash of lightning struck
a house in Southwark, on the south side of London, and followed the
line of the bell wire. After the lightning had passed, the wire was no
longer to be found; but the path of the lightning was clearly marked
by patches of vapor which were left, here and there, adhering to the
surface of the wall. In the year 1754, the lightning fell on a bell
tower at Newbury, in the United States of America, and having dashed
the roof to pieces, and scattered the fragments about, it reached the
bell. From this point it followed an iron wire, about as thick as a
knitting needle, melting it as it passed along, leaving behind a black
streak of vapor on the surface of the walls.

Again, the electric discharge, passing through a bad conductor,
produces mechanical disturbance, and, if the substance be combustible,
often sets it on fire. So, too, as you know, the lightning flash,
falling on a church spire, dashes it to pieces, knocking the stones
about in all directions, while it sets fire to ships and wooden
buildings; and more than once it has caused great devastation by
exploding powder magazines.

Let me give you one or two examples: In January, 1762, the
lightning fell on a church tower in Cornwall, and a stone--three
hundred-weight--was torn from its place and hurled to a distance of 180
feet, while a smaller stone was projected as far as 1,200 feet from the
building. Again, in 1809, the lightning struck a house not far from
Manchester, and literally moved a massive wall twelve feet high and
three thick to a distance of several feet. You may form some conception
of the enormous force here brought into action, when I tell you that
the total weight of mason-work moved on this occasion was not less than
twenty-three tons.

The church of St. George, at Leicester, was severely damaged by
lightning on the 1st of August, 1846. About 8 o’clock in the evening
the rector of the parish saw a vivid streak of light darting with
incredible velocity against the upper part of the spire. “For the
distance of forty feet on the eastern side, and nearly seventy on the
west, the massive stonework of the spire was instantly rent asunder and
laid in ruins. Large blocks of stone were hurled in all directions,
broken into small fragments, and in some cases, there is reason to
believe, reduced to powder. One fragment of considerable size was
hurled against the window of a house three hundred feet distant,
shattering to pieces the woodwork, and strewing the room within with
fine dust and fragments of glass. It has been computed that a hundred
tons of stone were, on this occasion, blown to a distance of thirty
feet in three seconds. In addition to the shivering of the spire, the
pinnacles at the angles of the tower were all more or less damaged, the
flying buttresses cracked through and violently shaken, many of the
open battlements at the base of the spire knocked away, the roof of the
church completely riddled, the roofs of the side entrances destroyed,
and the stone staircases of the gallery shattered.”[17]

Lightning has been at all times the cause of great damage to property
by its power of setting fire to whatever is combustible. Fuller says,
in his Church History, that “scarcely a great abbey exists in England
which once, at least, has not been burned by lightning from heaven.”
He mentions, as examples, the Abbey of Croyland twice burned, the
Monastery of Canterbury twice, the Abbey of Peterborough twice; also
the Abbey of St. Mary’s, in Yorkshire, the Abbey of Norwich, and
several others. Sir William Snow Harris, writing about twenty years
ago, tells us that “the number of churches and church spires wholly or
partially destroyed by lightning is beyond all belief, and would be too
tedious a detail to enter upon. Within a comparatively few years, in
1822 for instance, we find the magnificent Cathedral of Rouen burned,
and, so lately as 1850, the beautiful Cathedral of Saragossa, in Spain,
struck by lightning during divine service and set on fire. In March of
last year a dispatch from our Minister at Brussels, Lord Howard de
Walden, dated the 24th of February, was forwarded by Lord Russell to
the Royal Society, stating that, on the preceding Sunday, a violent
thunderstorm had spread over Belgium; that twelve churches had been
struck by lightning; and that three of these fine old buildings had
been totally destroyed.”[18]

Even in our own day the destruction caused by fires produced through
the agency of lightning is very great--far greater than is commonly
supposed. No general record of such fires is kept, and consequently
our information on the subject is very incomplete and inexact. I
may tell you, however, one small fact which, so far as it goes,
is precise enough and very significant. In the little province of
Schleswig-Holstein, which occupies an area less than one-fourth of the
area of Ireland, the Provincial Fire Assurance Association has paid in
sixteen years, for damage caused by lightning, somewhat over £100,000,
or at the rate of more than £6,000 a year. The total loss of property
every year in this province, due to fires caused by lightning, is
estimated at not less than £12,500.[19]


=Destruction of Ships at Sea.=--The destructive effects of lightning
on ships at sea, before the general adoption of lightning conductors,
seems almost incredible at the present day. From official records
it appears that the damage done to the Royal Navy of England alone
involved an expenditure of from £6,000 to £10,000 a year. We are
told by Sir William Snow Harris, who devoted himself for many years
to this subject with extraordinary zeal and complete success, that
between the year 1810 and the year 1815--that is, within a period of
five years--“no less than forty sail of the line, twenty frigates, and
twelve sloops and corvettes were placed _hors de combat_ by lightning.
In the merchant navy, within a comparatively small number of years,
no less than thirty-four ships, most of them large vessels with rich
cargoes, have been totally destroyed--been either burned or sunk--to
say nothing of a host of vessels partially destroyed or severely
damaged.”[20]

And these statements, be it observed, take no account of ships that
were simply reported as missing, some of which, we can hardly doubt,
were struck by lightning in the open sea, and went down with all hands
on board. A famous ship of forty-four guns, the _Resistance_, was
struck by lightning in the Straits of Malacca, and the powder magazine
exploding, she went to the bottom. Of her whole crew only three were
saved, who happened to be picked up by a passing boat. It has been well
observed that, were it not for these three chance survivors, nothing
would have been known concerning the fate of the vessel, and she would
have been simply recorded as missing in the Admiralty lists.

Nothing is more fearful to contemplate than the scene on board a ship
when she is struck by lightning in the open sea, with the winds howling
around, the waves rolling mountains high, the rain coming down in
torrents, and the vivid flashes lighting up the gloom at intervals, and
carrying death and destruction in their track. I will read you one or
two brief accounts of such a scene, given in the pithy but expressive
language of the sailor. In January, 1786, the _Thisbe_, of thirty-six
guns, was struck by lightning off the coast of Scilly, and reduced to
the condition of a wreck. Here is an extract from the ship’s log: “Four
A. M., strong gales; handed mainsail and main top-sail; hove to with
storm staysails; blowing very heavy, S. E. 4.15, a flash of lightning,
with tremendous thunder, disabled some of our people. A second flash
set the mainsail, main-top, and mizen staysails on fire. Obliged to cut
away the mainmast; this carried away mizen top-mast and fore top-sail
yard. Found foremast also shivered by the lightning. Fore top-mast went
over the side about 9 A. M. Set the foresail.”[21]

A few years later, in March, 1796, the _Lowestoffe_ was struck in the
Mediterranean, and we read as follows in the log of the ship: “North
end of Minorca; heavy squalls; hail, rain, thunder, and lightning.
12.15, ship struck by lightning, which knocked three men from the
masthead, one killed. 12.30, ship again struck; main top-mast shivered
in pieces; many men struck senseless on the decks. Ship again struck,
and set on fire in the masts and rigging; mainmast shivered in pieces;
fore top-mast shivered; men benumbed on the decks, and knocked out
of the top; one man killed on the spot. 1.30, cut away the mainmast;
employed clearing wreck. 4, moderate; set the foresail.”[22]

Again, in 1810, the _Repulse_, a ship of seventy-four guns, was struck,
off the coast of Spain. “The wind had been variable in the morning--and
at 12.35 there was a heavy squall, with rain, thunder, and lightning.
The ship was struck by two vivid flashes of lightning, which shivered
the maintop-gallant mast, and severely damaged the mainmast. Seven men
were killed on the spot; three others only survived a few days; and ten
others were maimed for life. After the second discharge the rain fell
in torrents. The ship was more completely crippled than if she had been
in action, and the squadron, then engaged on a critical service, lost
for a time one of its fastest and best ships.”[23]


=Destruction of Powder Magazines.=--Not less appalling is the
devastation caused by lightning when it falls on a powder magazine.
Here is a striking example: On the eighteenth of August, 1769, the
tower of St. Nazaire, at Brescia, was struck by lightning. Underneath
the tower about 200,000 pounds of gunpowder, belonging to the Republic
of Venice, were stored in vaults. The powder exploded, leveling to
the ground a great part of the beautiful city of Brescia, and burying
thousands of its inhabitants in the ruins. It is said that the tower
itself was blown up bodily to a great height in the air, and came down
in a shower of stones. This is, perhaps, the most fearful disaster of
the kind on record. But we are not without examples in our own times.
In the year 1856 the lightning fell on the Church of St. John, in the
Island of Rhodes. A large quantity of gunpowder had been deposited in
the vaults of the church. This was ignited by the flash; the building
was reduced to a mass of ruins, a large portion of the town was
destroyed, and a considerable number of the inhabitants were killed.
Again, in the following year, the magazine of Joudpore, in the Bombay
Presidency, was struck by lightning. Many thousand pounds of gunpowder
were blown up, five hundred houses were destroyed, and nearly a
thousand people are said to have been killed.[24]


=Experimental Illustrations.=--And now, before proceeding further,
I will make one or two experiments, with a view of showing that the
electricity of our machines is capable of producing effects similar
to those produced by lightning, though immeasurably inferior in point
of magnitude. Here is a common tumbler, about three-quarters full of
water. Into it I introduce two bent rods of brass, which are carefully
insulated below the surface of the water by a covering of india-rubber.
The points, however, are exposed, and come to within an inch of one
another, near the bottom of the tumbler. Outside the tumbler, the
brass rods are mounted on a stand, by means of which I can send the
full charge of this Leyden jar battery through the water, from point
to point. Since water is a bad conductor of electricity, as compared
with metals, the charge encounters great resistance in passing through
it, and in overcoming this resistance produces considerable mechanical
commotion, which is usually sufficient to shiver the glass to pieces.

To charge the battery will take about twenty turns of this large Holtz
machine. Observe how the pith ball of the electroscope rises as the
machine is worked, showing that the charge is going in. And now it
remains stationary; which is a sign that the battery is fully charged,
and can receive no more. You will notice that the outside coating of
the battery has been already connected with one of the brass rods
dipping into the tumbler of water. By means of this discharger I will
now bring the inside coating into connection with the other rod. And
see, before contact is actually made, the spark has leaped across, and
our tumbler is violently burst asunder from top to bottom.

[Illustration: GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY.]

This will probably appear to you a very small affair, when compared
with the tearing asunder of solid masonry, and the hurling about of
stones by the ton weight. No doubt it is; and that is just one of
the lessons we have to learn from the experiment we have made. For,
not only does it show us that effects of this kind may be caused by
electricity artificially produced, but it brings home forcibly to the
mind how incomparably more powerful is the lightning of the clouds than
the electricity of our machines.

The property which electricity has of setting fire to combustible
substances may be easily illustrated. This india-rubber tube is
connected with the gas pipe under the floor, and to the end of the
tube is fitted a brass stop-cock which I hold in my hand. I open
the cock, and allow the jet of gas to flow toward the conductor of
Carré’s machine, while my assistant turns the handle; a spark passes,
and the gas is lit. Again, my assistant stands on this insulating
stool, placing his hand on the large conductor of the machine, while
I turn the handle. His body becomes electrified, and when he presents
his knuckle to this vessel of spirits of wine, which is electrically
connected with the earth, a spark leaps across, and the spirits of wine
are at once in a blaze. Once more; I tie a little gun-cotton around
one knob of the discharging rod, and then use it to discharge a small
Leyden jar; at the moment of the discharge the gun-cotton is set on
fire.

It would be easy to explode gunpowder with the electric spark, but the
smoke of the explosion would make the lecture-hall very unpleasant for
the remainder of the lecture. I propose, therefore, to substitute for
gunpowder an explosive mixture of oxygen and hydrogen, with which I
have filled this little metal flask, commonly known as Volta’s pistol.
By a very simple contrivance, the electric spark is discharged through
the mixture, when I hold the flask toward the conductor of the machine.
A cork is fitted tightly into the neck of the flask, and at the moment
the spark passes you hear a loud explosion, and you see the cork driven
violently up to the ceiling.

[Illustration: GUN-COTTON SET ON FIRE BY ELECTRIC SPARK.]


=Destruction of Life.=--The last effect of lightning to which I shall
refer, and which, perhaps, more than any other, strikes us with terror,
is the sudden and utter extinction of life, when the lightning flash
descends on man or on beast. So swift is this effect, in most cases,
that death is, in all probability, absolutely painless, and the victim
is dead before he can feel that he is struck. I cannot give you, with
any degree of exactness, the number of people killed every year by
lightning, because the record of such deaths has been hitherto very
imperfectly kept, in almost all countries, and is, beyond doubt, very
incomplete. But perhaps you will be surprised to learn that the number
of deaths by lightning actually recorded is, on an average, in England
about 22 every year, in France 80, in Prussia 110, in Austria 212, in
European Russia 440.[25]

So far as can be gathered from the existing sources of information,
it would seem that the number of persons killed by lightning is, on
the whole, about one in three of those who are struck. The rest are
sometimes only stunned, sometimes more or less burned, sometimes
made deaf for a time, sometimes partially paralyzed. On particular
occasions, however, especially when the lightning falls on a large
assembly of people, the number of persons struck down and slightly
injured, in proportion to the number killed, is very much increased.

An interesting case of this kind is reported by Mr. Tomlinson. “On
the twenty-ninth of August, 1847, at the parish church of Welton,
Lincolnshire, while the congregation were engaged in singing the hymn
before the sermon, and the Rev. Mr. Williamson had just ascended the
pulpit, the lightning was seen to enter the church from the belfry,
and instantly an explosion occurred in the centre of the edifice. All
that could move made for the door, and Mr. Williamson descended from
the pulpit, endeavoring to allay the fears of the people. But attention
was now called to the fact that several of the congregation were lying
in different parts of the church, apparently dead, some of whom had
their clothing on fire. Five women were found injured, and having their
faces blackened and burned, and a boy had his clothes almost entirely
consumed. A respected old parishioner, Mr. Brownlow, aged sixty-eight,
was discovered lying at the bottom of his pew, immediately beneath one
of the chandeliers, quite dead. There were no marks on the body, but
the buttons of his waistcoat were melted, the right leg of his trousers
torn down, and his coat literally burnt off. His wife in the same pew
received no injury.”[26]

[Illustration: VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK.]

Not less striking is the story told by Dr. Plummer, surgeon of the
Illinois Volunteers, in the _Medical and Surgical Reporter_ of June 19,
1865: “Our regiment was yesterday the scene of one of the most terrible
calamities which it has been my lot to witness. About two o’clock a
violent thunderstorm visited us. While the old guard was being turned
out to receive the new, a blinding flash of lightning was seen,
accompanied instantly by a terrific peal of thunder. The whole of the
old guard, together with part of the new, were thrown violently to the
earth. The shock was so severe and sudden that, in most cases, the rear
rank men were thrown across the front rank men. One man was instantly
killed, and thirty-two men were more or less severely burned by the
electric fluid. In some instances the men’s boots and shoes were rent
from their feet and torn to pieces, and, strange as it may appear, the
men were injured but little in the feet. In all cases the burns appear
as if they had been caused by scalding-hot water, in many instances the
skin being shriveled and torn off. The men all seem to be doing well,
and a part of them will be able to resume their duties in a few days.”


=The Return Shock.=--It sometimes happens that people are struck down
and even killed at the moment a discharge of lightning takes place
between a cloud and the earth, though they are very far from the
point where the flash is actually seen to pass; while others, who are
situated between them and the lightning, suffer very little, or perhaps
not at all. This curious phenomenon was first carefully investigated by
Lord Mahon in the year 1779, and was called by him the “return shock.”
His theory, which is now commonly accepted, may be easily understood
with the aid of the sketch before you.

[Illustration: THE RETURN SHOCK ILLUSTRATED.]

Let us suppose ABC to represent the outline of a thundercloud which
dips down toward the earth at A and at C. The electricity of the cloud
develops by inductive action a charge of the opposite kind in the earth
beneath it. But the inductive action is most powerful at E and F, where
the cloud comes nearest to the earth. Hence, bodies situated near these
points may be very highly electrified as compared with bodies at a
point between them, such as D. Now, when a flash of lightning passes at
E, the under part of the cloud is at once relieved of its electricity,
its inductive action ceases, and, therefore, a person situated at F
suddenly ceases to be electrified. This sudden change from a highly
electrified to a neutral state involves a shock to his system which
may be severe enough to stun or even to kill him. Meanwhile, people
at _D_, having been also electrified to some extent by the influence
of the thundercloud, must in like manner undergo a change in their
electrical condition when the flash of lightning passes, but this
change will be less violent because they were less highly electrified.

Many experiments have been devised to illustrate this theory of Lord
Mahon. But the best illustration I know is furnished by this electric
machine of Carré’s. If you stand near one end of the large conductor
when the machine is in action and sparks are taken from the other end,
you will feel a distinct electric shock every time a spark passes.
The large conductor here takes the place of the cloud, the spark that
passes at one end represents the flash of lightning, and the observer
at the other end gets the return shock, though he is at a considerable
distance from the point where the flash is seen.

An experiment of this kind, of course, cannot be made sensible to a
large audience like the present. But I can give you a good idea of the
effect by means of this tuft of colored papers. While the machine is in
action I hold the tuft of papers near that end of the conductor which
is farthest from the point where the discharge takes place. You see the
paper ribbons are electrified by induction, and, in virtue of mutual
repulsion, stand out from one another “like quills upon the fretful
porcupine.” But, when a spark passes, the inductive action ceases, the
paper ribbons cease to be electrified, and the whole tuft suddenly
collapses into its normal state.

While fully accepting Lord Mahon’s theory of the return shock as
perfectly good so far as it goes, I would venture to point out another
influence which must often contribute largely to produce the effect in
question, and which is not dependent on the form of the cloud. It may
easily happen, from the nature of the surface in the district affected
by a thundercloud, that the point of most intense electrification--say
E in the figure--is in good electrical communication with a distant
point, such as F, while it is very imperfectly connected with a much
nearer point, D. In such a case it is evident that bodies at F will
share largely in the highly-electrified condition of E, and also share
largely in the sudden change of that condition the moment the flash of
lightning passes; whereas bodies at D will be less highly electrified
before the discharge, and less violently disturbed when the discharge
takes place.

This principle may be illustrated by a very simple experiment. Here
is a brass chain about twenty feet long. One end of it I hand to any
one among the audience who will kindly take hold of it; the other end
I hold in my hand. I now stand near the conductor of the machine; and
will ask some one to stand about ten feet away from me, near the middle
of the chain, but without touching it. Now observe what happens when
the machine is worked and I take a spark from the conductor: My friend
at the far end of the chain, twenty feet away, gets a shock nearly
as severe as the one I get myself, because he is in good electrical
communication with the point where the discharge takes place. But
my more fortunate friend, who is ten feet nearer to the flash, is
hardly sensible of any effect, because he is connected with me only
through the floor of the hall, which is, comparatively speaking, a bad
conductor of electricity.


=Summary.=--Let me now briefly sum up the chief destructive effects
of lightning. First, with regard to good conductors: though it passes
harmlessly through them if they be large enough to afford it an easy
passage, it melts and converts them into vapor if they be of such
small dimensions as to offer considerable resistance. Secondly,
lightning acts with great mechanical force on bad conductors; it is
capable of tearing asunder large masses of masonry, and of projecting
the fragments to a considerable distance. Thirdly, it sets fire to
combustible materials. And lastly, it causes the instantaneous death of
men and animals.


=Franklin’s Lightning Rods.=--The object of lightning conductors is to
protect life and property from these destructive effects. Their use was
first suggested by Franklin, in 1749, even before his famous experiment
with the kite; and immediately after that experiment, in 1752, he set
up, on his own house, in Philadelphia, the first lightning conductor
ever made. He even devised an ingenious contrivance, by means of which
he received notice when a thundercloud was approaching. The contrivance
consisted of a peal of bells, which he hung on his lightning conductor,
and which were set ringing whenever the lightning conductor became
charged with electricity.

Franklin’s lightning rods were soon adopted in America; and he himself
contributed very much to their popularity by the simple and lucid
instructions he issued every year, for the benefit of his countrymen,
in the annual publication known as “Poor Richard’s Almanac.” It is
very interesting at this distance of time to read the homely practical
rules laid down by this great philosopher and statesman; and, though
some modifications have been suggested by the experience of a hundred
and thirty years, especially as regards the dimensions of the lightning
conductor, it is surprising to find how accurately the general
principles of its construction, and of its action, are here set forth.

“It has pleased God,” he says, “in His goodness to mankind, at length
to discover to them the means of securing their habitations and other
buildings from mischief by thunder and lightning. The method is this:
Provide a small iron rod, which may be made of the rod-iron used by
nailors, but of such a length that one end being three or four feet in
the moist ground, the other may be six or eight feet above the highest
part of the building. To the upper end of the rod fasten about a foot
of brass wire, the size of a common knitting needle, sharpened to a
fine point; the rod may be secured on the house by a few small staples.
If the house or barn be long, there may be a rod and point at each end,
and a middling wire along the ridge from one to the other. A house thus
furnished will not be damaged by lightning, it being attracted by the
points and passing through the metal into the ground, without hurting
anything. Vessels also having a sharp-pointed rod fixed on the top of
their masts, with a wire from the foot of the rod reaching down round
one of the shrouds to the water, will not be hurt by lightning.”


=Introduction of Lightning Rods into England.=--The progress of
lightning conductors was more slow in England and on the Continent of
Europe, owing to a fear, not unnatural, that they might, in some cases,
draw down the lightning where it would not otherwise have fallen.
People preferred to take their chance of escaping as they had escaped
before, rather than invite, as it were, the lightning to descend on
their houses, in the hope that an iron rod would convey it harmless
to the earth. But the immense amount of damage done every year by
lightning, soon led practical men to entertain a proposal which offered
complete immunity from all danger on such easy terms; and when it was
found that buildings protected by lightning conductors were, over and
over again, struck by lightning without suffering any harm, a general
conviction of their utility was gradually established in the public
mind.

The first public building protected by a lightning rod in England was
St. Paul’s Cathedral, in London. On the eighteenth of June, 1764, the
beautiful steeple of Saint Bride’s Church, in the city, was struck by
lightning and reduced to ruin. This incident awakened the attention of
the dean and chapter of St. Paul’s to the danger of a similar calamity,
which seemed, as it were, impending over their own church. After long
deliberation, they referred the matter to the Royal Society, asking for
advice and instruction. A committee of scientific men was appointed by
the Royal Society to consider the question. Benjamin Franklin himself,
who happened to be in London at the time, as the representative of the
American States in their dispute with England, was nominated a member
of the committee. And the result of its deliberation was that, in the
year 1769, a number of lightning conductors were erected on St. Paul’s
Cathedral.

It was on this occasion that arose the celebrated controversy about
the respective merits of points and balls. Franklin had recommended a
pointed conductor; but some members of the committee were of opinion
that the conductor should end in a ball and not in a point. The
decision of the committee was in favor of Franklin’s opinion, and
pointed conductors were accordingly adopted for St. Paul’s Cathedral.
But the controversy did not end here. The time was one of great
political excitement, and party spirit infused itself even into the
peaceful discussions of science. The weight of scientific opinion was
on the side of Franklin; but it was hinted, on the other side, that the
pointed conductors were tainted with republicanism, and pregnant with
danger to the empire. As a rule, the whigs were strongly in favor of
points; while the Tories were enthusiastic in their support of balls.

For a time the Tories seemed to prevail. The king was on their side.
Experiments on a grand scale were conducted in his presence, at the
Pantheon, a large building in Oxford street; he was assured that these
experiments proved the great superiority of balls over points; and to
give practical effect to his convictions, his majesty directed that a
large cannon ball should be fixed on the end of the lightning conductor
attached to the royal palace at Kew. But the committee of the Royal
Society remained unconvinced. In course of time the heat of party
spirit abated; experience as well as reason was found to be in favor of
Franklin’s views; and the battle of the balls and points has long since
passed into the domain of history.[27]


=Functions of a Lightning Conductor.=--A lightning conductor fulfills
two functions. First, it favors a silent and gradual discharge of
electricity between the cloud and the earth, and thus tends to prevent
that accumulation which must of necessity take place before a flash
of lightning will pass. Secondly, if a flash of lightning come, the
lightning conductor offers it a safe channel through which it may pass
harmless to the earth.

These two functions of a lightning conductor may be easily illustrated
by experiment. When our machine is in action, if I present my closed
hand to the large brass conductor, a spark passes between them, and I
feel, at the same moment, a slight electric shock. Here the conductor
of the machine, as usual, holds the place of the electrified cloud;
my closed hand represents, as it were, a lofty building that stands
out prominently on the surface of the earth; the spark is the flash of
lightning, and the electric shock just suggests the destructive power
of the sudden disruptive discharge.

Now let me protect this building by a lightning conductor. For this
purpose, I take in my hand a brass rod, which I connect with the
earth by a brass chain. In the first instance, I will have a metal
ball on the end of my lightning conductor. You see the effect; sparks
pass rapidly, but I feel no shock. I can increase the strength of
the discharge by hanging this condensing jar on the conductor of the
machine. Sparks pass now, much more brilliant and powerful than before,
but still I get no shock. It is evident, therefore, that my lightning
rod does not prevent the flash from passing, but it conveys it harmless
to the ground.

I next take a rod which is sharply pointed, and connecting it as before
with the earth by a brass chain, I present the sharp point to the
conductor of the machine. Observe how different is the result; there is
no disruptive discharge; no spark passes; no shock is felt. Electricity
still continues to be generated in the machine, and electricity is
generated, by induction, in the brass rod, and in my body. But these
two opposite electricities discharge themselves silently, by means of
this pointed rod, and no sensible effect of any kind is exhibited.

These experiments are very simple, but they really put before us, in
the clearest possible way, the whole theory of lightning conductors.
In particular, they give us ocular demonstration that an efficient
lightning rod not only makes the lightning harmless when it comes,
but tends very much to prevent its coming. A remarkable example, on a
large scale, of this important property, is furnished by the town of
Pietermaritzburg, the capital of the colony of Natal, in South Africa.
This town is subject to the frequent visitation of thunderstorms,
at certain seasons of the year, and much damage was formerly done
by lightning, but since the erection of lightning conductors on the
principal buildings, the lightning has never fallen within the town.
Thunderclouds come as before, but they pass silently over the city,
and only begin to emit their lightning flashes when they reach the
open country, and have passed beyond the range of the lightning
conductors.[28]

But it will often happen, even in the case of a pointed conductor,
that the accumulation of electricity goes on so fast that the silent
discharge is insufficient to keep it in check. A disruptive discharge
will then take place, from time to time, and a flash of lightning will
pass. Under these circumstances, the lightning conductor is called upon
to fulfill its second function, and to convey the lightning harmless to
the earth.


=Conditions of a Lightning Conductor.=--From the consideration of
the functions which it has to fulfill, we may now infer what are the
conditions necessary for an efficient lightning conductor. The first
condition is that the end of the conductor, projecting into the air,
should have, at least, one sharp point. Our experiments have shown us
that a pointed conductor tends, in a manner, to suppress the flash
of lightning altogether; whereas a blunt conductor, or one ending in
a ball, tends only to make it harmless when it comes. It is evident,
therefore, that the pointed conductor offers the greater security.

But a fine point is very liable to be melted when the lightning falls
upon it, and thus to be rendered less efficient for future service. To
meet this danger, it has recently been suggested, by the Lightning Rod
Conference, that the extreme end of the conductor should be a blunt
point, destined to receive the full force of the lightning flash, when
it comes; and that, a little lower down, a number of very fine points
should be provided, with a view to favor the silent discharge. This
suggestion, which appears admirably fitted to provide for the twofold
function of a lightning conductor, deserves to be recorded in the exact
terms of the official report.

“It seems best to separate the double functions of the point,
prolonging the upper terminal to the very summit, and merely beveling
it off, so that, if a disruptive discharge does take place, the full
conducting power of the rod may be ready to receive it. At the same
time, having regard to the importance of silent discharge from sharp
points, we suggest that, at one foot below the extreme top of the upper
terminal, there be firmly attached, by screws and solder, a copper
ring bearing three or four copper needles, each six inches long, and
tapering from a quarter of an inch diameter to as fine a point as can
be made; and with the object of rendering the sharpness as permanent
as possible, we advise that they be platinized, gilded, or nickel
plated.”[29]

The second condition of a lightning conductor is, that it should be
made of such material, and of such dimensions, as to offer an easy
passage to the greatest flash of lightning likely to fall on it;
otherwise it might be melted by the discharge, and the lightning,
seeking for itself another path, might force its way through bad
conductors, which it would partly rend asunder, and partly consume
by fire. Copper is now generally regarded as the best material for
lightning conductors, and it is almost universally employed in these
countries. If it is used in the form of a rope, it should not be less
than half an inch in diameter; if a band of copper is preferred--and it
is often found more convenient by builders--it should be about an inch
and a half broad and an eighth of an inch thick. In France it has been
hitherto more usual to employ iron rods for lightning conductors, but
since iron is much inferior to copper in its conducting power, the iron
rod must be of much larger dimensions; it should be at least one inch
in diameter.[30]

The third condition is that the lightning conductor should be
continuous throughout its whole length, and should be placed in good
electrical contact with the earth. This is a condition of the first
importance, and experience has shown that it is the one most likely
of all to be neglected. In a large town the best earth connection is
furnished by the system of water-mains and gas-mains, each of which
constitutes a great network of conductors everywhere in contact with
the earth. Two points, however, must be carefully attended to--first,
that the electrical contact between the lightning conductor and the
metal pipe should be absolutely perfect; and, secondly, that the pipe
selected should be of such large dimensions as to allow the lightning
an easy passage through it to the principal main.

If no such system of water-pipes or gas-pipes is at hand, then the
lightning rod should be connected with moist earth by means of a bed of
charcoal or a metal plate not less than three feet square. This metal
plate should be always of the same material as the conductor, otherwise
a galvanic action would be set up between the two metals, which in
course of time might seriously damage the contact. Dry earth, sand,
rock, and shingle are bad conductors; and, if such materials exist near
the surface of the earth, the lightning rod must pass through them and
be carried down until it reaches water or permanently damp earth.


=Mischief Done by Bad Conductors.=--If the earth contact is bad, a
lightning conductor does more harm than good. It invites the lightning
down upon the building without providing for it, at the same time, a
free passage to earth. The consequence is that the lightning forces
a way for itself, violently bursting asunder whatever opposes its
progress, and setting fire to whatever is combustible.

I will give you some recent and striking examples. In the month of May,
1879, the church of Laughton-en-le-Morthen, in England, though provided
with a conductor, was struck by lightning and sustained considerable
damage. On examination it was found that the lightning followed the
conductor down along the spire as far as the roof; then, changing its
course, it forced its way through a buttress of massive masonwork,
dislodging about two cartloads of stones, and leaped over to the leads
of the roof, about six feet distant. It now followed the leads until it
came to the cast-iron down-pipes intended to discharge the rain-water,
and through these it descended to the earth. When the earth contact
of the lightning conductor was examined, it was found exceedingly
deficient. The rod was simply bent underground, and buried in dry
loose rubbish at a depth not exceeding eighteen inches. This is a very
instructive example. The lightning had a choice of two paths--one by
the conductor prepared for it, the other by the leads of the roof
and the down-pipes--and, by a kind of instinct which, however we may
explain, we must always contemplate with wonder, it chose the path of
least resistance, though in doing so it had to burst its way at the
outset through a massive wall of solid masonry.[31]

On the 5th of June, in the same year, a flash of lightning struck the
house of Mr. Osbaldiston, near Sheffield, and, notwithstanding the
supposed protection of a lightning conductor, it did damage to the
amount of about five hundred pounds. The lightning here followed the
conductor to a point about nine feet from the ground, then passed
through a thick wall to a gas-pipe at the back of the drawing-room
mirror. It melted the gas-pipe, set fire to the gas, smashed the
mirror to atoms, broke the Sevres vases on the chimney-piece, and
dashed the furniture about. In this case, as in the former, it was
found that the earth contact was bad; and, in addition, the conductor
itself was of too small dimensions. Hence, the electric discharge
found an easier path to earth through the gas-pipes, though to reach
them it had to force for itself a passage through a resisting mass of
non-conductors.[32]

Again in the same year, on the 28th of May, the house of Mr. Tomes, of
Caterham, was struck by lightning, and some slight damage was done.
After a careful examination it was found that the greater part of
the discharge left the lightning conductor with which the house was
provided, and passed over the slope of the roof to an attic room, into
which it forced its way through a brick wall, and reached a small iron
cistern. This cistern was connected by an iron pipe of considerable
dimensions with two pumps in the basement story; and through them the
lightning found an easy passage to the earth, and did but little harm
on its way. When the earth contact of the lightning conductor was
examined, it was discovered that the end of the rod was simply stuck
into a dry chalky soil to a depth of about twelve inches. Thus in this
case, as in the two former, it was made quite clear that the lightning
conductor failed to fulfill its functions because the earth contact was
bad.[33]

Cases are not uncommon in which builders provide underground a
carefully constructed reservoir of water, into which the lower end
of the lightning rod is introduced. The idea seems to prevail that a
reservoir of water constitutes a good earth contact; and this is quite
true of a natural reservoir, such as a lake, where the water is in
contact with moist earth over a considerable area. But an artificial
reservoir may have quite an opposite character, and practically
insulate the lightning conductor from the earth. One which came under
my notice lately, in the neighborhood of this city, consists of a large
earthenware pipe set on end in a bed of cement, and kept half full of
water. Now, the earthenware pipe is a good insulator, and so is the bed
of cement in which it rests; and the whole arrangement is identical,
in all essential features, with the apparatus of Professor Richman, in
which he introduced his lightning rod into a glass bottle, and by which
he lost his life a hundred and thirty years ago.

A conductor mounted in this manner will, probably enough, draw down
lightning from the clouds; but it is more likely to discharge it, with
destructive effect, into the building it is intended to guard, than
to transmit it harmlessly to the earth. An example is at hand in the
case of Christ Church, in the town of Clevedon, in Somersetshire.
This church was provided with a very efficient system of lightning
conductors, five in number, corresponding to the four pinnacles and the
flagstaff, on the summit of the principal tower. The five conductors
consisted of good copper-wire rope; all were united together inside the
tower, through which they were carried down to earth, and there ended
in an earthenware drain. This kind of earth contact might be pretty
good as long as water was flowing in the drain; but whenever the drain
was dry the conductor was practically insulated from the earth. On the
fifteenth of March, 1876, the church was struck by lightning, which
for some distance followed the line of the conductor; then finding its
passage barred by the earthenware drain, which was dry at the time, it
burst through the walls of the church, displacing several hundredweight
of stone, and making its way to earth through the gas-pipe.[34]

Another very instructive example is furnished by the lightning
conductor attached to the lighthouse of Berehaven, on the south-west
coast of Ireland. It consists of a half-inch copper-wire rope, which
is carried down the face of the tower “until it reaches the rock
at its base, where it terminates in _a small hole, three inches by
three inches, jumped out of the rock, about six inches under the
surface_.” Here, again, we have a good imitation of Professor Richman’s
experiment, with only this difference, that a small hole in the rock
is substituted for a glass bottle. A lightning conductor of this kind
fulfills two functions: it increases the chance of the lightning coming
down on the building, and it makes it positively certain that, having
come, it cannot get to earth without doing mischief.

The lightning did come down on the Berehaven Lighthouse, about five
years ago. As might have been expected, it made no use of the lightning
conductor in finding a path to earth, but forced its way through the
building, dealing destruction around as it descended from stage to
stage. The Board of Irish Lights furnished a detailed report of this
accident to the Lightning Rod Conference, in March, 1880, from which
the above particulars have been derived.[35]


=Precaution Against Rival Conductors.=--But it is not enough to
provide a good lightning conductor, which is itself able to convey
the electric discharge harmless to the earth; we must take care that
there are no rival conductors near at hand in the building, to draw
off the lightning from the path prepared for it, and conduct it by
another route in which its course might be marked with destruction.
This precaution is of especial importance at the present day, owing to
the great extent to which metal, of various kinds, is employed in the
construction and fittings of modern buildings. I will take a typical
case which will bring home this point clearly to your minds.

A great part of the roof of many large buildings is covered with lead.
The lead, at one or more points may come near the gutters intended
to collect the rain water; the gutters are in connection with the
cast-iron down-pipes into which the water flows, and these down-pipes
often pass into the earth, which, under the circumstances, is generally
moist, and, therefore, in good electrical contact with the metal pipes.
Here, then, is an irregular line of conductors, which, though it has
gaps here and there, may, under certain conditions, offer to the
lightning discharge a path not less free than the lightning conductor
itself. What is the consequence? The flash of lightning, or a part of
it, will quit the lightning rod, and make its way to earth through the
broken series of conductors, doing, perhaps, serious mischief, as it
leaps across, or bursts asunder, the non-conducting links in the chain.

Another illustration may be taken from the gas and water-pipes, with
which almost all buildings in great cities are now provided, and which
constitute a network of conductors, spreading out over the walls and
ceilings, and stretching down into the earth, with which they have
the best possible electrical contact. Now, it often happens that a
lightning conductor, at some point in its course, comes within a short
distance of this network of pipes. In such a case, a portion of the
electrical discharge is apt to leave the lightning conductor, force
its way destructively through masses of masonry, enter the network of
pipes, melt the leaden gas-pipe, ignite the gas, and set the building
on fire.

These are not merely the speculations of philosophers. All the various
incidents I have just described have occurred, over and over again,
during the last few years. You will remember, in some of the examples
I have already set before you, when the electric discharge failed to
find a sufficient path to earth through the lightning rod, it followed
some such broken series of chance conductors as we are now considering.
But this broken series of conductors seems to bring with it a special
danger of its own, even when the lightning conductor is otherwise in
efficient working order. I will give you just one case in point.

On the fifth of June, 1879, the Church of Saint Marie, Rugby, was
struck by lightning and set on fire, and narrowly escaped being
burned to the ground. A number of workmen were engaged on that day in
repairing the spire of the church. About three o’clock they saw a dense
black cloud approaching, and they came down to take shelter within the
building. In a few minutes they heard a terrific crash just overhead;
at the same moment the gas was lighted under the organ loft and the
woodwork was set in a blaze. The men soon succeeded in putting out the
fire, and the church escaped with very little damage.

Now, in this case there was no reason to suppose that the lightning
conductor was in any way defective. But about half-way up the spire
there was a peal of eight bells. Attached to these bells were iron
wires, about the eighth of an inch in diameter, leading from the
clappers down to the organ-loft, where they came within a short
distance of a gas-pipe fixed in the wall. It would seem that a great
part of the discharge was carried safely to earth by the lightning
conductor. But a part branched off at the bells in the spire, descended
by the iron wires, and forced its way into the organ loft, to reach
the network of gas-pipes, through which it passed down to the earth,
melting the soft leaden gas-pipe in its course and lighting the gas.

The remedy for this danger is obvious. All large masses of metal used
in the structure of a building--the leads and gutters of the roof,
the cast-iron down-pipes, the iron gas and water mains--should be put
in good metallic connection with the lightning conductor, and, as
far as may be, with one another. Connected in this way they furnish
a continuous and effective line of conductors leading safely down to
earth; and, instead of being a dangerous rival, they become a useful
auxiliary to the lightning rod.

I would observe, however, that the lightning conductor ought not to
be connected directly with the soft leaden pipes which are commonly
employed to convey gas and water to the several parts of a building.
Such pipes, as we have seen, are liable to be melted when any
considerable part of the lightning discharge passes through them; and
thus much harm might be done, and the building might even be set on
fire by the lighting of the gas. Every good end will be attained if the
conductor is put in metallic connection with the iron gas and water
_mains_ either inside or outside the building.


=Insulation of Lightning Conductors.=--It is a question often asked
whether a lightning rod should be insulated from the building it
is intended to protect. I believe that this practice was formerly
recommended by some writers, and I have observed that glass insulators
are still employed not infrequently by builders in the erection of
lightning conductors; but, from the principles I have set before
you to-day, it seems clear that any insulation of this kind is, to
say the least, altogether useless. The building to be protected is
itself in electrical communication with the earth, and the lightning
conductor, if efficient, is also in electrical communication with the
earth--therefore, the lightning conductor and the building are in
electrical communication with each other through the earth, and any
attempt at insulating them from one another above the earth is only
labor thrown away.

Further, I have just shown you that the masses of metal employed in
the structure or decoration of a building ought to be electrically
connected with each other and with the lightning conductor. Now, if
this be done, the lightning conductor is, by the fact, in direct
communication with the building, and the glass insulators are utterly
futile. Again, the building itself, during a thunderstorm, becomes
highly electrified by the inductive action of the cloud, and needs to
be discharged through the conductor just as the surrounding earth needs
to be discharged; therefore, the more thoroughly it is connected with
the conductor, the more effectively will the conductor fulfill its
functions.


=Personal Safety in a Thunderstorm.=--I suppose there is hardly any one
to whom the question has not occurred, at some time or another, what
he had best do to secure his personal safety during a thunderstorm.
This question is of so much practical interest that I think I shall
be excused if I say a few words about it, though perhaps, strictly
speaking, it is somewhat beside the subject of lightning conductors.

At the outset, perhaps, I shall surprise you when I say that you would
enjoy the most perfect security if you were in a chamber entirely
composed of metal plates, or in a cage constructed of metal bars, or
if you were incased, like the knights of old, in a complete suit of
metal armor. This kind of defense is looked upon as so perfect, among
scientific men, that Professor Tait does not hesitate to recommend
his adventurous young friends devoted to the cause of science to
provide themselves with a light suit of copper, and, thus protected,
take the first opportunity of plunging into a thundercloud, there
to investigate, at its source, the process by which lightning is
manufactured.[36]

The reason why a metal covering affords complete protection is that,
when a conductor is electrified, the whole charge of electricity
exists on the outside surface of the conductor; and therefore, when a
discharge takes place, it is only the outside surface that is affected.
Thus, if you were completely incased in a metal covering, and then
charged with electricity by the inductive action of a thundercloud, it
is only the metal covering that would undergo any change of electrical
condition; and when the lightning flash would pass, it is only the
metal covering that would be discharged.

Let me show you a very pretty and interesting experiment to illustrate
this principle: Here is a hollow brass cylinder, open at the ends,
mounted on an insulating stand. On the outside is erected a light brass
rod with two pith balls suspended from it by linen threads. Two pith
balls are also suspended by linen threads from the inner surface of
the cylinder. You know that these pith balls will indicate to us the
electrical condition of the surfaces to which they are attached. If the
surface be electrified, the pith balls attached to it will share in
its electrical condition, and will repel each other; if the surface be
neutral, the pith balls attached to it will be neutral, and will remain
at rest.

I now put this apparatus under the influence of our thundercloud, that
is, the large brass conductor of our machine. The moment my assistant
turns the handle, the electricity begins to be developed on the
conductor, and you see, at once, the effect on the brass cylinder. The
pith balls attached to the outer surface fly asunder; those attached
to the inner surface remain at rest. And now a spark passes; our
thundercloud is discharged; the inductive action ceases; the pith balls
on the outside suddenly collapse, while those on the inside are in no
way affected.

[Illustration: PROTECTION FROM LIGHTNING FURNISHED BY A CLOSED
CONDUCTOR.]

It is not necessary that the brass cylinder should be insulated. To
vary the experiment, I will now connect it with the earth by a chain;
you will observe that the effect is precisely the same as before.
Flash after flash passes while the machine continues in action; the
outside pith balls fly about violently, being charged and discharged
alternately; the inside pith balls remain all the time at rest. Thus
you see clearly that, if you were sitting inside such a metal chamber
as this, or covered with a complete suit of metal armor, you would
be perfectly secure during a thunderstorm, whether the chamber were
electrically connected with the earth or insulated from it.


=Practical Rules.=--But it rarely happens, when a thunderstorm comes,
that an iron hut or a complete suit of armor is at hand, and you will
naturally ask me what you ought to do under ordinary circumstances.
First, let me tell you what you ought not to do. You ought not to take
shelter under a tree, or under a haystack, or under the lee of a house;
you ought not to stand on the bank of a river, or close to a large
sheet of water. If indoors, you ought not to stay near the fireplace,
or near any of the flues or chimneys; you ought not to stand under a
gasalier hanging from the ceiling; you ought not to remain close to the
gas pipes or water-pipes, or any large masses of metal, whether used in
the construction of the building, or lying loosely about.

The necessity for these precautions is sufficiently evident from the
principles I have already put before you. You want to prevent your body
from becoming a link in that broken chain of conductors which, as we
have seen, the electric discharge between earth and cloud is likely to
follow. Now a tree is a better conductor than the air; and your body is
a better conductor than a tree. Hence, the lightning, in choosing the
path of least resistance, would leave the air to pass through the tree,
and would leave the tree to pass through you. A like danger would await
you if you stood under the lee of a haystack or of a house.

The number of people who lose their lives by taking refuge under trees
in thunderstorms is very remarkable. As one instance out of many, I
may cite the following case which was reported in the _Times_, July
14, 1887: “Yesterday the funeral of a negress was being conducted in a
graveyard at Mount Pleasant, sixty miles north of Nashville, Tennessee,
when a storm came on, and the crowd ran for shelter under the trees.
Nine persons stood under a large oak, which the lightning struck,
killing everyone, including three clergymen, and the mother and two
sisters of the girl who had been buried.”

Again, every large sheet of water constitutes practically a great
conductor, which offers a very perfect medium of discharge between the
earth round about and the cloud. Therefore, when a thundercloud is
overhead, the sheet of water is likely to become one end of the line of
the lightning discharge; and if you be standing near it, the line of
discharge may pass through your body.

When lightning strikes a building, it is very apt to use the stack
of chimneys in making its way to earth, partly because the stack of
chimneys is generally the most prominent part of the building, and
partly because, on account of the heated air and the soot within the
chimney, it is usually a moderately good conductor. Therefore, if
you be indoors, you must keep well away from the chimneys; and for a
similar reason, you must keep as far as you can from large masses of
metal of every kind.

Having pointed out the sources of danger which you must try to avoid
in a thunderstorm, I have nearly exhausted all the practical advice
that I have at my command. But there are some occasions on which it may
be possible, not only to avoid evident sources of danger, but to make
special provision for your own security. Thus, for example, in the open
country, if you stand a short distance from a wood, you may consider
yourself as practically protected by a lightning conductor. For a
wood, by its numerous branches and leaves, favors very much a quiet
discharge of electricity, thus tending to suppress altogether the flash
of lightning; and if the flash of lightning does come, it is much more
likely to strike the wood than to strike you, because the wood is a far
more prominent body, and offers, on the whole, an easier path to earth.
In like manner, if you place yourself near a tall solitary tree, some
twenty or thirty yards outside its longest branches, you will be in a
position of comparative safety. If the storm overtake you in the open
plain, far away from trees and buildings, you will be safer lying flat
on the ground than standing erect.

In an ordinary dwelling house, the best situation is probably the
middle story, and the best position in the room is in the middle of
the floor; provided, of course, that there is no gasalier hanging from
the ceiling above or below you. Strictly speaking, the _middle of the
room_ would be a still safer position than the middle of the floor;
and nothing could be more perfect than the plan suggested by Franklin,
to get into “a hammock, or swinging bed, suspended by silk cords, and
equally distant from the walls on every side, as well as from the
ceiling and floor, above and below.” An interesting case has been
recently recorded, by a resident of Venezuela, which illustrates in a
remarkable way the excellence of this advice. “The lightning,” he says,
“struck a _rancho_--a small country house, built of wood and mud, and
thatched with straw or large leaves--where one man slept in a hammock,
another lay under the hammock on the ground, and three women were busy
about the floor; there were also several hens and a pig. The man in
the hammock did not receive any injury whatever, while the other four
persons and the animals were killed.”[37]

But, as I can hardly hope that many of you when the thunderstorm
actually comes will find yourselves provided with a hammock, I would
recommend, as more generally useful, another plan of Franklin’s, which
is simply to sit on one chair in the middle of the floor and put your
feet up on another. This arrangement will approach very nearly to
absolute security if you take the further precaution, also mentioned by
Franklin, of putting a feather bed or a couple of hair mattresses under
the chairs.[38]


=Security Afforded by Lightning Rods.=--You might, perhaps, be inclined
to infer hastily, from the examples I have set before you, in the
course of this lecture, of buildings which were struck and severely
injured by lightning though provided with lightning conductors, that a
lightning rod affords a very imperfect protection to life and property.
But such an idea would be entirely at variance with the evidence at
hand on the subject. In all the cases to which I have referred, and in
many others which might easily have been cited, the damage was done
simply because the lightning rods were deficient in one or more of the
conditions on which I have so much insisted. Where these conditions are
fulfilled, the lightning flash will either not come down at all upon
the building, or, if it do come, it will be carried harmless to the
earth.

Perhaps there is no one fact that so forcibly brings home to the
mind the complete protection afforded by lightning conductors as the
change which followed their introduction into the Royal Navy. I have
already told you that in former times the damage done by lightning to
ships of the Royal Navy was a regular source of expenditure, amounting
every year to several thousand pounds sterling. But, after the general
adoption of lightning conductors about forty years ago, through the
indefatigable exertions of Sir William Snow Harris, this source of
expenditure absolutely disappeared, and injury to life and property has
long been practically unknown in Her Majesty’s Fleet.

I should say, however, that the trial of lightning conductors in the
Navy, though it lasted long enough to prove their perfect efficiency,
has almost come to an end in our own days. The great iron monsters
which in recent times have taken the place of the wooden ships of
Old England are quite independent of lightning rods in the common
sense of the word. Their ponderous masts are virtually lightning rods
of colossal dimensions, and their unsightly hulls are, so to speak,
earth-plates of enormous size in perfect electrical contact with the
ocean. To add to such structures lightning conductors of the common
kind would be nothing better than “wasteful and ridiculous excess.”

As regards buildings on land, I may refer to the little province
of Schleswig-Holstein, of which I have already spoken to you. From
some cause or other this small peninsula is singularly exposed to
thunderstorms, and of late years it has been more abundantly provided
with lightning conductors than, perhaps, any other district of equal
extent in Europe. Now, as a simple illustration of the protection
afforded by these lightning conductors, I may mention that, on the
26th of May, 1878, a violent thunderstorm burst over the little town
of Utersen. Five several flashes of lightning fell in different parts
of the town, but not the slightest harm was done, each flash being
safely carried to earth by a lightning conductor. Further, it appears
from the records of the fire insurance company that, out of 552
buildings injured by lightning during a period of eight years--from
1870 to 1878--only four had lightning conductors; and in these four
cases it was found, on examination, that the lightning conductors were
defective.[39]

It would be easy to multiply evidence on this subject. But as I have
already trespassed, I fear, too far on your patience, I will content
myself with saying, in conclusion, that according to all the highest
authorities, both practical and theoretical, any structure provided
with a lightning conductor properly fitted up in conformity with the
principles I have set before you to-day is perfectly secure against
lightning. The lightning, indeed, may fall upon it, but it will pass
harmless to the earth; and the experience of more than a hundred years
has fully justified the simple and modest words of the great inventor
of lightning conductors: “It has pleased God, in His goodness to
mankind, at length to discover to them the means of securing their
habitations and other buildings from mischief by thunder and lightning.”


NOTE I.

ON THE LIGHTNING CONDUCTOR AT BEREHAVEN.[40]

 It is satisfactory to know that the lightning conductor referred to
 in my lecture as attached to the lighthouse at Berehaven has been
 put in good order under the best scientific guidance. The following
 interesting letter from Professor Tyndall, which appeared in the
 _Times_, August 31, 1887, gives the history of the matter very
 clearly, and fully bears out the views put forward in my lecture:

 “Your recent remarks on thunderstorms and their effects induce me to
 submit to you the following facts and considerations. Some years ago
 a rock lighthouse on the coast of Ireland was struck and damaged by
 lightning. An engineer was sent down to report on the occurrence;
 and, as I then held the honorable and responsible post of scientific
 adviser to the Trinity House and Board of Trade, the report was
 submitted to me. The lightning conductor had been carried down the
 lighthouse tower, its lower extremity being carefully embedded in a
 stone perforated to receive it. If the object had been to invite the
 lightning to strike the tower, a better arrangement could hardly have
 been adopted.

 “I gave directions to have the conductor immediately prolonged, and
 to have added to it a large terminal plate of copper, which was to be
 completely submerged in the sea. The obvious convenience of a chain as
 a prolongation of the conductor caused the authorities in Ireland to
 propose it; but I was obliged to veto the adoption of the chain. The
 contact of link with link is never perfect. I had, moreover, beside
 me a portion of a chain cable through which a lightning discharge had
 passed, the electricity in passing from link to link encountering
 a resistance sufficient to enable it to partially fuse the chain.
 The abolition of resistance is absolutely necessary in connecting
 a lightning conductor with the earth, and this is done by closely
 embedding in the earth a plate of good conducting material and of
 large area. The largeness of area makes atonement for the imperfect
 conductivity of earth. The plate, in fact, constitutes a wide door
 through which the electricity passes freely into the earth, its
 disruptive and damaging effects being thereby avoided.

 “These truths are elementary, but they are often neglected. I watched
 with interest some time ago the operation of setting up a lightning
 conductor on the house of a neighbor of mine in the country. The
 wire rope which formed part of the conductor was carried down the
 wall and comfortably laid in the earth below without any terminal
 plate whatever. I expostulated with the man who did the work, but
 he obviously thought he knew more about the matter than I did. I am
 credibly informed that this is a common way of dealing with lightning
 conductors by ignorant practitioners, and the Bishop of Winchester’s
 palace at Farnham has been mentioned to me as an edifice ‘protected’
 in this fashion. If my informant be correct, the ‘protection’ is a
 mockery, a delusion, and a snare.”


NOTE II.

BOOKS OF REFERENCE.

 As some of my readers may wish to pursue the study of lightning and
 lightning conductors beyond the limits to which a popular lecture
 must, of necessity, be confined, I subjoin a list of the books which
 I think they would be likely to find most useful for the purpose.
 Among ordinary text-books on physics--Jamin, Cours de Physique,
 vol. i., pp. 470-494; Mascart, Traité d’Electricité Statique, vol.
 ii., pp. 555-579; De Larive, A Treatise on Electricity, in three
 volumes, London, 1853-8, vol. iii., pp. 90-201; Daguin, Traité
 de Physique, vol. iii., pp. 209-280; Riess, Die Lehre von der
 Reibungs-Elektricität, vol. ii., pp. 494-564; Müller-Pouillet,
 Lehrbuch der Physik, Braunschweig, 1881, vol. iii., pp. 210-225;
 Scott, Elementary Meteorology, chap. x. Of the numerous special
 treatises and detached papers on the subject, I would recommend
 Instruction sur les Paratonnerres adopté par l’Académie des Sciences,
 Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago,
 Sur le Tonnerre, Paris, 1837; also his Meteorological Essays,
 translated by Sabine, London, 1855; Sir William Snow Harris, On the
 Nature of Thunderstorms, London, 1843; also by the same writer,
 A Treatise on Frictional Electricity, London, 1867; and various
 papers on lightning conductors, from 1822 to 1859; Tomlinson, The
 Thunderstorm, London, 1877; Anderson, Lightning Conductors, London,
 1880; Holtz, Ueber die Theorie, die Anlage, und die Prüfung der
 Blitzableiter, Greifswald, 1878; Weber, Berichte über Blitzschläge
 in der Provinz Schleswig-Holstein, Kiel, 1880-1; Tait, A Lecture
 on Thunderstorms, delivered in the City Hall, Glasgow, in 1880,
 Nature, vol. xxii.; Report of the Lightning Rod Conference, London,
 1882. This last-mentioned volume comes to us with very high
 authority, representing, as it does, the joint labors of several
 eminent scientific men selected from the following societies: The
 Meteorological Society, the Royal Institute of British Architects, the
 Society of Telegraph Engineers and Electricians, the Physical Society.

 Since the above was in print, two lectures given before the Society
 of Arts by Professor Oliver Lodge, F. R. S., have appeared in the
 _Electrician_, June and July, 1888, in which some new views are put
 forward respecting lightning conductors, that seem deserving of
 careful consideration.


FOOTNOTES:

[17] The Thunderstorm, by Charles Tomlinson, F. R. S., Third Edition,
pp. 153-4.

[18] Two Lectures on Atmospheric Electricity and Protection from
Lightning, published at the end of his Treatise on Frictional
Electricity, p. 273.

[19] See Report of Lightning Rod Conference, p. 119.

[20] _Loco citato._

[21] Sir William Snow Harris, _loco citato_, p. 274.

[22] _Id._, p. 275.

[23] The Thunderstorm, by Charles Tomlinson, F.R.S., Third Edition, p.
172.

[24] See for these facts, Anderson, Lightning Conductors, p. 197;
Tomlinson, The Thunderstorm, pp. 167-9; Harris, _loco citato_, pp.
273-4.

[25] See Anderson, Lightning Conductors, pp. 170-5.

[26] The Thunderstorm, pp. 158-9. See also an account of four persons
who were struck on the Matterhorn, in July, 1869, all of whom were
hurt, and none killed: Whymper’s Scrambles Among the Alps, pp. 414, 415.

[27] See Philosophical Transactions of the Royal Society, 1773, p. 42,
and 1778, part i., p. 232; Anderson’s Lightning Conductors, pp. 40-2;
Lighting Rod Conference, pp. 76-9.

[28] See A Lecture on Thunderstorms, by Professor Tait of Edinburgh,
published in Nature, vol. xxii., p. 365.

[29] Report of the Lightning Rod Conference, p. 4.

[30] The dimensions here set forth are greater in some respects than
those “recommended as a minimum” in the report of the Lightning Rod
Conference, page 6. But it will be observed by those who consult the
report that the minimum recommended is just the size which, in the
preceding paragraph of the report, is said to have been actually melted
by a flash of lightning; and, therefore, it seems not to be a very
safe minimum. It will be also seen that there is some confusion in the
figures given, and that they contradict one another. For the dimensions
of iron rods, see the instructions adopted by the Academy of Science,
Paris, May 20, 1875; Lightning Rod Conference, pp. 67-8.

[31] See letter of Mr. R. S. Newall, F. R. S., in the _Times_, May 30,
1879.

[32] See Nature, June 12, 1879, vol. xx., p. 146.

[33] See letter of Mr. Tomes in Nature, vol. xx., p. 145; also
Lightning Rod Conference, pp. 210-15.

[34] See Anderson, Lightning Conductors, pp. 208-10.

[35] See Lightning Rod Conference, pp. 208-10; see also the note at the
end of this Lecture, p. 52.

[36] Lecture on Thunderstorms, Nature, vol. xxii., pp. 365, 437. See,
also, a very interesting paper by the late Professor J. Clerk Maxwell,
read before the British Association at Glasgow in 1876, and reprinted
in the report of the Lightning Rod Conference, pp. 109, 110.

[37] Nature, vol. xxxi., p. 459.

[38] See further information on this interesting subject in the Report
of the Lightning Rod Conference, pp. 233-5.

[39] See “Die Theorie, die Anlage, und die Prüfung der Blitzableiter,”
von Doctor W. Holtz, Griefswald, 1878.

[40] See page 44.




APPENDIX.

RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.


The lecture on lightning conductors contained in this volume fairly
represents, I think, the theory hitherto received on the subject. It
is, moreover, entirely in accord with the report of the Lightning Rod
Conference, brought out in 1883, by a committee of most eminent men,
representing several branches of science, who were specially chosen to
consider this question some ten years ago.


=Lectures of Professor Lodge.=--But, in the month of March, 1888,
two lectures were given before the Society of Arts, in London, by
Professor Oliver Lodge, in which this theory was directly challenged,
and attacked with cogent arguments, supported by striking and original
experiments. These lectures gave rise to an animated controversy, which
culminated in a formal discussion at the recent meeting of the British
Association in Bath. The discussion was carried on with great spirit,
and most of the leading representatives of physical and mechanical
science took an active part in it. The greater portion of this volume
was printed off before the meeting of the British Association took
place. But the discussion on the theory of lightning conductors seemed
to me so interesting and important that I thought it right, in the form
of an Appendix, to give some account of the questions at issue, and of
the opinions expressed upon them.

Professor Lodge maintains[41] that the received theory of lightning
rods is open to two objections. First, it takes account only of the
conducting power of the lightning rod, and takes no account of the
phenomenon known as self-induction, or electrical inertia. Secondly,
it assumes that the whole substance of a lightning rod acts as a
conductor, in all cases of lightning discharge; whereas there is reason
to believe that, in many cases, it is only a thin outer shell that
really comes into action. I will deal with these two points separately.


=The Effect of Self-Induction.=--When an electric discharge begins
to pass through a conductor, a momentary back electro-motive force
is developed in the conductor, which obstructs its passage. This
phenomenon is called by some self-induction, by others electrical
inertia; but its existence is admitted by all. Now, when a flash
of lightning, so to say, falls on a lightning rod, the back
electro-motive force developed is very considerable; and it may
offer so great an obstruction that the discharge will find an easier
passage by some other route, such as the stone walls and woodwork, and
furniture of the building.

According to this view, the obstruction which a flash of lightning
encounters in a conductor consists partly of the resistance of the
conductor, in the ordinary sense of the word resistance, and partly of
the back electro-motive force due to self-induction. The sum of these
two Professor Lodge calls the _impedance_ of the lightning rod; and he
considers that the impedance may be enormously great, even when the
resistance, in the ordinary sense, is comparatively small.

In support of this view he has devised the following extremely
ingenious and remarkable experiment. A large Leyden jar, L, was
arranged in such a manner that, while it received a steady charge from
an electrical machine, it discharged itself, at intervals, across the
air space at A, between two brass balls. The discharge had then two
alternative paths before it; one through a conducting wire, C, the
other across a second air space, between two brass balls at B. During
the experiment, the two balls at A were kept at a fixed distance of one
inch apart; but the distance between the two balls at B was varied.
The conductor, C, used in the first instance, was a stout copper wire,
about forty feet long, and having a resistance of only one-fortieth of
an ohm.

[Illustration: INDUCTION EFFECT OF LEYDEN JAR DISCHARGE.

  M Electrical Machine.
  L Leyden Jar.
  A B Air Spaces between Brass Knobs.
  C Conducting Wire.]

It was found that, so long as the distance between the B knobs was less
than 1.43 inches, all the discharges passed across between the knobs,
in the form of a spark. When the distance exceeded 1.43 inches, all
the discharges passed through the conductor, C, and no spark appeared
between the balls at B. And when the distance was exactly 1.43 inches,
the discharge sometimes took place between the knobs, and sometimes
followed the conductor, C. The interpretation given to these facts
is that the obstruction offered by the conductor C was about equal
to the resistance of 1.43 inches of air; and it is proposed to call
this distance, under the conditions of the experiment, the _critical
distance_.

Coming now to the application of these results, Professor Lodge argues
that the conductor C, in his experiment, represents a lightning rod
of unimpeachable excellence; and yet, in certain cases, the discharge
refuses to follow the conductor, and prefers to leap across a
considerable space of air, notwithstanding the enormous resistance it
there encounters. In like manner, he says, a flash of lightning may,
in certain cases, leave a lightning rod fitted up in the most orthodox
manner, and force its way to earth through resisting masses of mason
work and such chance conductors as may come across its path.

This conclusion, he admits, is altogether at variance with the received
views on the subject; but he contends that it is perfectly in accord
with the scientific theory of an electrical discharge. The moment
the discharge begins to pass in the conductor, it encounters the
obstruction due to self-induction; and this obstruction is so great
that the bad conductors offer, on the whole, an easier path to earth.


=Variation of the Experiment.=--When the experiment was varied by
substituting a thin iron wire for the stout copper wire at first
employed, a very curious result was obtained. The wire chosen was of
the same length as the copper, but had a resistance about 1,300 times
as great; its resistance being, in fact, 33.3 ohms. Nevertheless, in
this experiment, when the B knobs were at a distance of 1.43 inches,
no spark passed, which showed that the discharge always followed the
line of the conductor, and therefore that the conductor offered less
obstruction than 1.43 inches of air. The knobs were then brought
gradually nearer and nearer; and it was not until the distance was
considerably reduced that the sparks began to pass between them. When
the distance was exactly 1.03 inches, the discharge sometimes passed
between the knobs, and sometimes through the conductor; this was,
therefore, the _critical distance_, in the case of the iron wire. Thus
it appeared that the obstruction offered to the discharge by the iron
wire was much less than that offered by the copper, the one being equal
to a resistance of only 1.03 inches of air, the other to a resistance
of 1.43 inches.

It does not appear that Professor Lodge undertakes to offer any
satisfactory explanation of this result. He has come to the conclusion,
from his various experiments, that, in the case of a sudden discharge,
difference of conducting power between fairly good conductors is a
matter of practically no account; and that difference of sectional area
is a matter of only trifling account. But he does not see why a thin
iron wire should have a _smaller_ impedance than a much thicker wire of
copper. He proposes to repeat the experiments so as to confirm or to
modify the result, which for the present seems to him anomalous.[42]


=The Outer Shell only of a Lightning Rod Acts as a Conductor.=--As a
consequence of self-induction or electrical inertia, Professor Lodge
contends that a lightning discharge in a conductor consists of a
series of oscillations. These oscillations follow one another with
extraordinary rapidity--there may be a hundred thousand in a second,
there may be a million. Now it has been shown that, when a current
starts in a conductor, it does not start at once all through its
section; it begins on the outside, and then gradually, but rapidly,
penetrates to the interior. From this he infers that the extremely
rapid oscillations of a lightning discharge have not time to penetrate
to the interior of a conductor. The electricity keeps surging to
and fro in the superficial layer or outer shell, while the interior
substance of the rod remains inert and takes no part in the action. A
conductor, therefore, will be most efficient for carrying off a flash
of lightning if it present the greatest possible amount of surface; a
thin, flat tape will be more efficient than a rod of the same mass; and
a number of detached wires more efficient than a solid cylinder. As for
existing lightning conductors, the greater part of their mass would,
in many cases, have no efficacy whatever in carrying off a flash of
lightning.


=The Discussion.=--The discussion at the meeting of the British
Association was opened by Mr. William H. Preece, F.R.S., Electrician
to the Post Office, who claimed to have 500,000 lightning conductors
under his control. He expressed his conviction that a lightning rod,
properly erected and duly maintained, was a perfect protection against
injury from lightning; and in support of this conviction he urged
very strongly the report of the Lightning Rod Conference. This report
represented the mature judgment of the most eminent scientific men,
who had devoted years to the study of the question; and he wished
particularly to bring before the meeting their clear and decisive
assertion--an assertion he was there to defend--that “there is no
authentic case on record where a properly constructed conductor failed
to do its duty.”

The new views put forward by Professor Lodge were based, in great
measure, on his theory that a lightning discharge consisted of a series
of rapid oscillations. But this theory should be received with great
caution. It seemed to be nothing more than a deduction from certain
mathematical formulas, and was not supported by any solid basis of
observation or experiment. Besides, there were many facts against
it. They all knew that a flash of lightning magnetized steel bars,
deranged the compasses of ships at sea, and transmitted signals on
telegraph wires. But such effects could not be produced by a series of
oscillations, which, being equal and opposite, would neutralize each
other. It was alleged that these rapid oscillations occurred in the
discharge of a Leyden jar. That might be true, and probably was true;
but they were not dealing with Leyden jars, they were dealing with
flashes of lightning. If there was any analogy between the discharge of
a Leyden jar and a flash of lightning, it was to be found, not in the
external discharge employed by Professor Lodge in his experiments, but
in the bursting of the glass cylinder between the two coatings of the
jar.

Lord Rayleigh thought the experiments of Professor Lodge were likely
to have important practical applications to lightning conductors. But
though these experiments were valuable as suggestions, they did not
furnish a sufficient ground for adopting any new system of protection.
It was only by experience with lightning conductors themselves that the
question could be finally settled.

Sir William Thomson hoped for great fruit from the further
investigation of self-induction in the case of sudden electrical
discharges. He warmly encouraged Professor Lodge to continue his
researches; but he expressed no decided opinion on the question at
issue. Incidentally he observed that the best security for a gun-powder
magazine was an iron house; no lightning conductor at all, but an iron
roof, iron walls, and an iron floor. Wooden boards should, of course,
be placed over the floor to prevent the danger of sparks from people
walking on sheet-iron. This iron magazine might be placed on a dry
granite rock, or on wet ground; it might even be placed on a foundation
under water; it might be placed anywhere they pleased; no matter what
the surroundings were, the interior would be safe. He thought that was
an important practical conclusion which might safely be drawn from the
consideration of these electrical oscillations and the experiments
regarding them.

Professor Rowland, of the Johns Hopkins University, America, said that
the question seemed to be whether the experiment of Professor Lodge
actually represented the case of lightning. He was very much disposed
to think it did not. In the experiment almost the whole circuit
consisted of good conductors; whereas, in the case of lightning, the
path of the discharge was, for the most part, through the air, and
therefore it might be an entirely different phenomenon. The air being a
very bad conductor, a flash of lightning might, perhaps, not consist of
oscillations, but rather of a single swing. Moreover, it was not at all
clear that the length of the spark, in the experiment, could be taken
as a measure of the obstruction offered by the conductor. Professor
George Forbes was greatly impressed with the beauty and significance of
Professor Lodge’s experiments, but he did not think the result so clear
that they should be warranted in abandoning the principles laid down by
the Lightning Rod Conference.

M. de Fonvielle, of Paris, supported the views of Mr. Preece. He cited
the example of Paris, where they had erected a sufficient number
of lightning conductors, according to the received principles, and
calamities from lightning were practically unknown. He suggested that
the Eiffel Tower, which they were now building, and which would be
raised to the height of a thousand feet, would furnish an unrivalled
opportunity for experiments on lightning conductors.

Sir James Douglass, Chief Engineer to the Corporation of Trinity House,
had a large experience with lighthouse towers. The lightning rods on
these towers had been erected and maintained during the last fifty
years entirely according to the advice of Faraday. They never had a
serious accident; and such minor accidents as did occur from time to
time were always traced to some defect in the conductor. They had now
established a more rigid system of inspection, and he, for one, should
feel perfectly safe in any tower where this system was carried out.

Mr. Symons, F.R.S., Secretary to the Meteorological Society, had taken
part in a discussion on lightning conductors as long ago as 1859. It
had been a hobby with him all his life to investigate the circumstances
of every case he came across in which damage was done by lightning,
and the general impression left by his investigations entirely
coincided with the views just expressed by Sir James Douglass. He had
been a member of the Lightning Rod Conference, and was the editor of
their report; and he wished to enter his protest against the idea of
rejecting all that had hitherto been done in connection with lightning
conductors on the strength of mere laboratory experiments.

Professor Lodge, in reply, said he could perfectly understand the
position of those who held that a lightning rod properly fitted up
never failed to do its duty, because, whenever it failed, they said
it was not properly fitted up. The great resource in such cases was
to ascribe the failure to bad earth contact. He thought a good earth
contact was a very good thing, but he could not understand why such
extraordinary importance should be attached to it. A lightning rod
had two ends--an earth end and a sky end--and he did not see why good
contact was more necessary at one end than at the other. If a few sharp
points sticking out from the conductor were sufficient for a good sky
contact, why were they not sufficient also for a good earth contact?

Besides, though a bad earth contact might explain why a certain amount
of disruption should take place at the earth where the bad contact
existed, he did not see how it accounted for the flash shooting off
sideways half-way down the conductor. Again, what does a bad earth
contact mean? If an electrical engineer finds a resistance of a
hundred ohms, he will rightly pronounce the earth contact to be very
bad indeed. But why should the lightning flash leave a conductor
with a resistance of a hundred ohms in order to follow a line of
non-conductors where it encounters a resistance of many thousand ohms?

He accepted the statement of Mr. Preece that his whole theory depended
on the existence of oscillations in the lightning discharge; but there
was good reason to believe they existed, because they were proved to
exist in the discharge of a Leyden jar. Mr. Preece objected that an
oscillating discharge could not produce magnetic effects, as a flash of
lightning was known to do. He confessed he was unable to explain how
an oscillating discharge produced such effects;[43] but that it could
produce them there was no doubt whatever, for the discharge of a Leyden
jar produces magnetic effects, and we have ocular demonstration that
the discharge of a Leyden jar is an oscillating discharge.

As to the assurances we had received from electrical engineers that
a properly fitted lightning conductor never fails, he should like to
ask them how the Hotel de Ville, in Brussels, had been set on fire by
lightning on the 1st of last June. The system of lightning conductors
on this building had been erected in accordance with the received
theory, and had been held up by writers on the subject as the most
perfect in Europe. Unless some explanation were forthcoming to account
for its failure, we could no longer regard lightning conductors as a
perfect security against danger.

The President of Section A, Professor Fitzgerald, in bringing the
discussion to a close, observed that one result of this meeting would
be to give a new interest to the phenomena of static electricity and
its practical applications. He was inclined himself to think that the
experiments of Professor Lodge were not quite analogous to the case of
a flash of lightning. In comparing the discharge of a Leyden jar with
a flash of lightning they should look for the analogy, not so much in
the external discharge through a series of conductors, but rather,
as Mr. Preece had observed, in the bursting of the glass between the
two coatings of the jar. As regarded the oscillations in a Leyden jar
discharge, he did not think such oscillations were at all necessary
to account for the phenomena observed in the experiments. Many of
the results which Professor Lodge seemed to think would require some
millions of oscillations per second would be produced by a single
discharge lasting for a millionth of a second. Improvements, perhaps,
were possible in our present system of lightning conductors, but
practical experience had shown, however we might reason on the matter,
that, on the whole, lightning conductors had been a great protection to
mankind from the dangers of lightning.


=Summary.=--I will now try to sum up the results of this interesting
discussion, and state briefly the conclusions which, as it seems to
me, may be deduced from it. First, I would remind my readers that a
lightning rod has two functions to fulfill. Its first function is to
promote a gradual, but rapid, discharge of electricity according as it
is developed, and thus to prevent such an accumulation as would lead
to a flash of lightning. Its second function is to convey the flash
of lightning, when it does come, harmless to the earth. Now, the new
views advanced by Professor Lodge in no way impugn the efficiency of
lightning rods as regards their first function; and it is evident that
the greater the number of lightning rods distributed over a given area,
the more perfectly will this function be fulfilled. This is a point of
great practical importance which seemed to me, in some degree, lost
sight of during the progress of the discussion.

Secondly, it was practically admitted by the highest authorities that
the experiments and reasoning of Professor Lodge afford good grounds
for reconsidering the received theory of lightning conductors as
regards their second function--that of carrying the lightning flash
harmless to the earth. But there was undoubtedly a general feeling
that it would be rash to set aside, all at once, the received theory
on the strength of laboratory experiments made under conditions widely
different from those which actually exist in a lightning discharge.
Experiments are wanted on a larger scale; and, if possible, experiments
with lightning rods themselves.

Thirdly, the testimony of electrical engineers who have had large
experience with lightning conductors seems almost unanimous that a
lightning conductor erected and maintained in accordance with the
conditions prescribed by the Lightning Rod Conference gives perfect
protection. It was certainly unfortunate that the Hotel de Ville, in
Brussels, which was reputed the best protected building in Europe,
should have been damaged by lightning just two months before the
discussion took place; but no certain conclusion can be drawn from
this catastrophe until we know exactly the conditions under which it
occurred.

So the matter stands, awaiting further investigation.


FOOTNOTES:

[41] See his Lectures, published in the _Electrician_, June 22, June
29, July 6, and July 13, 1888.

[42] See paper read at the meeting of the British Association, in Bath,
1888, published in the _Electrician_, page 607. September 14.

[43] See a very ingenious hypothesis, to account for this phenomenon,
suggested by Professor Ewing in the _Electrician_, p. 712. October 5,
1888.




Transcriber’s Notes

Errors and omissions in punctuation have been corrected.

Page 11: “continuous inpression” changed to “continuous impression”

Page 41: “full conconducting power” changed to “full conducting power”

Page 44: “it base” changed to “its base”

Page 58: “follow one an-another” changed to “follow one another”