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WAR DEPARTMENT

OFFICE OF THE CHIEF SIGNAL OFFICER

MANUAL No. 6


VISUAL SIGNALING

SIGNAL CORPS UNITED STATES ARMY

1910

[Illustration]

    WASHINGTON
    GOVERNMENT PRINTING OFFICE
    1910




    WAR DEPARTMENT,
    Document No. 366.
    OFFICE OF THE CHIEF SIGNAL OFFICER.




    WAR DEPARTMENT,
    OFFICE OF THE CHIEF OF STAFF,
    _Washington, April 20, 1910_.

The following Manual of Visual Signaling, prepared in the Office of
the Chief Signal Officer, is approved and herewith published for the
information and guidance of the Regular Army and the Organized Militia
of the United States, and supersedes all other pamphlets or similar
instructions heretofore issued upon the subject. Officers and men
of the Signal Corps will thoroughly familiarize themselves with the
instructions and suggestions contained herein.

By order of the Secretary of War.

                TASKER H. BLISS,
                    _Brig. General, General Staff,
                              Acting Chief of Staff_.




TABLE OF CONTENTS.



                                                          Page.
  CHAPTER I.--INTRODUCTION                                   9

  CHAPTER II.--_Visual signaling equipment._
  The wand                                                  11
  The flag kit:
    The 2-foot flag kit                                     12
    The 4-foot flag kit                                     12
    Care of flag material                                   13
    Powers and limitations of flag signaling                13
  The heliograph:
    Historical                                              14
    Description                                             14
    Assembling                                              17
    Adjustment                                              20
    Operation                                               21
    Care of apparatus                                       22
    Powers and limitations of the heliograph                22
  The signal lantern:
    Acetylene                                               23
    Calcium carbide                                         23
    Method of gas generation                                24
    Description                                             25
    Operation and care                                      30
    Powers and limitations of the signal lantern            35
  Rockets and shells:
    Description                                             35
    Operation                                               38
    Employment                                              40
  The semaphore: Description                                40
  The searchlight: Methods of employment                    41
  The Coston signals                                        41
  Very's night signals                                      42
  The Ardois system of signaling                            42
  Sound signals                                             44
  Improvised signal methods                                 44

  CHAPTER III.--_Alphabets or systems of signals._
  Signal alphabets:
    American Morse                                          45
    Continental Morse                                       45
    Army and navy                                           45
    Abbreviations                                           46
  Code calls                                                47
  Execution of signal alphabets                             47
    The army and navy alphabet                              47
    The Morse alphabets                                     49
  International code of signals:
    Description                                             51
    Two-arm semaphore                                       51
  The Ardois system                                         52
  Coston signals                                            54
  Very's night signals                                      54
  Rocket signaling                                          55
  Two-arm semaphore alphabet, U. S. Navy                    57
  Summary of signals, army and navy alphabet                60

  CHAPTER IV.--_The field message._
  Definition                                                64
  The blank form                                            64
  Writing the message                                       66
  Instructions to operators:
    Use of message blank                                    66
    Duties of sending operators                             66
    Order of transmission                                   66
    Duties of receiving operators                           67
    Communications confidential                             67
    Checking the message                                    67

  CHAPTER V.--_The signal station._
  Location of stations:
    General considerations                                  68
    Backgrounds                                             70
    Azimuth of stations                                     71
    Altitude                                                71
    Determination of background color                       72
    Choice of apparatus                                     73
    Miscellaneous considerations                            73
    Intervisibility table                                   74
  Finding a station                                         75
  Operation of stations:
    Personnel                                               76
    Calls and personal signals                              78
    Opening communication                                   79
    Commencing the message                                  80
    Sending and receiving                                   80
    Breaking                                                80
    Discontinuance of transmission                          81
    Acknowledgment of receipt                               81
    Station records                                         81
    Formation of signals                                    82
    Repeating the message                                   83
    Signal practice                                         83

  CHAPTER VI.--_Codes and ciphers._
  Codes in use                                              84
  Employment of codes                                       84
  Cipher code                                               85
  The War Department Code                                   86
  Cipher code in field work                                 87
  Field ciphers:
    Description and use                                     87
    Forms of field cipher                                   88
    Inversions                                              88
    Concealment of terminations                             88
  Cipher apparatus: The cipher disk                         89
  The mathematical cipher                                   93
  The route cipher                                          94
  Cipher detection: Employment of cipher disk               96

  CHAPTER VII.--_Field glasses and telescopes._
  Reflection                                                98
  Refraction                                                98
  Lenses                                                    98
  Focus                                                     99
  Optical center                                            99
  Image                                                     99
  Conjugate foci                                            99
  Law of foci                                              100
  Formation of image                                       101
  Spherical aberration                                     102
  Chromatic aberration                                     102
  Telescopes                                               104
  Galilean field glasses and telescopes                    106
  Porro prism field glasses and telescopes                 106
  Field glasses                                            108
  Properties of telescopes and field glasses               109
    Power                                                  109
    Light                                                  111
    Field                                                  114
    Definition                                             115
  Field glasses and telescopes issued by the Signal Corps  119
    Type A                                                 121
    Type B                                                 124
    Type C                                                 125
    Type D                                                 125
  Field-glass specifications                               126




CHAPTER I.

INTRODUCTION.


While, in consequence of the development of electrical invention and
improvement, visual signaling will be less frequently resorted to in
future than heretofore in the service of field lines of information,
it should be appreciated that the necessity for an adequate supply
of apparatus of this kind, and the need for skilled manipulators to
operate it, has in no wise diminished. The great celerity with which
electric signals can be exchanged and their usual entire independence
of local conditions has placed systems of this class foremost among
the signaling methods of the world. There is scarcely any commercial
industry whose successful existence does not vitally depend upon some
one, perhaps several systems of signaling, and improvements of old and
inventions of new signal devices are continually necessary to meet the
requisite needs demanded by the progress of art and science. Railways
are probably the greatest of all commercial users of signals. With them
the great mass of intelligence is transmitted by the electric telegraph
and telephone, but the flag, the semaphore, the signal light, and
many other contrivances furnish indispensable visual adjuncts. Visual
signaling is and always will be a most valuable means of transmitting
information in peace and war, and it is not to be imagined that it
will ever be supplanted in its particular function by the introduction
of other methods. Occasions will frequently occur in the field when
no other means will be practicable, and then, if not before, will the
value of the system be fully emphasized.

Strictly speaking, a visual signal is any visible sign by which
intelligence is communicated, but in a military sense the term
visual signaling has a broader meaning and includes other methods of
transmitting information than those which appeal to the sense of sight.

In most systems of signals suitable for military use, each signal is
composed of one or more separate units, known as elements. Having
prescribed a certain number of elements, the various signals are
formed by having these elements appear singly or together in different
arrangements or combinations. The continental system is one of two
elements, namely the dot and the dash, while the Morse system employs
three elements, the dot, the dash, and the space. Having agreed upon
a certain number of combinations of elements, a system of signals is
formed by giving a meaning to each combination. These meanings usually
include the letters of the alphabet and numerals, combinations of which
being used to formulate necessary information. Combinations of elements
of any system can also, however, be used to indicate any desired
meaning.

With reference to period of visibility, signals are of two kinds,
transient and permanent. A transient signal is one which disappears as
soon as completed; a permanent signal is one that remains in view for
some time. Heliograph signals are transient signals, while signals made
by code flags are permanent signals. Signals are divided into classes
in accordance with the number of elements employed in their formation.
Thus, signals using two elements are signals of the second class,
signals using three elements signals of the third class, etc.

The standard apparatus used in visual signaling is fully described in
a succeeding chapter. Some of the instruments employed are used wholly
for day, and some wholly for night, signaling. Some devices, either
with or without slight variations, are equally well adapted to day or
night work. Visual signaling presents a great field for ingenious and
resourceful work, and emergency will often demand the advantageous
employment of other methods than those described herein.




CHAPTER II.

VISUAL SIGNALING EQUIPMENT.


THE WAND.

The wand is a stick of light wood about 18 inches long and one-half
inch in diameter. It is held loosely between the thumb and forefinger
and waved rapidly to the right or left to indicate the elements of the
alphabet. It is used for practice purposes and the signals made by it
are only intended to be read at very short distances.


THE FLAG KIT.

Two kinds of flag kits, the 2-foot kit and the 4-foot kit, are issued
by the Signal Corps.

_The 2-foot kit._--This kit consists of one white and one red signal
flag, two three-jointed staffs, and a suitable carrying case to contain
the outfit. The white flag is made of white muslin 2 feet square, with
an 8-inch turkey-red muslin center. The red flag is of similar size
and material, the only difference being an alternation of colors in
the body and center. The means of attachment to the staff consists
of a loop at the center, and two ends of white tape at each edge,
of the back of the flag body. The staff is made of hickory in three
joints, each 23 inches long, and is assembled by telescoping into brass
ferrules. Brass eyes are provided on the first and second joints to
receive the tape ends at the edge of the flag. The carrying case, of
convenient size and shape to contain the two flags and staffs complete,
is made of 8-ounce standard khaki bound with leather and fitted with a
shoulder strap.

The 2-foot kit is essentially a practice kit, although under favorable
conditions of weather and terrain it may be used to advantage as a
short distance service signaling outfit. Two of these kits are issued
to each troop, battery, and company for the purpose of disseminating
general instruction in military signaling throughout the army.

_The 4-foot kit._--This kit is of essentially the same description as
the 2-foot kit except as regards size. The flags are 3 feet 9 inches
square with 12-inch centers and the staffs are considerably heavier,
the joints being each 36 inches long. The 4-foot kit is the standard
field flag kit and the range at which signals can be exchanged with it
depends on a variety of factors, such as the condition of the weather,
the location of stations, the proficiency of signalmen, etc. The speed
for continuous signaling is seldom greater than five to six words per
minute.

_Care of flag material._--Signal flags should be examined at the close
of drill or practice and repairs made to any rents or loose ties
discovered. Flags, when soiled, should be thoroughly washed and dried
in the sun. Signals made by clean flags are much more easily read
than those made by dirty ones. Staffs should be handled with care,
especially when jointing or unjointing. Care should be taken not to
bruise the ends of the brass ferrules. If a ferrule becomes loose on a
staff it should be tightened without delay.

_Powers and limitations of flag signaling._--The advantages which may
be claimed for this method of signaling are portability of apparatus,
adaptability to varied weather conditions, and great rapidity of
station establishment. The disadvantages are the lack of celerity
of the signals, their impenetrability to dust or smoke, and the
comparatively short ranges at which they can be read.


THE HELIOGRAPH.

The heliograph is an instrument designed for the purpose of
transmitting signals by means of the sun's rays.

_Historical._--Experiments with the heliograph with a view to its
adoption as a part of the visual signaling equipment of the United
States Army were commenced as early as 1878. The reported successful
use of the instrument by the British in India about this time led to
the importation of two heliographs of the Mance pattern. A series
of experiments with these machines conducted for the purpose of
eliminating certain objectionable features finally resulted in the
evolution of the present type of service heliograph.

The early English heliograph was not provided with a shutter, the flash
being directed on the distant station by means of a movable mirror
controlled by a key. The great objection to this type of instrument
was the impossibility of maintaining accurate adjustment during the
transmission of signals due to the fact that the manipulation of the
mirror tended to throw the flash constantly out of alignment. To
overcome this, the American heliograph has been provided with a screen
designed to operate as a shutter and control the flash reflected from
an immobile mirror.

_Description._--The service heliograph equipment of the Signal Corps
consists of:

     A sole-leather pouch with shoulder strap containing--

  1 sun mirror.     }
  1 station mirror. } Inclosed in a wooden box.
  1 screen, 1 sighting rod, 1 screw-driver.

     A small pouch, sliding by 2 loops upon the strap of the larger
     pouch, containing 1 mirror bar.

     A skeleton leather case containing 2 tripods.

[Illustration: FIG. 1.--Heliograph assembled.]

The mirrors are each 4½-inch squares of plate glass supported by
sheet brass and cardboard backings, and mounted in brass retaining
frames. At the center of each mirror there is an unsilvered spot three
thirty-seconds of an inch in diameter and holes corresponding to these
spots are drilled in the backing. The sun mirror differs from the
station mirror only in that it has a paper disk pasted upon its face
covering the unsilvered spot. The mirror frames are carried by brass
supports provided at the bases with conical projections accurately
turned to fit the sockets of the mirror bar and grooved at the ends
to receive the clamping spring. Each support is fitted with a tangent
screw and worm wheel attachment functioned to control the motion of the
mirror frame about its horizontal axis.

[Illustration: FIG. 2.--Mirror and mirror bar case.]

The mirror bar is a bronze casting provided at the center with a clamp
threaded to fit the screw of the tripod. By releasing the clamp the bar
may be moved independently of the screw and adjusted to any desired
position. Conical sockets for the reception of the mirror supports are
provided at the ends of the mirror bar. These sockets work freely in
the bar and, being actuated by a tangent screw and worm wheel, serve
to regulate the motion of the mirror frame about its vertical axis.
Clamp springs, for engaging and securing the ends of the mirror frame
supports, are attached at each end of the bar.

The screen is a brass frame 6½ inches square, in which six segments
or leaves are mounted in such a way as to form a shutter. The leaves
are designed to turn through arcs of 90° on horizontal axes, unanimity
of movement being secured by connections made with a common crank bar.
The crank bar is operated by a key and retractile spring which serve to
reveal and cut off the flash. A set screw and check nut at the lower
edge of the screen frame limits the motion of the crank bar and the
opening of the leaves. A threaded base support furnishes the means of
attaching the screen frame to the tripod.

The sighting rod is a brass rod 6½ inches long, carrying at the
upper end a front sight and a movable disk. About the rod is fitted
a movable bronze collar, coned and grooved to take the socket and
clamping spring of the mirror bar. A milled edged bronze washer serves
to clamp the collar to the rod at any desired point.

[Illustration: FIG. 3.--Heliograph tripods.]

The tripods are similar in all respects, the screw of either threading
into the mirror bar or screen frame. Each tripod is provided with a
hook at the base of the head, allowing the suspension of a weight when
great stability is required.

_Assembling._--There are two ways of assembling the heliograph and
the position of the sun is the guide in determining which of the two
should, in any given case, be employed. When the sun is in front of
the operator (that is, in front of a plane through his position at
right angles to the line joining the stations) the sun mirror only is
required; with the sun in rear of this plane both mirrors should be
used. With one mirror the rays of the sun are reflected directly from
the sun mirror to the distant station; with two mirrors, the rays are
reflected from the sun mirror to the station mirror, and thence to the
distant station.

_With one mirror_: Firmly set one of the tripods upon the ground;
attach the mirror bar to the tripod; insert and clamp in the sockets
the sun mirror and sighting rod, the latter having the disk turned
down. At a distance of about 6 inches, sight through the center of
the unsilvered spot in the mirror and turn the mirror bar, raising or
lowering the sighting rod until the center of the mirror, the extreme
point of the sighting rod, and the distant station are accurately
in line. Firmly clamp the mirror bar to the tripod, taking care not
to disturb the alignment, and turn up the disk of the sighting rod.
The mirror is then moved by means of the tangent screws until the
"shadow spot" falls upon the paper disk in the sighting rod, after
which the flash will be visible at the distant station. The "shadow
spot" is readily found by holding a sheet of paper or the hand about 6
inches in front of the mirror, and should be constantly kept in view
until located upon the disk. The screen is attached to a tripod and
established close to, and in front of, the sighting disk, in such a way
as to intercept the flash.

_With two mirrors_: Firmly set one of the tripods on the ground; clamp
the mirror bar diagonally across the line of vision to the distant
station; clamp the sun mirror facing the sun to one end of the mirror
bar and the station mirror facing the distant station. Stooping down,
the head near and in rear of the station mirror, turn the sun mirror
by means of its tangent screws until the whole of the station mirror
is seen reflected in the sun mirror and the unsilvered spot and the
reflection of the paper disk accurately cover each other. Still looking
into the sun mirror, adjust the station mirror by means of the tangent
screws until the reflection of the distant station is brought exactly
in line with the top of the reflection of the disk and the top of the
unsilvered spot of the sun mirror; after this the station mirror must
not be touched. Now step behind the sun mirror and adjust it by means
of the tangent screws so that the "shadow spot" falls upon the center
of the paper disk on the station mirror. The flash will then be visible
at the distant station. The screen and its tripod are established as
described in the single mirror assembling.

_Alternate method with two mirrors_: Clamp the mirror bar diagonally
across the line of vision to the distant station, with the sun mirror
and the station mirror approximately facing the sun and distant
station, respectively.

Look through small hole in sun mirror and turn the station mirror on
its vertical and horizontal axes until the paper disk on the station
mirror accurately covers the distant station.

Standing behind sun mirror, turn it on its horizontal and vertical axes
by means of the tangent screw attachments until the shadow spot falls
upon the paper disk on station mirror.

_Adjustment._--Perfect adjustment is maintained only by keeping the
"shadow spot" uninterruptedly in the center of the paper disk, and
as this "spot" continually changes its position with the apparent
movement of the sun, one signalman should be in constant attendance
on the tangent screws of the sun mirror. Movement imparted by these
screws to the mirror does not disturb the alignment, as its center (the
unsilvered spot) is at the intersection of the axes of revolution.
Extra care bestowed upon preliminary adjustment is repaid by increased
brilliancy of flash. With the alignment absolutely assured and the
"shadow spot" at the center of the disk, the axis of the cone of
reflected rays is coincident with the line of sight and the distant
station receives the greatest intensity of light. Remember the distant
observer is unquestionably the better judge as to the character of the
flash received; and if therefore, adjustment is called for when the
"shadow spot" is at the center of the disk, the alignment is probably
at fault and should be looked after at once. In setting up the tripods
always see that the legs have a sufficient spread to give a secure base
and on yielding soil press firmly into the ground. Keep the head of the
tripod as nearly level as possible and in high wind ballast by hanging
a substantial weight to the hook. See that the screen completely
obscures the flash; also that the flash passes entire when the screen
is opened. This feature of the adjustment is partially regulated by
the set screw attached to the screen frame. The retractile spring
should sharply return all the leaves of the screen to their normal
positions when the key is released. Failure to respond promptly is
obviated by strengthening or replacing the spring.

_Operation._--It is of the utmost importance that uniformity in
mechanical movement of the screen be cultivated, as lack of rhythm in
the signals of the sender entails "breaks" and delay on the part of the
receiver. Dark backgrounds should, when practicable, be selected for
heliograph stations, as the signals can be most easily distinguished
against them.

To find a distant station, its position being unknown, reverse the
catch holding the station mirror and with the hand turn the mirror
very slowly at the horizon over the full azimuth distance in which
the distant station may possibly lie. This should be repeated not
less than twice, after which, within a reasonable time, there being
no response, the mirror will be directed upon a point nearer the home
station and the same process repeated. With care and intelligence it
is quite probable that, a station being within range and watching for
signals from a distant station with which it may be desired to exchange
messages, this method will rarely fail to find the sought-for station.

The exact direction of either station searching for the other being
unknown, that station which first perceives that it is being called
will adjust its flash upon the distant station to enable it when this
light is observed to make proper adjustments. If the position of each
station is known to the other, the station first ready for signaling
will direct a steady flash upon the distant station to enable the
latter to see not only that the first station is ready for work, but to
enable the distant station to adjust its flash upon the first station.

Smoked or colored glasses are issued for the purpose of relieving the
strain on the eyes produced by reading heliograph signals.

_Care of apparatus._--Minor parts of the instrument should be
dismounted only to effect repairs, for which spare parts are furnished
on requisition. Steel parts should be kept oiled and free from rust.
Tangent screws and bearings should be frequently inspected for dust or
grit. Mirrors should invariably be wiped clean before using. In case of
accident to the sun mirror, the station mirror can be made available
for substitution therefor by removing the paper disk. If the tripod
legs become loose at the head joints, tighten the assembling screws
with the screw-driver.

_Powers and limitations of the heliograph._--Portability, great range,
comparative rapidity of operation, and the invisibility of the signals
except to observers located approximately on a right line joining the
stations between which communication is had, are some of the advantages
derived from using the heliograph in visual signaling.

The principal disadvantage results from the entire dependence of the
instrument upon the presence of sunlight. The normal working range
of the heliograph is about 30 miles, though instances of its having
attained ranges many times greater than this are of record. The
heliograph can be depended upon to transmit from five to twelve words
per minute.


THE ACETYLENE LANTERN.

The signal lantern is an instrument designed for the purpose of
transmitting signals by means of intermittent flashes of artificial
light. It is the standard night visual signaling equipment furnished by
the Signal Corps and depends for its illumination upon the combustion
of acetylene gas.

_Acetylene._--Acetylene is a pure hydrocarbon gas, producible in
various ways, the commoner of which are: (_a_) By dropping calcium
carbide into water; (_b_) by dropping water upon calcium carbide.
This gas gives, when burning, high penetrative power, and was first
described by Mr. Edmund Davy, professor of chemistry to the Royal
Dublin Society, in 1836.

_Calcium carbide._--In the manufacture of calcium carbide for
commercial purposes the best quality of coke and quicklime are
used. These two substances are powdered thoroughly, mixed in proper
proportions, and then placed in an electrical furnace. Under the action
of the intense heat (5,500° F.) these two refractory substances unite
and form calcium carbide. Calcium carbide is of a grayish-white color,
crystal in appearance, and is nonexplosive and noncombustible, being,
except for its affinity for water, an absolutely inert substance. A
pound of commercial carbide will produce approximately 5 cubic feet
of gas. When water is brought in contact with calcium carbide, the
generation of acetylene is rapid; owing to its strong affinity for
water it will become air slacked and slowly lose its strength if
exposed to the action of the moisture in the atmosphere; consequently,
when stored or being transported it should be kept in air-tight cans.

When calcium carbide is brought in contact with water, the following
occurs:

As is known, the principal components of water are oxygen and hydrogen,
and calcium carbide is calcium and carbon. When brought in contact,
the oxygen in the water decomposes the calcium in the carbide, and in
this decomposition the hydrogen in the water is liberated and unites
with the carbon of the carbide, forming a hydrocarbon gas which is
acetylene. It is a pure white light of intense brilliancy and high
candlepower. The spectrum analysis of acetylene shows that it is almost
identical with sunlight, and in consequence delicate shades of color
appear according to their true value as under the light of the sun,
consequently it penetrates fog to a greater distance than other lights.
Acetylene is like other gases--explosive when mixed with air in proper
proportions, confined, and ignited--and the same precautions should
therefore be taken in its use as would be in the handling of coal or
water gas, gasoline vapor, etc. As acetylene is very rich in carbon,
it will not burn in its pure state without smoking. To avoid this,
burners have been constructed so that the gas is mixed with the proper
proportion of air at the burner tip, to insure perfect combustion. The
burners for acetylene are different from those for other gases. In
order to get a flat flame, the gas is brought through two perfectly
round holes at an angle which causes the two flames to impinge upon
each other and thus form a flat flame.

_Method of gas generation._--The method employed for producing
acetylene in the signal lantern is by bringing water into contact
with calcium carbide. The disadvantage of this method is that when
the water is not in excess and does not entirely surround and touch
each piece of carbide the heat of generation will so change the
chemical properties of the gas that combustion at the burners is not
satisfactory.

This change is technically known as "polymerization," or the breaking
up of acetylene into other hydrocarbons, such as vapors of benzine,
benzole, etc. These form a tarry substance which is apt to condense
at the burner tip and clog the openings. Also they deposit carbon on
the burners, as they require more air for perfect combustion than does
pure acetylene. Another disadvantage of this system is that after the
carbide and water are in contact, generation of gas will continue
until all the water is absorbed. Where, however, portability of the
generating apparatus is desired and resort to this method is necessary,
the objections are not important, if the apparatus is well constructed
and care is taken in its use.

_Description._--This equipment consists of a signal lantern with
cartridge generator attached. The lantern is equipped with a special
aplanatic lens mirror, 5 inches in diameter and about 3 inches focus.
The lantern is packed complete in a wooden case with shoulder straps
and the following extra parts are included, each part having its own
receptacle in the case: 2 burners; 1 cover glass; 3 cartridges of
calcium carbide of 5 ounces each; 1 pair of gas pliers; 1 tube white
lead; 1 extra filter bag; 1 screw-driver.

[Illustration: FIG. 4.--The signal lantern.]

The lantern is made of brass, all parts of which are riveted. The
burner is of the double tip form, consuming three-quarters of a cubic
foot per hour. The lantern is fitted with a hood to provide proper
ventilation and at the same time to prevent the flickering of the
light by the wind. The front door of the lantern is hinged and fastens
with a spring clasp; it is so arranged that it can be entirely removed
if necessary. The cover glass is made in three sections and is not
affected by the expansion and contraction of the metal due to changes
in temperature. The glass is fastened by the aid of a spring wire, so
that it can be readily removed if it is necessary to replace a broken
section. In the base of the lantern is a key and the adjustment for
regulating the height of the flame. The key is so arranged that when
not depressed but little gas is admitted through the by-pass to the
burner and the flame is low. By depressing the key as much gas as can
be entirely consumed is admitted to the burner, which gives a bright
flash. At the back of the lantern there is an adjustable handle, so
that the equipment can be used as a hand lantern if desired. This
form of lantern can be used with the regular heliograph tripod,
the generator being either attached to the back of the lantern or
suspended, as shown in figure 4. When practicable it is better to
attach the generator to the lantern, as shown in figure 5. The
candlepower of this lantern is about 1,900.

[Illustration: FIG. 5.]

_The generator_ used is known as "the cartridge generator," and while
constructed on the water-feed principle, the disadvantages incident to
this method are eliminated as far as possible. It is constructed of
brass and has a removable top. Attached to the inside of the top is
a flexible frame with a spring latch, the spring latch being hinged.
(Fig. 8.) At the top of the frame is a tube or cylinder, the bottom of
which is conical in shape and covered by a rubber plug. At the bottom
of the frame is a hollow tube, which is the water inlet. The cartridge
proper consists of a tin cylinder, having an opening at either end. A
small cylinder of wire mesh extends from and connects these openings.
The carbide lays around this mesh on the inside of the cartridge. The
rubber plug before mentioned fits into the upper opening, and the water
tube into the lower opening. (See figs. 7, 8, and 9.) Inside the tube,
at the top of the frame, is a filter, the function of which is to
remove the dust and moisture from the gas. The outlet from this chamber
is by a brass bent tube having a stopcock attached thereto.

Figure 6 gives a sectional view of the generator with the cartridge
in place. _D F G H_ represent the valve frame and _I_ the cartridge
attached. The reservoir _A_ is filled with water, and when the frame is
immersed, with the valve _R_ closed, the air contained in the cartridge
and tubing can not escape, the water seal preventing, while the
confined air prevents the water from rising in the tube _N_. When the
valve at _R_ is opened and the air is allowed to escape, part of the
water from the reservoir rises into the tube _N_ and then out through
the small hole _O_ to the carbide. Gas is immediately generated, the
pressure of which prevents further ingress of the water from the tube
_N_, and the generation of gas is suspended.

As the gas passes out through the valve at _R_ the pressure decreases,
permitting the water to again rise in the tube and flow through _O_.
Gas is again generated, which at once exerts its pressure and cuts off
the supply of water. This is the automatic action by which water is
brought in contact with the calcium carbide. Thus it will be observed
that the use or escape of the gas regulates the generation by the
simple device of the rise and fall of a water column. There is a cap
_M_ screwed over the tube _N_. This is used to deflect the course
of the water downward, so that the carbide in the lower part of the
cartridge is first attacked. There is a needle inside of cap _M_,
which can be used for cleaning the hole _O_. When the gas is generated
it passes through the filter _D_ on its way to the burner through
_R_. This filter consists of a tube loosely packed with ordinary
nonabsorbent cotton, which should never cover the escape pipe leading
to the valve _R_. In passing through this cotton filter moisture and
dust are removed from the gas. In the latest model a felt filter is
used instead of cotton.

[Illustration: FIG. 6.--Signal lantern generator.]

The escape pipe _F_ provides a means for the escape of gas generated
and not used or generated more rapidly than consumed. Should an excess
be generated, it passes down through the tube _F_, and, finding its
way through some small holes in the bottom of this tube, escapes
through the water seal and the opening at _C_. It will be noted that
if escaping gas at _C_ should become accidentally lighted, the flame
can not strike back into the filter and cartridge because of the
water seal. The principal things to observe in the operation of this
generator are the following:

(1) To see that the rubber plugs _fit tightly_ into the openings of the
cartridge.

(2) That the tube _N_, the cap _M_, and water hole _O_ are not stopped
up.

(3) That the cotton in the filter is changed frequently.

(4) That the _stopcock R is closed before inserting the frame in the
water_. If this latter instruction is not complied with, it can be
readily seen that the water will have free access to the carbide and
excessive generation will occur.

When the charge is exhausted, the entire cartridge is taken out
and thrown away. This eliminates the handling of carbide and the
disagreeable task of cleaning out the residuum after the gas has been
extracted.

Connection is made from the stopcock _R_ to the hose connection on
the lantern proper, and this is the passageway of the gas from the
generator to the burner. As soon as the stopcock is opened the water
rises through the tube and flows to the carbide. The advantage of the
cartridge being submerged in the water is to reduce and absorb as much
of the heat liberated by generation as is possible. These lanterns have
been tested up to a distance of 10 miles with the naked eye, and under
favorable conditions can be used over a range somewhat in excess of
this. With a 30-power telescope the flash can be read at a distance of
30 miles.

_Operation and care._--Take the lamp and generator from the case by
aid of the handle attached to the lamp; screw the complete outfit on
a heliograph tripod, or stand the outfit on a level object; remove the
cover of generator, to which is attached the flexible frame (fig. 9);
detach spring from the catch of the flexible frame; tear off flaps from
the ends of carbide cartridge (or pry off small caps) and attach the
cartridge as shown in figure 9. Then attach to frame as shown in figure
10, being careful to see that both rubber plugs fit tightly into the
holes in the cartridge; fasten the latch of the spring over the metal
catch; close stopcock _R_ on service pipe; completely fill the outer
can of generator with water, the object being to have the generator
level full of water when the lamp is in service, then immerse the frame
and cartridge, pressing the top of the generator down tight. In doing
this the water will overflow the sides of the generator tank. Now
connect by rubber tubing the stopcock with the gas inlet at the bottom
of the lamps, as shown in figure 4; then (1) open front door of the
lamp, (2) light a match, (3) open stopcock, and (4) light the gas at
the burner. In doing this hold the key open. In the new model the key
and hose connection are on the side of bottom of lamp.

When the gas is ignited, the lamp is ready for signaling, and the key
can be operated as is the Morse telegraph instrument, but of course not
so rapidly.

In the event of the flame being too high when the key is closed,
adjustment can be made by loosening the set screw (fig. 4, indicated
by an arrow) and adjusting the light by turning screw _b_. When at the
proper height, tighten the set screw which locks the by-pass in its
proper position. In the new model this is accomplished by aid of the
regulator by-pass valve at the left-hand side of bottom of lamp. The
lamp is properly adjusted when shipped and should not be changed unless
absolutely necessary. Connect the rubber tube to the burner before
opening the stopcock on the generator.

To recharge the generator, take the frame and the old cartridge from
the case, throw away the old case and replace with a fresh one,
proceeding as before. See that fresh water is put in the generator each
time a new cartridge is used.

[Illustration: FIG. 7.]

In the tube through which the service pipe passes is a felt filter for
taking the dust out of the gas. If the filter clogs, unscrew the cap to
which the service pipe is attached, clean the felt, or replace it with
a new filter, binding it in place by a stout thread or string.

If the burner of the lamp does not produce a perfectly flat flame
it has become clogged and should be cleaned with the burner cleaner
furnished, or a new burner should be substituted, care being taken to
put a little white lead on the nipple, if practicable, so as to insure
a tight joint.

In repacking the outfit in the case, throw out the water and wipe the
can and generator parts dry. You can not be too careful to keep the
apparatus clean. This is especially true of the small pipe that passes
up through the bottom of the cartridge, with a cap over it. The cap
should always be screwed in place, as its object is to prevent the
water from squirting to the top of the cartridge.

[Illustration: FIG. 8.]

[Illustration: FIG. 9.]

The back of the lamp can be removed by turning the small thumbscrew on
the top and drawing out the pin which holds the shell into which is
fitted the lens. It is not necessary to take the back out except to
replace a lens, as the latter can be cleaned by opening the front door.

If it is desirable to use the lamp as a hand lantern the flame can be
turned on full by turning the button in a vertical position; this locks
the key open. In the new model depress the key and lock it with the
latch above the key.

One charge of calcium carbide will supply gas to burn about one hour
with the light turned on full, or for approximately three hours'
signaling.

[Illustration: FIG. 10.]

If signaling is to be suspended for some hours, empty the water out of
the generator and close valve _R_.

The glass front can be replaced by taking out the wire spring. The
glass cuts should be mounted in a horizontal position and, to prevent
breaking, should be protected from rain when the lamp is hot. If a
glass should be broken and an extra one is not available to replace it,
signaling can be continued by turning the flame on full and using the
heliograph shutter, a cap or piece of board in front of the lantern to
obscure and reveal the flash. Without the protection of the cover the
flame is easily blown out when turned low, but will not be extinguished
even in a strong wind if the gas is turned full on.

Old model lamps are serially numbered from 1 to 200, inclusive; the new
model lamps are serially numbered from 201 upward.

_Powers and limitations of the acetylene signal lantern._--As
conditions are usually more uniform at night than in the daytime, the
signal lantern is probably the most reliable of all visual signaling
outfits. The advantages of this form of apparatus are its portability,
speed of operation, and comparatively great range. The principal
disadvantages are due to the interference caused by rain, fog, and
moonlight. The speed attainable with the lantern is about the same as
that attainable with the heliograph.


ROCKETS AND SHELLS.

Two distinct kinds of rockets and shells are issued, one of which is
adapted to day and the other to night signaling. Shells and rockets of
the amber smoke type with parachutes are used in the daytime, while
shells (red and white) and sequence rockets are used at night.

[Illustration: FIG. 11.--Signaling rocket and accessories.]

[Illustration: FIG. 12.--Signaling shells.]

_Description._--The shells are all single shot and are fired from a
5-inch portable mortar, attaining a height of about 550 feet. The
report of explosion can be heard at varying distances up to 5 miles,
depending on weather conditions. The parachute attached to the smoke
shell suspends a small light wooden tube which, after ignition,
emits smoke for from four to six seconds. The red and white shells,
on bursting, discharge a shower of red and white fire which can be
observed for some time, in fact almost until the sparks fall to the
ground.

Rockets for both day and night signaling are equipped with parachutes.
The smoke rocket is of similar construction to the smoke shell. The
sequence rocket is so arranged at the base that threaded sections of
combustible material burning either red or white can be attached to it.
Rockets ascend about 700 feet.

Each rocket and shell is supplied in a cylindrical sealed tin can,
which also contains a port fire, wind matches, and for the rockets
a stick in four sections. On the outside of the can is a label
designating the kind of shell or rocket therein contained. These cans
are easily opened by pulling a ring and require no special opening tool.

_Operation._--In firing shells the mortar should be surrounded by earth
or sand, preferably placed in sacks. The fuse for all shells is very
rapid and should be ignited by attaching the port fire to a long stick.

All of the old type Signal Corps mortars, originally designed to
withstand a pressure of 1,000 pounds per square inch, and made of
ordinary iron pipe, are considered unsafe and should be immediately
destroyed. The new mortars, recently made for the Signal Corps by
the Ordnance Department, are of cold-drawn steel having a tensile
strength of 6,000 pounds per square inch, which is more than the
maximum pressure for firing any of the Signal Corps bombs. They are
stamped "Signal Corps, U. S. A., Model 1907," or "Rocket Gun, Watertown
Arsenal, 1907."

The sequence rocket is prepared for use by attaching red or white
sections to the base in such a combination as to form letters of the
alphabet which it is desired to use. Letters containing the same color
in sequence are very difficult to read and should be avoided whenever
possible. If necessary to use them, blank sections furnished for the
purpose should be inserted between the units. The base of the rocket
will secure six units.

When rockets are to be fired the sticks must be firmly attached, the
rocket placed upright in a trough, upon a frame, or against a post.
If the fuse is beneath the paper covering the "choke" orifice, the
paper should be torn off and the rocket ignited by a port fire. In
the rockets now used the fuse extends through the covering and can
be lighted direct. If the night be damp this fuse should be exposed
only a moment before the rocket is fired. If several rockets are to
be fired in succession it is well to prepare them all at the same
time, and to have them all stood upright, but each separated from the
other at a distance of at least 6 feet, else one may ignite the other
accidentally. In firing for chronosemic signals, one rocket ought to be
kept ready upon the frame and in reserve, to be fired in place of one
that fails.

If a rocket misses fire it is to be taken from the stand and laid on
the ground. Its place is at once supplied by a similar rocket, fired
in its stead. The failing rocket is laid on the ground pointed away
from the station in order that if it has only hung fire and should
afterwards ignite it may not disarrange the signal shown or injure any
one of the party. If the wind blows freshly the rocket to be fired
should be inclined slightly against the wind.

Signal rockets and shells are furnished in sealed cans and should not
be removed therefrom until ready for use. Strict economy should be
observed in the use of these articles and on no account should they be
used for purposes of display.

_Employment._--Rockets and shells are especially valuable in making
preconcerted or emergency signals. On account of the great amount
of ammunition required it is impracticable to spell out messages
with them. These articles should be supplied to outposts, detached
stations, etc., to be used for signaling the approach of the enemy or
the happening of unexpected events, the necessity for promptly knowing
which is important.


THE SEMAPHORE.

If signal stations are to be permanently occupied, and it is
impracticable to electrically connect them, communication may be
facilitated by erecting semaphores.

Semaphores, while primarily used for day signaling, can be
advantageously used at night by attaching lights to the arms.

The navy semaphore consists of four arms pivoted at the ends, three
on one side of the upright, or pole, and one on the other side. These
arms have three positions: Horizontal; upward at an angle of 45° to the
horizontal; downward at an angle of 45° to the horizontal.

Full instructions for the operation of the semaphore, and also for the
use of balls, cones, drums, pennants, and whefts as distant signals,
are given in the International Code of Signals.


THE SEARCHLIGHT.

The electric searchlight, when available, can often be successfully
employed for night signaling, frequently affording efficient means of
communication between ships and shore stations, when wireless working
is impracticable. This system of visual signaling is practicable and
especially valuable where the stations are, on account of the terrain,
not intervisible.

_Methods of employment._--In signaling with the searchlight the usual
method of handling the shaft or beam is identical with that employed
with the flag. In the first position the beam is shown vertically,
while motions to the right, the left, and directly serve to indicate
the elements of the alphabet. Chronosemic signals may also be used in
searchlight signaling, the shaft of light being directed intermittently
on some conspicuous object, such as a cloud, balloon, or high mountain
top.


COSTON SIGNALS.

These signals are pyrotechnic compositions which burn with great
intensity of light and color. The colors red, white, and green are
found best suited for signaling. The signals are prepared in the form
of cartridges and are burned from a holder. The colors burned may
indicate the elements of any alphabet, or such other special signals as
may be desired.


VERY'S NIGHT SIGNALS.

The Very system employs projected red, white, and green stars, which
are shot from pistols held in the hand.

_Description._--The Very pistol is a breechloading, single-shot pistol
with an 8-inch steel barrel chambered to receive a 12-gauge commercial
shotgun shell. Brass shells are used and are packed in boxes colored
to indicate the character of stars employed in loading. The color of
the star fired may indicate an element of any alphabet or any special
signal which may be desired. The stars rise to a height of about 200
feet and remain visible for some time.


THE ARDOIS SYSTEM.

The Ardois system is a special system of night signaling designed
to utilize combinations of red and white signal lights in forming
the elements of any desired alphabet. Four signal lamps capable of
displaying either red or white lights are attached at convenient
intervals to a vertical cable or staff rigged between the top of a mast
and the deck, if on shipboard, or the ground, if on shore. Illumination
is furnished by electrical means and any desired combination of lights
is automatically obtainable by operating a keyboard.

This system is valuable on vessels or at permanent shore stations, but
the great expense of installation precludes its general use. Wiring
diagrams and technical instructions relative to this apparatus are in
all cases furnished when the same is issued.

[Illustration: FIG. 13.--The Very pistol.]


SOUND SIGNALS.

When recourse to any method of sight signals can not be had on account
of weather conditions or lack of suitable apparatus, sound signals
may often be advantageously used. The commoner means of furnishing
sound signals are the horn and the whistle, though many other kinds of
apparatus are practicable. The necessary elements of any system can be
indicated by one short, two shorts, and a long blast. The advantage
of this system of signaling is that it can be used in any kind of
weather, both in daytime and at night. On the other hand, sound signals
are generally more difficult to read than sight signals and tend to
disclose the presence of stations to hostile forces.


IMPROVISED SIGNALING METHODS.

The object of this chapter has been to describe only the standard
visual signaling equipment issued and generally utilized. Besides the
methods detailed, there are many others which may be successfully
employed by the ingenious signalman when the necessity for them arise.
The use of any means of transmitting signals whatever is justifiable
when for any reason the regular apparatus is not available. Special
conventional scout signals are given in paragraph 82, Field Service
Regulations.

In the field many instances will occur where it will be necessary
to transmit information rapidly without recourse to the authorized
equipment. This will be especially true of outposts, detached stations,
patrols, and other small bodies of troops, and it will devolve upon
individual commanders to improvise methods of signaling best suited to
the occasion and the conveniences at hand.




CHAPTER III.

ALPHABETS OR SYSTEMS OF SIGNALS.



    SIGNAL ALPHABETS.

                 AMERICAN        CONTINENTAL        ARMY AND
    Letters--     MORSE.            MORSE.           NAVY.

    A            · --             · --                  22

    B            -- · ·  ·        -- · ·  ·           2112

    C            · ·   ·          -- · -- ·            121

    D            -- · ·           -- · ·               222

    E            ·                ·                     12

    F            · -- ·           · ·  -- ·           2221

    G            -- -- ·          -- -- ·             2211

    H            · ·  · ·         · ·  · ·             122

    I            · ·              · ·                    1

    J            -- · -- ·        · -- -- --          1122

    K            -- · --          -- · --             2121

    L            ----             · -- · ·             221

    M            -- --            -- --               1221

    N            -- ·             -- ·                  11

    O            ·  ·             -- -- --              21

    P            · ·  · ·  ·      · -- -- ·           1212

    Q            · ·  -- ·        -- -- · --          1211

    R            ·  ·  ·          · -- ·               211

    S            · ·  ·           · ·  ·               212

    T            --               --                     2

    U            · ·  --          · ·  --              112

    V            · ·  · --        · ·  · --           1222

    W            · -- --          · -- --             1121

    X            · -- · ·         -- · ·  --          2122

    Y            · ·   · ·        -- · -- --           111

    Z            · ·  ·  ·        -- -- · ·           2222

    &            ·  ·  · ·

    tion                                              1112

    Numerals--   AMERICAN          CONTINENTAL      ARMY AND
                  MORSE.              MORSE.         NAVY.

    1            · -- -- ·        · -- -- -- --       1111

    2            · ·  -- · ·      · ·  -- -- --       2222

    3            · ·  · -- ·      · ·  · -- --        1112

    4            · ·  · ·  --     · ·  · ·  --        2221

    5            -- -- --         · ·  · ·  ·         1122

    6            · ·  · ·  · ·    -- · ·  · ·         2211

    7            -- -- · ·        -- -- · ·  ·        1222

    8            -- · ·  · ·      -- -- -- · ·        2111

    9            -- · ·  --       -- -- -- -- ·       1221

    0            ------           -- -- -- -- --      2112


    Punctuation--

    . Period    · ·  -- -- · ·    · ·   · ·   · ·

    : Colon       Ko              -- -- -- · ·  ·

    ; Semicolon   Si              -- · -- · -- ·

    , Comma       · -- · --       · -- · -- · ---

    ? Interrogation  -- · ·  -- ·   · ·  -- -- · ·

    ! Exclamation  -- -- -- ·       -- -- · ·  -- --

    Fraction line  -

    - Hyphen       Hx               -- · ·  · ·  --

    ' Apostrophe                    · -- -- -- -- ·

    £ Pound Sterling                -- · -- · ·

    () Parenthesis  Pn              -- · -- -- · --

    " Quotation marks  Qn          · -- · -- · -- · --

    Paragraph       -- -- -- --

    Brackets        Bn

    Dollar mark     Sx

    Dash            Dx

    Underline       Ux

The following abbreviations, conventional signals, and code calls are
authorized in visual signaling:


    ABBREVIATIONS.

    a     after.

    b     before.

    c     can.

    h     have.

    n     not.

    r     are.

    t     the.

    u     you.

    ur    your.

    w     word.

    wi    with.

    y     yes.


    CODE CALLS.

    International Code use               ICU
    (Navy) telegraph dictionary use      TDU
    (Navy) geographical list use         GLU
    (Navy) general signal use            GSU
    Navy list use                        NLU
    Vessel's numbers use                 VNU
    Cipher "A" use[a]                    CAU
    Cipher "B" use[a]                    CBU
    Cipher "C" use[a]                    CCU

FOOTNOTE:

[a] These calls are for preconcerted use in or with the navy.


Although the use of but one alphabet is authorized in visual signaling
in the U. S. Army, emergencies may arise where it may be imperative to
use either the Army and Navy, the Continental Morse, or the American
Morse alphabet. Instructions for the use of either alphabet under such
conditions are given.


EXECUTION OF SIGNAL ALPHABETS.

THE ARMY AND NAVY ALPHABET.

SIGNALING WITH FLAG OR TORCH, HAND LANTERN, BEAM OF SEARCHLIGHT, AND
HELIOGRAPH.

There is one position and three motions. The position is with the flag
or other appliance held vertically, the signalman facing directly
toward the station with which it is desired to communicate, his body
erect and feet sufficiently separated to insure stable equilibrium. The
first motion ("one" or "1") is to the right of the sender, and will
embrace an arc of 90°, starting with the vertical and returning to it,
and will be made in a plane at right angles to the line connecting the
two stations. The second motion ("two" or "2") is a similar motion to
the left of the sender. The third motion ("front," "three," or "3") is
downward directly in front of the sender and instantly returned upward
to the first position.

The beam of searchlight will be ordinarily used exactly as the flag,
the first position being a vertical one.

To use the torch or hand lantern, a footlight must be used as a point
of reference to the motion. The lantern is more conveniently swung out
upward to the right of the footlight for "1," to the left for "2," and
raised vertically for "3."

In using the heliograph, the first position is to turn a steady
flash on the receiving station. The signals are made by short and
long flashes. Use short flashes for "1," two short flashes in quick
succession for "2," and a long, steady flash for "3." The elements for
a letter should be slightly longer than in sound signals.

Each word, abbreviation, or conventional signal is followed by "3."

The full address of a message is considered as one sentence and will be
followed by the signal "33."

The signal to indicate that "cipher follows" and "cipher ends" is
with the flag and torch "XC3," and with other methods, except the
International Code, by "XC." It will always precede and follow a cipher
message or such part of a plain text message as is enciphered.

The following conventional signals are authorized in the use of the
army and navy alphabet:

    End of a word                                 3
    End of a sentence                            33
    End of a message                            333
    Numerals follow (or) numerals end           xx3
    Signature follows                        sig. 3
    Error                                   12 12 3
    Acknowledgment (or) I understand        22 22 3
    Cease signaling                    22 22 22 333
    Cipher follows (or) cipher ends      2122 121 3
    Wait a moment                            1111 3
    Repeat after (word)       121 121 3 22 3 (word)
    Repeat last word                     121 121 33
    Repeat last message             121 121 121 333
    Move a little to the right            211 211 3
    Move a little to the left             221 221 3
    Signal faster                            2212 3


THE MORSE ALPHABETS.

TO SIGNAL WITH THE FLAG, TORCH, HAND LANTERN, OR BEAM OF SEARCHLIGHT.

The dot is made by a motion to the right of the sender embracing an arc
of 90°, starting from the vertical and returning to it, in a plane at
right angles to the line connecting the two stations.

The dash is made by a similar motion to the left.

The space which occurs only between dots is made by prolonging the
signal for the last dot for an interval of time equal to the time of
an additional dot, the staff of the flag, the beam of the searchlight,
etc., being maintained in a horizontal position for the time specified.
The signal so made would therefore represent a dot and space.

The letter "C" is accordingly made thus: Right, right prolonged, right.

The long dash ("L") is distinguished from the short dash ("t") by
prolonging the signal to the left for a period of time equal to
one dot. The long dash representing "naught" is similarly made by
prolonging the signal to the left for a period of time equal to two
dots.

The "front" signal is made by lowering the flag from the vertical
position to the front and immediately returning it to the vertical
position.

A slight pause is made between each signal.

The following conventional signals are authorized, using the Morse
alphabets:

    End of word            one front.
    End of sentence       two fronts.
    End of message       three fronts.


TO SIGNAL WITH THE HELIOGRAPH OR FLASH LANTERN.

The dot is made by pressing down the key of the shutter and immediately
releasing the same.

The short dash is made by pressing down the key and holding it down for
a period equal to two dots.

The long dash ("L") is made by holding down the key for a period equal
to three dots while the longer dash (naught) requires the key to be
held down for a period equal to four dots.

The space is made on the heliograph as in ordinary telegraphy by the
absence of any signals for a period equal to the time of one dot.

On the heliograph the letter "C" is made as follows: Short flash, short
flash, interval, short flash.

[Illustration: Fig. 14.]

[Illustration: Fig. 15.]

When the call of a station is acknowledged, both stations will adjust
each on the flash of the other. When adjustments are satisfactory,
the station called will acknowledge and cut off its flash, and the
calling station will proceed with its message.


INTERNATIONAL CODE OF SIGNALS.

_Description._--By means of the International Code of Signals people
of different nationalities may communicate with each other, although
neither party has knowledge of any language save his own native
language. The code is, as its name indicates, international, and every
seagoing vessel of every nation is equipped with its flags. The Code
of Signals contemplates the use of 26 flags (figs. 14 and 15); one for
each letter of the alphabet and a code pennant. Complete instructions
relative to the use of this code are contained in a book issued by
the Hydrographic Office, Navy Department, and known as the "The
International Code of Signals." In using this system the signals are
displayed by hoisting combinations of two, three, or four flags. All
possible combinations represent words, expressions, or phrases, which
may be found in the "International Code of Signals," referred to above.

_Two-arm semaphore._--This system is frequently used by the United
States Navy, the following instructions covering the use of the system:

1. To communicate with a station:

Face the station and wave the flags over the head to attract
attention, making at frequent intervals the call letter of the
station. When the station called is ready to receive the message, it
answers by displaying its own call letter until the sender makes the
"alphabetical" or "numeral," as the case may be. Then proceed with the
message. At the end of each word bring the flags across the lower part
of the body.

2. To call a ship:

Hoist International Code letter J and make code letter of ship; then
proceed as in article 1.

3. To make a general semaphore signal:

Hoist cornet; all ships answer by answering pennant; then make signal.

4. At the end of the message extend the arms horizontally and wave the
flags until the receiver answers in the same manner, showing that the
message is understood.

Should the receiver miss a word, he signifies the fact by waving the
flag over his head. The sender will then cease signaling and wave his
flags similarly to show that he understands. The receiver then makes
"repeat last word," or whatever he wishes to say.

Should the sender make a mistake, he will make the "error" signal until
answered by the receiver with the same signal. He then proceeds with
the message.


THE ARDOIS SYSTEM.

In using this system in connection with the Army and Navy Code, the red
lamp indicates "1" and the white lamp "2." Four lamps are placed on a
vertical staff and electrically illuminated to indicate the numerals
of the Myer Code, which represents the letters of the alphabet.
For instance, white-white, or "22," represents the letter "A," and
white-red-red-white, or "2112," represents the letter "B," etc. In this
system the lights indicating the letters of the alphabet are read from
the top downward.

When the lamps are placed horizontally, they are read from the sender's
right to his left, and consequently from the receiver's left to his
right.

When the letters of the alphabet are to be used to indicate the meaning
set opposite them in the following tabulation, the upper light of the
display is pulsated. This is effected by means of a special pulsating
key. Special signification is not given "I" and "T," they being
represented by a single lamp.

    -----------+----------------------------------------
     Steady    | Upper light pulsated.
      display. |
    -----------+----------------------------------------
     A         | Cipher "A" use.
     B         | 0 (naught).
     C         | Repeat (following rule for conventional
               |  signals under wigwag code).
     D         | Telegraphic dictionary use.
     E         | Error.
     F         | 4.
     G         | 6.
     H         | Compass signals use.
     I         |
     J         | 5.
     K         | Negative.
     L         | Geographical list use.
     M         | 9.
     N         | Cipher "B" use.
     O         | Cipher "C" use.
     P         | Affirmative.
     Q         | Interrogatory.
     R         | International code use.
     S         | General signals use.
     T         |
     U         | Navy list use.
     V         | 7.
     W         | Annulling.
     X         | Numerals.
     Y         | Vessels' number use.
     Z         | 2.
     Letters   | 3.
     Code call | 8.
     Interval  | Boat signals use.
    -----------+----------------------------------------

Before numerals are made, the distinctive signal for "numerals" "X"
is shown and the upper light is pulsated, which serves still further
to distinguish them from letters. The resumption of letters after
using numerals will be indicated by the upper light being no longer
pulsated, but the display "letters" ("3") will be turned on as an
additional indication.

The acknowledgment of the correct receipt of a message will be
indicated by the letter "R." If the message has not been fully
received, or if it is not understood, indication thereof will be made
by signaling the letter "G."

The end of a word is indicated by 2212.


COSTON SIGNALS.

Letters of the army and navy alphabet may be represented at night by
Coston lights, port fires, or other colored pyrotechnical lights by
displaying the "red" for one and the "white" for two.

In using the Morse alphabet the "red" represents the dot and the
"white" the dash.

Coston signals and other similar lights are best suited for
preconcerted signals.


VERY'S NIGHT SIGNALS.

The navy signal book is used, to which the following explanation refers:

The letter R stands for red and the letter G for green, and each letter
designates a separate star or cartridge. Bracketed stars are a pair of
different colors, discharged together from two pistols. The system is
based on the Army and Navy Code, red representing "1" and green "2."

     1--RRRR.
     2--GGGG.
     3--RRRG.
     4--GGGR.
     5--RRGG.
     6--GGRR.
     7--RGGG.
     8--GRRR.
     9--RGGR.
    10--GRRRG.

  Affirmative, or "Yes"                                RGRG
  Negative, or "No"                                    GRGR
  Numeral                                              GRGG
  Interrogatory                                        RGRR
  Annulling                                            RRGR
  Divisional point, date, designator, or interval      GGRG
                          {R}
  Telegraphic dictionary, { } bracketed.
                          {G}
                     {R}
  Geographical list, { } followed by a rocket.
                     {G}
                                   {R}
  Boat signals, rocket followed by { }
                                   {G}
                                                    {R} {R}
  Navy list                                         { } { }
                                                    {G} {G}
  General call, rocket followed by G.
  Message call, G without the rocket.
  The squadron, division, section, or ship's call, the
    "number" of squadron, division, section, or ship.
  Answering, or "I understand"                            R
  Repeating, or "I do not understand"                     G
  Danger or distress, R repeated several times in
    quick succession.


ROCKET SIGNALING.

In general, rockets and shells are best used in displaying preconcerted
signals.

Sequence rockets may also be used to display different colored lights
in sequence to represent letters or numerals of the army and navy
alphabet. The method of attaching the sections in the base of the
sequence rocket is described in Chapter III. In using sequence rockets
in this manner, the element "1" of the army and navy alphabet is
represented by a red star, while a white star represents the element
"2." To send the letter "A" a rocket showing two white stars is sent
up. If "B" is to be sent, a rocket showing white-red-red-white is
discharged. Each star burns for four to six seconds, and there is
a slight interval between the visibility of each star. Between two
or more stars of the same color, as "A," "N," "D," "dummies," which
show no light and carry the fire to the next star to be ignited, are
employed.

In the preparation of codes for signals with rockets or bombs there
should always be arranged a "preparatory signal" which means "Are you
ready?" etc., and an "answering signal," which means "Repeat your last
signal," etc., a signal "annul," which means "Disregard last signal,"
and a signal to signify the correct receipt of the complete message, or
"Signal seen and understood."

[Illustration: TWO-ARM SEMAPHORE ALPHABET, U. S. NAVY.]

[Illustration]

[Illustration]

    CONVENTIONAL SIGNALS.

    End of word            see instructions.
    End of message         see instructions.
    Error                  see instructions.
    Repeat last word       =C=, 'end of word', once.
    Repeat last message    =C=, 'end of word', 3 times.
    Use paper and pencil   =P=, 'end of word', twice.

    ABBREVIATIONS.

    =A= "end of word"  after
    =B=   "  "   "     before
    =C=   "  "   "     can
    =H=   "  "   "     have
    =N=   "  "   "     not
    =R=   "  "   "     are
    =T=   "  "   "     the
    =U=   "  "   "     you
    =UR=  "  "   "     your
    =W=   "  "   "     word
    =WI=  "  "   "     with
    =Y=   "  "   "     yes
    =PG=  "  "   "     permission granted
    =NG=  "  "   "     permission not granted
    =XX=  "  "   "     numerals follow

[Illustration: SUMMARY OF SIGNALS, ARMY AND NAVY ALPHABET.]




CHAPTER IV.

THE FIELD MESSAGE.


_Definition._--The term "field message" is applied to all messages sent
over field lines of information. All field messages for transmission
over field lines of information by electrical or visual means should be
plainly written by the sender on the blank forms in the United States
Army Field Message Book. The practice of verbally delivering telegrams
to enlisted men for transmission should invariably be discouraged.

"In framing telegrams, all words not important to the sense will be
omitted. The last name of the officer addressed, or his title, and the
last name of the sender are generally sufficient." (Paragraph 1198,
Army Regulations.)

_The blank form._--The United States Army Field Message Book issued
by the Signal Corps is 4-5/8 inches wide by 6¾ inches long, and
contains 40 message blanks with duplicate tissue sheets and two sheets
of carbon paper.

The message is written on the yellow sheet, which can be torn out for
delivery. The carbon sheet is attached to the book, and contrary to the
custom in most carbon duplicating books, is placed _under_ the tissue
sheet when a message is being written. When not being used, the carbon
sheet should invariably be kept in the back of the book. When the upper
carbon sheet has become worn out, it should be torn out and the second
carbon sheet used instead. The blank form is shown in figure 16. The
back of the blank is ruled in squares and provided with scales for use
in making sketches.

[Illustration:

  +-------------------------------+-----+---------+--------+-------+-----+------
  |                               | No. | Sent by |  Time  | Rec'd |     |
  |  =U. S. ARMY FIELD MESSAGE.=  | _1_ |  K _Mo_ | _1.15  |  by   |Time |Check
  |                               |     |         | A. M._ |       |     |
  |                               |       [These spaces for Signal       |
  |                               |            Operators only]           |
  +-------------------------------+--------------------------------------+------
  |       Communicated by         |        [Name of sending detachment]
  | BUZZER, PHONE, TELEGRAPH,     | From___Headquarters 1st Corps_,_____________
  | WIRELESS, LANTERN, HELIO,     |
  | FLAG, CYCLIST, FOOT MESSENGER,|     [Location of sending detachment]
  | MOUNTED MESSENGER.            |At___Taylors School House, Kansas___________
  |   (Underscore means used)     |Date__May 1, 1909___Hour__12.45 P. M.___No.__
  +-------------------------------+
  |____To_______________________________________________________________________
  |_______________Signals_______________________________________________________
  |_________________Platte City, Mo.____________________________________________
  |_____________________________________________________________________________
  |_____Request ten miles buzzer wire be sent here quick________________________
  |_____________________________________________________________________________
  |____________________________________Jones____________________________________
  |_____________________________________________________________________________
  |    Received_____________________________________

FIG. 16.]

_Writing the message._--In writing the message the name of the sending
detachment should appear after the heading "from" on the upper line,
as "from Headquarters 1st Brigade," while the location of the sender
should appear on the second line after the heading "at." The heading
"hour" on the third line should show the hour the message was _written_
and not the hour the message was transmitted. The heading "received" at
the bottom of the page is filled in by the addressee and shows the time
of the receipt of the message by him.


INSTRUCTIONS TO OPERATORS.

_Use of message blank._--The field message blank will be used for field
messages both sent and received.

_Duties of sending operators._--The _sending_ operator will enter the
time when the message is handed him for transmission in the left-hand
corner at the bottom of the blank opposite the word "Received." He will
enter in the proper places, at the head of the blank, the number of
the message, the call letter of his station, with his personal signal,
the check (number of words or groups of cipher contained in message,
counting address and signature), and, after "OK" has been received, he
will enter the time the message was sent, and the call letter of the
receiving station, with the personal signal of the receiving operator.

_Order of transmission._--To transmit a message, the operator will
send: (1) The number of message and call letter of his station; (2) his
personal signal; (3) the check; (4) "fm" followed by name of sending
detachment; (5) "at" followed by location of sending detachment and
date; (6) "Ho" followed by hour (a. m. or p. m.) message was written;
(7) address in full; (8) period, (· · -- -- · ·); (9) body of message;
(10) "sig" (signature follows); (11) signature.

_Duties of receiving operators._--The _receiving_ operator will add to
the message received, the month, date, and year, and omit the "sig,"
"fm," and "at," and, after satisfying himself that the check and number
of words correspond will give "OK" followed by the call letter of his
station and his own personal signal. He will then enter in the proper
places, at the head of the blank, the call letter of his own station,
with his personal signal and the time the message was received.

_Communications confidential._--Communications transmitted by telegraph
or signals are always confidential and will only be revealed to those
officially entitled to receive them.

_Checking the message._--In preparing the "check" of the message, all
words and figures written in the address, body of the message, and the
signature will be counted.

In counting the check of a message, all words, whether in plain
English, code, or cipher, pronounceable or unpronounceable, or initial
letters, will be counted each as one word. The abbreviations for the
names of places, cities, towns, villages, States, Territories, and
Provinces, will be counted as if written in full. In the names of
towns, counties, countries, or States, all of the words will be counted.

Abbreviations of weights and measures in common use, and cardinal
points of the compass, will be counted each as one word.

To prevent liability to error, numbers and amounts should be written in
words, and when not so written, the receiving operator will request
that it be done. If the writer declines to write the amounts in words,
the message will be accepted as written, and each figure will be
counted as one word.

Figures, decimal points, and bars of division, and letters will be
counted each separately as one word.

In ordinal numbers, the affixes, st, d, nd, rd, and th, will each be
counted as one word.

In transmitting the telegram shown in figure 16, the following would be
sent by the operator:

     No 1 K Mo CK 14 OB fm Headquarters 1st Corps at Taylor's School
     House Kan 1 ho 1245 PM to Signals Platte City Mo. Request ten
     miles buzzer wire be sent here quick sig Jones




CHAPTER V.

THE SIGNAL STATION.


LOCATION OF STATIONS.

In field operations tactical considerations will usually prescribe
within certain limits the number and general location of signal
stations. The general directions for deployment being given, the
signalman will be called upon to demonstrate his skill in the selection
of particular locations most conducive to the efficient service of
information.

[Illustration: FIG. 17.--Field signal station.]

_General considerations._--Considering all things, the best location
for a signal station is one which affords maximum visibility and at the
same time minimum exposure to hostile observation. These conditions,
apparently paradoxical, can be more or less reconciled by the exercise
of ingenuity on the part of the signalist. A good theoretical knowledge
of the special requisites of signal sites, together with the ability
to apply it to the conditions arising in any given case, will result
in securing the best obtainable locations.

The first essential of the signal station is visibility, the second
being that of concealment from hostile observation. In acquiring a mean
between conflicting requirements, the following special considerations
in the selection of stations should be considered.

_Backgrounds._--Backgrounds are important factors in the selection of
signaling sites.

Sky backgrounds are desirable as affording strong contrast and are
therefore conducive to celerity in the transmission of signals. They
are rare and can only be secured when stations are located on the exact
crest of ridges, on mountain peaks, or on lands which bound the horizon
of view from the other stations. Stations with sky backgrounds, while
affording the best facilities for transmission, are little adapted to
the requirement of secrecy.

Dark backgrounds are far more common and more easily obtainable than
sky exposures. They afford the maximum means of concealment from
hostile observation, but materially reduce the range, speed, and
accuracy of signal transmission.

Mixed or broken backgrounds are those which display varied colors
behind the signals. Backgrounds of this description do not accord with
either of the essential requirements of the signal station and should
be avoided whenever possible.

In general, sky backgrounds should always be selected for signal
stations when conditions are such that the requirement of secrecy can
be dispensed with; if, on the other hand, there is reason to fear that
the signals may be intercepted by the enemy, dark backgrounds should
invariably be chosen, even though the disadvantages they impose, render
them less desirable visually.

_Azimuth of stations._--The azimuth of signal stations should, if
possible, be such that the visual lines of information should intersect
the vertical plane through the apparent course of the sun, at a
considerable angle. Stations located so as to be unavoidably viewed
from these directions during portions of the day are very liable to
appear enveloped in a haze, and telescopes, if turned upon them, are
filled with dazzling light. If the location of stations on or close to
the sun line is unavoidable, sites affording sky exposures should be
chosen. Exposures of this kind obviate to a great extent the difficulty
of sun haze and should be secured when this difficulty is encountered
and it is impracticable to change the azimuth of the station.

_Altitude._--The location of signal stations at high altitudes will
tend to obviate difficulties arising from smoke, haze, and dust. The
undulation of the atmosphere noticeable on a hot summer's day is
always less at a distance from the earth's surface, and it is often
practicable to read signals from a tree or housetop when they would be
unintelligible from the ground. This air undulation is less over spots
well shaded than those exposed to the glare of the sun, a fact that
should be borne in mind in all telescopic examinations. Another reason
for locating stations at high altitudes is because the cool night air,
the smoke and dust of the day, and heavy mists lie close to the ground,
filling the depressions and lowlands, while the higher points remain
in view. Stations on high ground are then equally well adapted to day
and night signaling. Sites and selections of this kind of terrain will
not only often preclude the necessity for changes of location, but also
will allow the continuous working of the station when signals made from
lower positions would be invisible. In foggy or murky weather peaks and
mountain tops are usually enveloped in mist, and under these conditions
stations should be situated on lower ground.

_Determination of background color._--The color of the background of a
station is that color against which the signals appear to be displayed
when viewed from the distant station. Having chosen a point entirely
in view of the station or stations to be communicated with, and having
fixed the exact position of the signaling apparatus, the color of the
background should be determined as carefully as conditions of terrain
will permit. If the elevation of the distant station is without doubt
greater than that of the home station it is safe to assume that the
color of the background will be that of the objects directly around and
behind it. On the other hand, if the distant station unquestionably
occupies the lower position, a sky exposure will usually result. In
locating stations it is very difficult, if not impossible, especially
at long ranges, to determine the color of the background as viewed from
the distant station when the stations are approximately on the same
level. This can only be done by proceeding in front of the home station
and taking such a position that it can be viewed with the eye on the
line of sight between the stations. The telescope should be established
over the initial point of the home stations and directed on the distant
station. The observer for background should proceed to a point where
his head is in the center of the field of the telescope. Looking back
at the home station from this point, the color of the objects about and
just behind the initial point will be the color of the background. The
correct determination of background color from the vicinity of home
stations is usually difficult and unsatisfactory, and it is considered
the best method to establish communication with the distant station by
simultaneously using several kinds of signaling apparatus, that kind
producing the most intelligible signals being retained for continued
use.

_Choice of apparatus._--Sunlight conditions permitting, the heliograph
will ordinarily be used for day signaling on account of the advantages
of the great range and speed afforded by it. When its use is prohibited
by weather conditions, the flag will be substituted for it. The white
flag will be used against dark and the red against sky or broken
backgrounds. The distant station is the better judge as to which color
flag is best suited to given conditions and the color indicated by it
should invariably be used. For night signaling, the acetylene lantern
is usually employed. Long-range night signaling should be done with
the searchlight if available. The employment of the semaphore, in
daytime, and the Ardois system, at night, will be confined to more or
less permanent stations. Rockets, shells, night fires, etc., are only
employed for special or emergency signals.

_Miscellaneous considerations._--For various reasons stations should
not be located at or near camp grounds. These localities usually
afford mixed backgrounds, and the presence of dust and smoke and
the interference caused by moving bodies of troops and trains will
militate against the efficient transmission of signals. Stations
located in vicinities of this kind are also subject to annoyance from
noise and visits of unauthorized persons. Signal stations should be
convenient for messenger service and hence as near commonly traveled
roads as the physical contour of the country will permit. Locations
for signal stations should be so selected that the visual lines do not
cross traveled roads, camps, etc., as dust and smoke in the daytime and
lights at night are factors in determining the visibility of signals.
Signal stations can if necessary be artificially concealed by erecting
screens constructed of limbs of trees, etc., about the flanks and rear.
Sheltered positions should be utilized in windy weather.

_Intervisibility table._--The following table shows the extent of
horizon for different heights above the sea level--that is, it shows
how far one can see an object which is itself at the level of the sea:

    ----------------------------+-----------
                                |  Distance
    Height of the eye above sea |in statute
             level.             |   miles.
    ----------------------------+-----------
     10 feet                    |      4
     15 feet                    |      5
     20 feet                    |      6
     30 feet                    |      7
     40 feet                    |      8
     50 feet                    |      9
     60 feet                    |     10
     70 feet                    |     11
     85 feet                    |     12
    100 feet                    |     13
    115 feet                    |     14
    130 feet                    |     15
    150 feet                    |     16
    200 feet                    |     18
    230 feet                    |     20
    300 feet                    |     23
    350 feet                    |     25
    500 feet                    |     30
    700 feet                    |     35
    900 feet                    |     40
    ----------------------------+-----------

A formula to determine approximately the limits of visibility from
a given height is as follows: The square root of the height of the
station in feet multiplied by 1.26 equals the distance in miles at
which the signal is visible.

Hence, an observer whose eye is 30 feet above the sea can distinguish
an object 7 miles distant, provided it is at the sea level; but if the
object is itself 15 feet above the sea he can make it out 7 + 5 = 12
miles off.


FINDING A STATION.

To find a signalman near any known station, note with the unaided eye
some prominent landmark near which the looked-for person or object is
supposed to be, and direct the telescope upon the place, as sight is
taken over a gun barrel, covering the object; if the eye is now placed
at the eyeglass of the telescope, the prominent or directing landmark
will be found in the field of view. It will be easy then to scale the
country near the marker until the signalman is found. This method is
often necessary at night, when only a point of light is seen far off
through the darkness, and the telescope must be turned upon it. When
the compass bearing of the object sought for is known, the telescope
may be aligned by a line drawn with the proper compass bearing.
Commencing then with the view at the horizon, the telescope is slowly
moved from side to side, taking in fresh fields of view each time a
little nearer to the observer, until the whole country shall have been
observed from the horizon to quite near the station. When the general
direction only of the object can be given and it is sought for, the
whole landscape in that direction to the horizon should be divided into
sections by imaginary lines, the limits of these sections being bounded
between visible landmarks through which the bounding lines are supposed
to pass. Each section should be scrutinized little by little until the
glass has been passed over every spot. Such search will seldom fail to
be successful.

The magnetic bearings of all stations with which another station has
worked should be carefully noted and made matter of record in the
office directly concerned, so that advantageous use may be made of this
data. In addition, guide lines may be established by driving two stakes
firmly into the ground and close to each other. A prolongation of a
line through the center of one post and marked on the adjacent one will
strike the distant station. Under each line should be written the name
of the station which it marks.

Signalers upon permanent or semipermanent stations will examine, from
time to time, every prominent point within signal distance, to see if
communication is attempted therefrom.

Attempts to attract the attention of a known station, in order to
be successful, must be persistent. They should never be abandoned
until every device has been exhausted, and they should be renewed
and continued at different hours of the day and night. It must be
remembered that efforts which have failed because the observer's
attention has been drawn in another direction may at any other moment
be successful if the observing glass chances to bear on the calling
signals.

During the whole time that signals are being made to attract attention
the calling station must watch closely with the telescope the station
called. The watch should not be relaxed until communication is
established or the station ordered abandoned.

[Illustration: FIG. 18.--Signal Corps heliograph station.]


OPERATION OF STATIONS.

_Personnel._--At signal stations where continued operation is required
at least a squad or "set of fours" is required. Physical and mental
exhaustion always result from continuous signal duty, and as
alertness of mind and body is an indispensable factor in the prevention
of errors, two reliefs of signalmen should be furnished each station
whenever practicable. The senior officer or enlisted man is in charge
of the station and is responsible for efficiency and discipline. He
will require from each man a strict and entire attention to his own
immediate duties, and permit no conversation that will distract the
men at work. He will be careful not to allow persons to loiter about
the station or within the hearing of the words called out to the
signaler. The assignment of men should be such that a continuous watch
for signals is kept and the responsibility for neglect to promptly
answer calls determined. Of the station men, one is the sender,
whose duty it is to transmit all signals to contiguous stations.
Another, the receiver, attends the telescope and reads and calls off
the signals displayed at the distant station. A third man acts as
recorder, alternately calling off the outgoing message to the sender or
transcribing the incoming message repeated by the receiver.

_Calls and personal signals._--Each station will be assigned a call
consisting of one or two letters. Each and every operator will also
have a personal signal of like character. Station calls or personal
signals when once given or assumed will not be changed except by order
of higher authority. Every station should at all times have on hand
a list of all calls and personal signals liable to be encountered in
station working. The general call suited to attract the attention of
any station whose regular call is unknown will always be a signal
represented by the letter "A" in the Morse or the letter "E" of the
Army and Navy Code.

_Opening communication._--To open communication with any distant
station whose call is known, signal the call repeatedly, occasionally
signing the call of the home station. If the regular call of the
station sought is unknown the general call above prescribed should
be used. As soon as the call is observed the called station will
acknowledge receipt by "ii ii," or "I understand," signing thereafter
its station call. These preliminaries completed, the stations are ready
for working.

It is sometimes difficult to secure the attention of stations
at unexpected hours. The force may not be strong enough for an
uninterrupted watch. To provide, so far as possible, for this
contingency, it may be concerted that if communication is required
at unusual time, or is of pressing importance, certain flags
shall be displayed, rockets discharged, smokes shown, or other
attention-compelling signals used.

When a number of stations are in view from one station and it is
desired to send a message to all or more than one station, some
preconcerted signal, as a rocket, a red light, or some peculiar flag or
torch signal, should be designated as a signal for general attention.
Upon noticing this signal all the called stations reply, and then
observe the calling station. This plan is useful when two or more
stations can, at the same time, read the signals from the one station,
and thus together receive any information to be transmitted from it.

When a signal station is to communicate with two or more stations, a
telescope should be firmly fixed bearing on each, when practicable, and
so far apart that those communicating with one station will not disturb
the other party.

_Commencing the message._--Every message is invariably commenced by the
signal "Hr" or "Anr." Sometimes at the commencement of communication
a preface will be sent in order to give some preparatory information
to the receiving station regarding the number or character of messages
about to be sent. For example, "Hr 8," means "I have eight for you" or
"Hr ck 300" means a three hundred word message follows.

_Sending and receiving._--Before the commencement of a message, care
should be taken that all the letters and characters thereof are
entirely and correctly understood by the signalman whose duty it is
to call the same to the sending operator. The message is read off by
the "reader," who first calls off a word and then spells it out letter
by letter. The "reader" should observe the signals of the operator
and invite his attention to any apparent errors. When the last letter
of a word is announced this fact will be communicated to the sending
operator.

At the receiving station the man at the telescope will call off each
letter as received and not wait until the completion of a word. On
reaching the end of a word announcement of this fact will be made to
the recorder.

_Breaking._--If the sending operator discovers that he has made
an error which will probably render the sense of the message
unintelligible at the receiving station, he will make the signal "BK"
and recommence the message, beginning at the last word correctly sent.
When the receiving station fails for any reason to get correctly what
is being sent, the sending station is interrupted by the signal "GA,"
followed by the last word correctly received. The message will then be
recommenced by the sending station at the point indicated.

_Discontinuance of transmission._--When all the messages on file at any
station have been sent the signal "NM" in Morse or "Cease signaling"
in the army and navy system, according to which code is authorized,
will be the concluding signal of the sending station. When a signal
station is operated only during the daytime, the signal "GN" will be
transmitted after all business filed up to the hour designated for
closing has been dispatched.

_Acknowledgment of receipt._--No message will be considered sent until
receipt for the same has been acknowledged. This is effected by making
either the "I understand" of the army and navy or the "OK" of one of
the Morse systems, depending upon the one authorized. In every case
the receiving operator's signal is signed after acknowledgment. When a
number of messages are continuously sent, one acknowledgment for all
will suffice and will be so understood. In receiving messages nothing
should be taken for granted and nothing considered as seen until it has
been positively and clearly in view.

_Station records._--Records kept at field signal stations will be
confined to original files of messages sent and carbon copies of
messages received. Ordinarily the only available stationery will be
the United States Army Field Message Book. Station records will be
invariably preserved as part of the station equipment until orders for
their disposition are given by higher authority. Whenever a station is
in imminent danger of capture, all records should be destroyed in the
discretion and under the direction of the operator in charge.

_Formation of signals._--Make signals with regularity; do not send one
word rapidly, the next slowly; adopt such a rate of speed as can be
read by the distant signaler without causing him to "break" frequently.
Make a distinct pause between letters. It is time gained to do so; it
is a loss of time and an annoyance to run letters together. Nothing
so distinguishes the good from the indifferent operator, visual or
telegraph, as this. When signals are being made with a flag, a fraction
of a second will be ample. In using the lantern or heliograph, the
pause between letters should be relative to the time of display of
the elements, longer than with the flag. To prevent any entangling of
the flag upon its staff, skillful handling, acquired by practice, is
necessary. It is accomplished by making a scoop of the flag against the
wind, the movement describing an elongated figure 8, thus ∞.
The motions should be made so as to display in the lateral waves the
whole surface of the flag toward the point of observation.

In using the heliograph, if the receiver sees that the sender's mirror
needs adjustment, he will turn on a steady flash until answered by a
steady flash. When the adjustment is satisfactory, the receiver will
cut off his flash and the sender will resume his message.

_Repeating the message._--It may happen that very important messages
received by signals must be verified by repeating back from the
receiving station, signal by signal, each signal used by the sending
station in conveying the message. There can be no error in signals thus
verified, and the correct transmission of the message is made certain.
For such verification each signal must be repeated by the receiving
station as soon as it is made at the sending station.

_Signal practice._--Full efficiency of the signaler can be maintained
only through constant practice, and those in charge of Signal Corps
troops should see that sufficient practice be had to insure that
accuracy and rapidity in handling messages which is so essential in
time of war.

Instruction should commence with the study of the principles of
signaling and the theories of their general use, and the pupil should
be well grounded in this study before practice is begun. He should
so memorize the alphabets to be used that no letter combination will
require thought to determine its meaning.

Daily inspections should be made to insure that all signaling
instruments, appliances, and materials are in readiness for instant
use. Defects in the apparatus annoy the sender; to a greater extent
they annoy the person to whom the messages are imperfectly sent, and
delays result that may have serious consequences.




CHAPTER VI.

CODES AND CIPHERS.


A code is a list or collection of arbitrary words or groups of letters
to each of which some ordinary word, proper name, phrase, or sentence
is assigned for meaning.

Ciphers embrace all means whereby writings may be transcribed
into occult terms. All ciphers employ some distinct method for
transcription, which method is termed a key. In practice the key is
usually applied directly in enciphering and reversed in deciphering
messages.


CODES IN USE.

The codes of the Western Union and Postal Telegraph companies are
examples of well-known codes suited to general commercial use. Besides
these, many special codes have been formulated, so as to embody
technical expressions especially adapted to use in particular lines of
industry. The War Department Code is a military code adapted to the
special needs of the military establishment in peace and war.


EMPLOYMENT OF CODES.

Codes are primarily intended for economy, but they may also be readily
employed to secure secrecy. When used solely for economy, the coded
message is said to be plain code; that is, the word or phrases of
the message are coded by direct reference to their respective code
equivalents. Thus plain code is readily translatable to anyone in
possession of a code book. When secrecy is desired, some method of
enciphering or key is employed in such a way that only persons in
possession of it can in conjunction with the code book decipher it. In
such case the message is said to be in cipher code.


CIPHER CODE.

In all codes each expression and its equivalent in plain language is
assigned a number. These numbers usually commence at unity and increase
consecutively to any desired figure. Messages may be enciphered by
means of a key number or series of numbers. An additive number, say
55 additive, requires that in enciphering a message, the fifty-fifth
word numerically greater than the proper code word shall be used; if 55
subtractive is used, the fifty-fifth word numerically smaller than the
proper code word is to be used. By agreement a single key number can
be used alternately additive and subtractive, that is, first additive,
second subtractive, third additive, etc.

The key numbers are used over and over until the entire message is
enciphered. The key number can sometimes be expressed by a single
word, as, for instance, "Grant," each letter having a value of tens in
accordance with its position in the alphabet; that is, G, the seventh
letter equals 70; R equals 180; A equals 10; N equals 140; and T
equals 200. Or by preconcerted arrangement letters may represent units
or hundreds. Security from translation by persons not having the key
number is greater when the key numbers are used alternately additive
and subtractive. If a cipher key word is used, it should be one of
an odd number of letters, as, for instance, "Jones," the numbers
corresponding to the positions of the letters in the alphabet. The
first number should be additive, the second subtractive, etc. By this
means the first letter of the key word is additive the first time it is
used, subtractive the second, additive the third, and so on. In some
instances the key number, when added to or subtracted from the code
number, gives a resulting number exceeding the highest code number
or less than unity. In cases of this kind it should be remembered in
enciphering that unity follows the highest code number in addition,
and that the highest code number follows unity in subtraction. In
deciphering a message the process of enciphering is reversed.


THE WAR DEPARTMENT CODE.

As previously stated, the War Department Code is the technical military
code and contains expressions numbered consecutively from 1 to 62,000.
All the code words are composed of 6 letters, which are so arranged
that the vowels and consonants invariably alternate. In the formation
of code words the following 13 letters only are used, viz, A, B, D, E,
F, G, I, K, M, N, S, U, and X. The body of the code book is arranged as
follows:

     (_a_) Army list, containing the name of every commissioned officer
     in the regular establishment.

     (_b_) Military organizations, giving all batteries, companies,
     troops, etc.

     (_c_) Military posts and stations, covering Alaska, Hawaii,
     Philippine Islands, Porto Rico, and the United States.

     (_d_) United States naval stations and vessels.

     (_e_) Geographical names.

     (_f_) Miscellaneous tables as follows:

    Numerals.
    Arrivals and departures.
    Dates.
    Indorsements.
    Letter acknowledgments.
    Requisitions.
    Telegram acknowledgments.
    Mails, shipments, and transports.
    Blanks for future additions as they may be needed.
    Ranks and grades of officers and men in the Army.
    Wireless stations of the Army and Navy.

     (_g_) Alphabetical list of code expressions arranged conveniently
     for use.

When it is desired to transmit some word or expression not to be
found in the code and no suitable synonym can be discovered the word
or expression should be sent in plain language or spelled out by the
equivalents for letters and endings to be found on page 589.

Complete instructions for the use of the code either as a code or
cipher are contained in the introductory pages of the book.


CIPHER CODE IN FIELD WORK.

The use of cipher code in enciphering field messages will usually be
practicable only between the several headquarters and other large
stations supplied with code books. This method, too, is prohibitive
for urgent messages when the time of enciphering and deciphering is an
important factor connected with delivery.


FIELD CIPHERS.

_Description and use._--Field ciphers include all systems and the
apparatus connected therewith which are ordinarily employed in
enciphering and deciphering field messages. Field ciphers are intended
for use when code books are not available, and hence the employment of
cipher code is precluded. Some methods of field cipher employ simple
forms of apparatus, while others require the use of no apparatus at all.

_Forms of field cipher._--There are two general classes of field
cipher. The first class employs the transposition or reversal of the
letters or words of a message according to some preconcerted rule as a
means of secrecy. The route cipher hereafter described is an example
of this class. The method used in ciphers of the second class consists
in the substitution of certain letters or symbols for each of the
individual letters composing the words of the message. Both classes of
cipher can be rendered more efficient by a judicious use of inversions
and by the concealment of terminations.

_Inversions._--By the inversions of the whole or certain parts of
messages, according to some preconcerted arrangement, the complications
of cipher can be greatly increased. If a message is to be inverted,
either as a whole or by clauses, it should be inverted before the
cipher letters are written over it. Messages may be further complicated
by sending the letters of each word backward in various other
prearranged combinations.

_Concealment of terminations._--To evade the discovery of the key or
keys employed, it is most important that the termination of the words
of a message should be concealed. The best method to conceal the
beginning, and at the same time the termination of words, is to divide
them into arbitrary groups of four or five letters each. This procedure
will add immeasurably to the strength of the cipher and should in no
way confuse one in possession of the key. For instance, the words
"sufficient time" would be divided "suff" "icie" "ntti" "me," and such
blind letters as may be agreed upon to fill the last two spaces of the
last group. All such artifices as this will surely delay a translator
not in possession of the key.


CIPHER APPARATUS.

_The cipher disk._--The cipher disk is composed of two disks of
cardboard, leather, or other material joined concentrically, the upper
disk revolving upon the lower. The alphabet, reading from left to
right, and such other signals, numerals, or combinations of letters, as
may be desired, are printed around the circumference of the lower disk.
On the upper disk are printed the alphabet and such other signals,
numerals, or combinations of letters as are printed on the lower disk.
On the lower disk they are printed from left to right, while on the
upper disk they are printed from right to left. If it is desired to
encipher a message, the key letter or the first letter of the key word
or words is set opposite "A." Let us assume it to be "J." The cipher
letters to be written are those opposite the text letter when the
letter "a" on the upper disk is set opposite "J" on the lower disk. For
example, "Send powder" would be written "rfwg uvngfs."

Having a cipher disk as above described, this mere transposition of
letters would delay but a short time the deciphering of a message by
one not knowing the key letter, as it would be necessary only to place,
in turn, opposite "a" each of the letters of the alphabet beginning
with "b" and noting the letters opposite the enciphered letters. But
this simple disk can be used with a cipher word, or preferably, cipher
words known only to the correspondents, and it is entirely improbable
that a message so enciphered could be deciphered in time to be of any
value to the enemy. Using the key words "permanent body" to encipher
the message "Reenforcements will reach you at daylight," we would
proceed as follows: Write out the message to be enciphered and above it
write the key word or key words, letter over letter, thus:

    PERMANENTBODYPERMANENTBODYPERMANENTB
    Reenforcementswillreachyouatdaylight
    yanzvznlppkqfxijbBpwanruqpeplomccwhmi

Now bring the "a" of the upper disk under the first letter of the
key word on the lower disk, in this case "P." The first letter of
the message to be enciphered is "R." "Y" is found to be the letter
connected with "R" and it is put down as the first cipher letter.
The letter "a" is then brought under "E," which is the second letter
of the key word. "E" is to be enciphered and "a" is found to be the
second cipher letter. Then bring "a" to "R" and the cipher letter
will represent "e," the third text letter of the message. Proceed in
this manner until the last letter of the cipher words is used, and,
beginning again with the letter "P," so continue until all letters of
the message have been enciphered. Divided into groups of four letters,
it will be as follows: "yanz vznl ppkq fxij bpwa nruq pepl omcc whmi."

[Illustration: FIG. 19.--Cipher disk.]

To decipher the message, reverse the proceedings above described; thus
the letter "a" on the upper disk is brought under the first letter
of the key word "P." Following these instructions, we find the first
cipher letter of the message; "a" is then brought to the next letter of
the key word. In this case "E" is, of course, the next letter of the
text. "R" is the next letter in the key and "a" is brought over it.
The cipher letter "n" gives us the next text letter, which is "e," and
so on until the completion of the message. If the letters of the key
word or phrase are exhausted, begin again with the first letter and so
continue until the entire message is deciphered.

With a key word, or, preferably, a key phrase of three or four words,
the deciphering of a message is extremely difficult.

In a military cipher message, it may be desired to transmit numerals,
the spelling out of which would require considerable time. This can
be done by an arrangement of the cipher disk so that the numerals of
which will appear in the same order as and follow the letters of the
alphabet. Thus on the lower disk 1 is placed opposite A; 2 opposite B;
3 opposite C; 4 opposite D; 5, 6, 7, 8, 9, and 0 opposite E, F, G, H,
I, and J, respectively.

On the upper disk the above numerals also appear, beginning numeral 1
opposite A; 2 opposite B, etc., 0 being opposite J.

The arbitrary sign XX will be used to indicate "numerals follow" and
"numerals end." Supposing then we wish to send the following message:
"Send 6,000 cavalry at once," and that the key word was "Washington."
Following the instructions heretofore given for enciphering, we would
place the words as follows:

    WASHINGTONWASHINGTONWASHI
    SENDXX6000XXCAVALRYATONCE
    EWFELQBKFEZDQHNNYCQNDMFFE

In place of a disk means may be extemporized by taking two strips of
paper, on one of which the alphabet, numerals, etc., are twice written
in succession. On the other, with equal spacing, the alphabet, etc.,
are written once, but in reverse order. By sliding these strips in
juxtaposition with each other they will replace the disk.

Cipher disks should never be allowed to fall into the hands of the
enemy or of anyone unauthorized to have and use them; to insure this,
special instructions should be issued for their care and keeping.


THE MATHEMATICAL CIPHER.

This cipher is a highly efficient one for the purpose of secrecy and at
the same time requires no apparatus whatever attendant upon its use.
The cipher is constructed as follows: Commit to memory the alphabet
by numbers, viz, A, 1; B, 2; etc. Take any key word, phrase, or
sentence desired; for example, "A discovery." Suppose the message to be
enciphered is "Send me powder tonight." The enciphering of the message
using the key given above will be as follows:

To encipher, first write out the key, letter by letter, placing the
message letter by letter beneath it. Then reduce the letters of the
key and the message to the numeral alphabetical equivalents. Add the
individual columns and subtract unity from each. From any result thus
found, which exceeds the number of letters in the alphabet, the number
26 must be subtracted. The final totals reduced to letters by numerical
alphabetical equivalents will then give the cipher.

    ADISCOVERYADISCOVER
    sendmepowdertonight

which reduced to numerical equivalents according to alphabetical
position of letters becomes:

     1 4  9 19  3 15 22  5 18 25 1  4  9 19  3 15 22 5 18
    19 5 14  4 13  5 16 15 23  4 5 18 20 15 14  9  7 8 20

Now add the columns and subtract unity from each. If any result
so found exceeds the number of letters in the alphabet 26 must be
subtracted from it.

In the example given the numerical totals are as follows:

  20  9 23 23 16 20 38 20 41 29 6 22 29 34 17 24 29 13 38
   1  1  1  1  1  1  1  1  1  1 1  1  1  1  1  1  1  1  1
  -------------------------------------------------------
  19  8 22 22 15 19 37 19 40 28 5 21 28 33 16 23 28 12 37
                    26    26 26      26 26       26    26
  -------------------------------------------------------
  19  8 22 22 15 19 11 19 14  2 5 21  2  7 16 23  2 12 11

which connected to letters gives:

    SHVVOSKSNBEUBGOWBLK

the cipher required.

Translation of cipher is had by reversing the processes described.


THE ROUTE CIPHER.

This is a cipher in which the words or a message are retained
unchanged, but are so disarranged by preconcerted rules that the
sense becomes unintelligible. The message as received seems to be a
number of disconnected words and without meaning, but by arrangement
in proper order in accordance with certain rules can be easily read.
Messages enciphered in this manner may be translated by persons not
in possession of the key, and therefore the information contained
therein should only be of such a character as to be of little value to
the enemy unless acted upon immediately. The usual method employed in
arranging a message for this cipher is to write the words in vertical
columns. The number of words in each column should always equal the
number of columns, being made so, if necessary, by the addition of
sufficient "blind" words. A preconcerted route is agreed upon, as up to
the first column, down the third, up the second, etc. The message is
then transmitted without reference to the columns, but is deciphered
at the receiving station by column arrangement and perusal along the
original route.

For example, to encipher the message "Move daylight. Enemy approaching
from north. Prisoners say strength one hundred thousand. Meet him as
planned," arrange as follows:

    Move         strength  planned  say
    daylight     one       as       prisoners
    enemy        hundred   him      north
    approaching  thousand  meet     from

Here the route is down the first column, up the fourth, down the
second, and up the third.


CIPHER DETECTION.

_General instructions._--In deciphering a message in which the same
cipher letter or symbol is uniformly used to represent the same text
letter, the following data will be of assistance.

The proportion of occurrence of letters of the alphabet in English
words is as follows: For every 2 of the letter Q there are 4 of the
letter X, 8 of K, 16 of B, 13 of C, 80 of I, N, O, and S; 85 of A, 90
of T, and 120 of letter E.

The compounds most frequently met with are NG EE LL MM TT DD and NN.

The order of frequency in which the letters of the alphabet occur as
initial letters in words is as follows:

    S, C, P, A, D, I, F, B, L, T.


_Employment of Cipher Disk._

If messages are enciphered by a mere transposition of the letters of
the alphabet, the cipher disk can be used to quickly decipher the
message, as the following example will show: Assuming that F is used to
represent A, G to represent B, H to represent C, I to represent D, J
to represent E, etc., in regular sequence, and that the message to be
enciphered is: "We are short of rifle ammunition; send 30,000 rounds at
once."

This would be enciphered if divided into groups of four letters as
follows:

    jbfo bnyr omra oxub fuls xmxr snbs cmjb smhm yrln
         fsco rlsc nfmr sdb.

Place "a" of the upper cipher disk under B of the lower disk and notice
whether the cipher letters jbfo--the first group--are intelligible.
They give "sawn," continue this for "saw," the first three letters, may
be the text word. Now the next group is B N Y R and these give A O D K.
We know that A does not represent B because the first 8 cipher letters
give the meaningless letters "sawnaodk." Turn "a" to C and we have for
the first group T B X O, which is without meaning. Turning "a" to D we
get U C Y P, a meaningless jumble. Turn "a" to E and we get V D Z Q,
which is meaningless. Now turn "a" under F and we find that JBFO mean
"Wear," which, so far at least, gives us a part of a word, or the word
"We" and part of another word. We continue to the next group B N Y R,
which gives us "esho." We now have these letters "Wearesho," which at a
glance we read "We are sho;" continuing to the next group O M R A the
cipher disk gives us "rtof," and we read "We are short of" and know we
have found the key letter, and the information hidden in the cipher
is ours. Continue deciphering with "a" under F until the end of the
message. Sometimes the key letter is changed after two, three, or four
letters.

It is a matter of minutes only to run through the alphabet and learn
the meaning of a message so enciphered.




CHAPTER VII.

FIELD GLASSES AND TELESCOPES.


Reflection--refraction--lenses.

[Illustration: FIG. 20.]

When light falls on a transparent body, part is reflected and part
is refracted. The angle which the ray makes with the normal, or
perpendicular, to the surface at the point of contact is known as the
angle of incidence, and the angles which the reflected and refracted
rays make with the same normal are known respectively as the angle
of reflection and refraction. The reflected ray makes the same angle
with the normal as the incident ray, while the refracted ray, when
passing from a rarer to a denser medium, is bent toward the normal,
and vice versa; the denser the medium into which the ray passes the
greater is the deviation. This law allows us at once to understand the
action of a lens, which may be defined as a transparent medium that
from the curvature of its surface causes the rays of light traversing
it to either converge or diverge. The ordinary lenses have either
spherical surfaces or a combination of spherical and plane surfaces.
This combination will give rise to six classes (fig. 20): (_a_) Double
convex; (_b_) plano convex; (_c_) double concave; (_d_) plano concave;
(_e_) converging, and (_f_) diverging meniscus. Those lenses which are
thicker at the center than at the edges are converging or concentrating
lenses, and those which are thicker at the edges than the center are
diverging.


FOCUS--OPTICAL CENTER.

The focus of a lens is the point where the refracted rays or their
prolongation meet; if the rays themselves intersect after refraction
the focus is real, and if their prolongations meet the focus is
virtual. The line passing through the centers of curvature of the two
surfaces of a lens is called the principal axis and contains a point
known as the optical center, which has the property by virtue of which,
if a ray passes through it, the ray will not be deviated. The optical
center can always be found by drawing two radii parallel to each other,
one from each center of the curvature of the surface until the radii
intersect their respective surfaces, then draw a line joining these two
points. The intersection of this last line with the principal axis will
give the optical center.


IMAGE--CONJUGATE FOCI.

Let AB be the section of a double convex lens and C and D (fig. 21) be
the centers of curvature of the two surfaces. Draw the lines CD´ and DE
from C and D parallel to each other, then join D´ and E by a straight
line. The point O will be the optical center of the lens. Let us take a
point R, on the principal axis as a source of light; the ray RD passes
through the optical center and is not deviated. The ray RK on striking
will be refracted in the direction KG toward the perpendicular to the
surface KD in accordance with the law of refraction, as glass is denser
than air. On emerging at G it is refracted away from the perpendicular
to the surface CG, since it passes from a denser to a rarer medium,
and will intersect the ray RD at the point R´. In a like way the ray
RK´ will be found to intersect the ray RD at the same point, R´, which
is the focus for all rays coming from R. The point R´ is said to be the
image of the object R, and when the two points are considered together
they are called conjugate foci. If the incident beam is composed of
parallel homogeneous light, the rays will all be brought to a focus at
a point on the principal axis, called the principal focus of the lens,
and the distance of this point from the optical center is the principal
focal length, which is always a fixed quantity for any given lens.

[Illustration: FIG. 21.]


LAW OF FOCI.

There is a fixed relation between the principal focal length of a
double convex lens and the position of the image of the object which
may be expressed as follows: 1/_i_ = 1/_f_ - 1/_o_, in which _i_ and
_o_ are the distances of the image and object, respectively, from the
optical center and _f_ the focal length, from which we see that for
all positions of the object from an infinite distance away from the
lens to double the principal focal distance, the image will be on the
other side, between a distance equal to the principal focal length and
double this length. These are the limits of the image and object in the
ordinary cases. If we place this expression in the following form: _i_
= _of_/(_o_ - _f_), and suppose the object to remain the same distance
from various lenses, it will be seen that the image will be closer
to the lens which has the shorter focal length. The principal focal
distance, or, briefly, the focal length of the lens, depends on the
curvature of the surfaces, and the greater the curvature the shorter
the focal length.


FORMATION OF IMAGE.

[Illustration: FIG. 22.]

Let us now see how an image is formed by a convex lens, and suppose
that CD is the section of a double convex lens (fig. 22), O the optical
center, and AB an object at a greater distance from the optical center
than double the focal length. Rays will pass out in all directions
from the object and some will fall on the lens. A ray from A will pass
through the optical center and will not be deviated; others will be
incident at various points, for example, E and G, and if we apply
the law of refraction we will find that AE and AG will intersect each
other and AO at the point A´, provided we do not consider the figure
of the lens, forming one point of the image A´ B´; similarly for rays
from other points of the object, as, for example, B, we can construct
the focus B´, and thus obtain the image A´ B´, which is inverted and
smaller than the object AB. The relative size of the image and object
will be directly as the conjugate foci, and these can be found at once
from the equation of the lens.


SPHERICAL ABERRATION.

If, however, we consider the form of the lens, we will find that all
the rays emerging from one point on the object are not brought to the
same focus, because the rays incident on the edges of the lens are
refracted to a greater extent than those falling on the center, and
will be brought to a focus at a shorter distance from the lens than
those passing through the central part. This confusion or wandering of
the foci from one point is called spherical aberration, or aberration
of form, and is due solely to the geometrical form of the lens.


CHROMATIC ABERRATION.

In what has been said about the visual image we have supposed that the
light was monochromatic, or homogeneous. Let us see what will happen if
the light is polychromatic, say, for example, sunlight, and let a beam
of sunlight be intercepted on a screen after passing through a double
convex lens. It will be observed, as in figure 23, that the violet
rays are brought to a focus nearest the lens, and the red farthest
away, and circles of light will be seen on the screen; this wandering
of the colored rays from a common focus is called chromatic aberration
and depends on the dispersive properties of the material of which the
lens is made. Here is a defect that can not be corrected by a stop,
but as the refractive and dispersive properties of a substance do not
vary together, it is possible to combine two substances, one with high
refractive and low dispersive properties and the other with the reverse
properties. If proper curves are given to them they will correct each
other, thereby producing coincidence of the visible and chromatic foci.
Such a combination gives an achromatic lens, which is usually composed
of a double convex of crown glass cemented to a diverging meniscus of
flint glass, as shown in section in figure 24. This combination is not
absolutely achromatic, but sufficiently so for all general purposes.

[Illustration: FIG. 23.]

[Illustration: FIG. 24.]


TELESCOPES.

The telescope is an optical instrument based on an object glass or
reflector to form a real image of a real and distant object, and of
an ocular to magnify and view the image. Telescopes are classified as
refracting or reflecting according as the object glass is a lens or a
reflector. The object glass must be essentially convex if the telescope
is a refractor, and if a reflector, the object mirror must be concave;
the ocular may be either concave or convex.

There are four types of refractive telescopes used for military
purposes, viz:

    1. The astronomical.
    2. The terrestrial.
    3. The galilean.
    4. The prismatic.

Figure 26 is a section of an astronomical telescope. The object glass
(_D_) is a combination consisting of a double convex and a double
concave lens cemented together with Canada balsam. The double concave
lens is added to correct for chromatic aberration. The ocular (_E_) is
a convex-concave lens.

Rays of light from some distant object are converged by the objective
(_D_) and form an inverted image (_ab_) at the _focal_ plane (_F_).
The eye lens (_E_) receives the divergent pencils from _a_ and _b_
and bend them so that they enter the eye as if coming apparently from
the direction of _a´ b´_ where the apparent image is seen. From the
eyepiece (_E_) the rays emerge in a cone of pencils of light smaller
than the pupil of the eye, which enables a telescope of this type to
have a large field of view. The image, however, is inverted and the
astronomical telescope in its original form is therefore not suitable
for military purposes. In a modified form it is much used, as will be
shown in a later paragraph.

[Illustration: _Figure 26_]

Figure 27 is a section of a terrestrial telescope much used for
military purposes. Glasses of this type are quite generally known as
"spyglasses."

As in the case of the astronomical telescope, the first inverted
image _ba_ is formed at the focal plane (_F_), and the first eyeglass
converges these pencils to _L_. Instead of placing the eye at _L_,
as in the astronomical telescope, the pencils are allowed to cross
and fall on a second eyeglass, by which the rays of each pencil are
converged to a point in the second erect image _a´ b´_, which image is
viewed by means of the third and last eyeglass.

[Illustration: _Figure 27_]

Terrestrial telescopes have a comparatively small field of view. The
barrels of this telescope are necessarily long on account of the
additional lenses.


GALILEAN FIELD GLASSES AND TELESCOPES.

Figure 28 is a section of a Galilean telescope which differs from the
astronomical telescope in having a double concave instead of a double
convex, eyepiece or ocular.

In this telescope the rays from an object are converged by the object
glass (_O_) and would normally focus at the focal plane (_C_) and
there form the inverted image _ba_ were it not that the double concave
eyeglass or ocular (_D_) is so located in the barrel of the telescope
as to intercept the pencils before they are focused. This double
concave eyeglass diverges these pencils and forms a magnified erect
image _a´ b´_ apparently at _E_. Due to the diverging action of this
concave eye lens, the cone of pencils entering the eye is larger than
the pupil of the eye, and therefore but a small part of the field
gathered by the object glass is utilized by the eye, which causes
telescopes of this type to have a comparatively small field of view.

[Illustration: _Figure 28_]


PORRO PRISM FIELD GLASSES AND TELESCOPES.

In 1850 a French engineer, Porro, discovered a combination of prisms
which, when inserted between the objective and the eyepiece of an
astronomical telescope, showed the image erect or in its natural
position, while the same telescope without the prisms showed the image
inverted. Practical use of this discovery was not made for many years
after. These prisms served a twofold purpose, viz, showing the image
of the object looked at in its natural position instead of reversed,
and second, the shortening of the telescope by twice turning the ray of
light upon itself. Each tube of the prism field glass contains two of
these double-reflecting prisms. The ray of light passing through the
object glass enters the first prism in such a manner as to be twice
totally reflected, each time at an angle of 90°, thus emerging parallel
to the entering ray, but in the opposite direction. It is thus caught
by the second prism and is similarly reflected and sent on its original
direction without change except in one very important point, viz, the
image of the object observed, which, without the intervention of the
prism, would be upside down, is now erect, and will be magnified by
the simple astronomical eyepiece just as the stars and planets are
magnified in large telescopes.

The field of view of the Porro prism glass is considerably larger than
that of the ordinary field glass. It decreases about 12½ per cent
with each magnifying power, a number 6-power glass giving a linear view
of 118 feet in a thousand, while in a number 10 glass the field is but
70 linear feet. This is explained as follows:

The rays of light emerging from the ocular of the Galilean telescope
are divergent and cover an area much greater than the size of the
pupil of the eye. As all rays falling outside the pupil of the eye are
lost, but a small field of view can be seen, as in looking through an
ordinary cone from the larger end. The prism glasses are constructed
on the opposite principle. The rays of light gathered by the objective
emerge from the eyepiece in a converging pencil of light small enough
to enter the pupil of the eye, thus giving a larger field of view;
theoretically, nine times the area given by the old-style instrument of
the same power. With these advantages, however, the Porro prism glass
has not been found in all respects satisfactory for field service.
With a clear atmosphere and the object which is being viewed well
illuminated, it is distinctly superior to the Galilean field-type glass
in respect to light, power, and definition. The prisms having once
been deranged, however slightly, satisfactory use of the glass can not
be had until the prisms have been readjusted, and until very recently
it was impracticable to have this done elsewhere than at the place of
manufacture of the glass.

[Illustration: FIG. 29.--Porro prism.]


FIELD GLASSES.

The field glass or binocular is a combination of two similar telescopes
and possesses mechanical adjustments capable of focusing the two
telescopes simultaneously or separately, depending upon the type
considered.

Field glasses are divided into two general classes, viz, the Galilean
glasses and the Porro prism glasses.


PROPERTIES OF TELESCOPES AND FIELD GLASSES.

Telescopes and field glasses have four properties, viz, power, light,
field, and definition. These properties are expressed in terms of the
corresponding qualities of the unaided eye.

Eyes are of very different capabilities. Some people have "short" sight
while others have "far" sight. There are normal, excellent, and weak
eyes. In the following discussion the capabilities of the normal eye
are assumed.

For each individual there is a certain distance at which objects may
be most distinctly seen. This is called the "visual distance." With
shortsighted eyes this distance is from 3 to 6 inches; with normal
eyes, from 8 to 14 inches, and with farsighted eyes, from 16 to 28
inches.

The capabilities of the normal unassisted eye may therefore be
expressed as follows: Power, 1; light, 1; field, 45°; definition, 40´´
to 3´.

_Power._--At the "visual distance," all objects seen by the unaided
normal eye appear in their natural size. At less than the "visual
distance" they appear indistinct, blurred, and imperfectly defined; at
greater than the "visual distance" objects are clear and well defined,
but diminish in size, the more so as they are farther removed.

The ability of a lens to magnify the apparent diameter of an object is
termed its power.

The power of a lens is defined as the ratio of the diameter of the
object as seen through the lens to the diameter as viewed by the
unaided eye.

The power is also defined as the ratio of the focal distance of the
object glass to that of the eyepiece.

The power of a field glass can be roughly determined by focusing the
instrument on a wall or a range rod, by looking at the object through
the instrument with one eye and at the same object directly with the
unaided eye. A comparison of the diameter of the two images gives the
ratio.

The power of a telescope or a field glass can more accurately be
measured by means of a dynameter, which is a microscope which can be
fitted over the eyepiece end of the instrument, and which magnifies the
image. The end of the dynameter next to the eyepiece of the instrument
is ruled with a series of lines one-hundredth of an inch apart. On
focusing the dynameter, the image of the emerging pencil appears as a
sharply defined ring of light with the magnified scale of the dynameter
across it.

The number of subdivisions covered by the diameter of the ring of light
is noted. The diameter of the object glass is similarly measured by
means of a pair of dividers and read to the hundredth part of an inch.

The ratio of the diameter of the object glass to that of the image as
seen in the dynameter gives the power of the instrument. This method is
not applicable in the case of the Galilean telescope or the field glass
consisting of two Galilean telescopes, due to the fact that the rays
from the eyepiece of the Galilean telescope are divergent.

Field glasses in which the image appears magnified from one to six
diameters are known as "low-power" glasses. Field glasses which produce
an image magnified over six diameters are termed "high power."

For the mounted man a glass of but 4, or at most 6, powers, can be
used with advantage; on foot, with free hand, instruments of not to
exceed 10 powers can be used. If more than 10 powers are desired, a
holder becomes necessary, and if the holder is intended to be portable
a greater power than 50 is not practicable, as the movement of the air
or the slightest touch of the hand sets up vibrations that render clear
vision impossible.

Field glasses with low magnifying power, which are usually preferred
by ordinary observers, have their chief value in the comparatively
extensive field of view; they should be used to observe extensive
movements, where large tracts of country must be taken in one field of
view or in sweeping the landscape to find the tents of the enemy, their
wagons, etc., or other objects, to be afterwards more closely examined
with the telescope.

They may be used on shipboard or in boats, where the rolling motion
interferes with the use of the telescope; also on horseback or in
hasty examination made on foot or in trees, and generally for all
observations not critical or those to be made under circumstances where
the telescope can not be conveniently handled. The field glass ought to
be held by both hands when in use, and to steady it the arms should be
kept close to the body.

For reading signals at short ranges, say, up to 5 miles, these glasses
are better than the telescope. Flag signals have frequently been read
with glasses of this description at a distance of 10 miles.

_Light._--The illumination of an object when observed with the
unaided eye is impressed upon the retina with a brightness in strict
proportion to that of the object itself. If an object be viewed under
equal illuminating conditions alternately with the naked eye and with
a glass, the brightness of the image seen with the naked eye may be
represented by 1, while that of the image in the glass will generally
differ, being greater or less.

The light of the telescope or field glass is expressed by the number
which shows how many times brighter the object appears through the
instrument than to the naked eye. Light is a function of the dimensions
of the object glass and of the power of the instrument, and is
sometimes determined by dividing the square of the objective aperture
(expressed in millimeters) by the square of the power.

The light of a telescope or field glass can also be determined by means
of the absorption apparatus shown in figure 30 (_a_) (_b_) (_c_).

This absorption apparatus operates on the principle of viewing an
object through a perfectly black liquid, which absorbs all colors
equally, and of increasing the thickness of the liquid layer until
the object becomes invisible. The thickness of the layer of liquid
will then be a measure of the relative brightness or intensity of the
illumination.

[Illustration: FIG. 30(a)

FIG. 30(b)

FIG. 30(c)]

The apparatus consists of two wedge-shaped vessels, made of brass,
with glass windows in the sides. One of these vessels is shown in
perspective in figure 30_a_. The sides _A_ and the one opposite are
of glass. _B_ is tubulure for filling the apparatus, and is stopped
with a cap. The operation of the apparatus is shown diagrammatically
in figures 30_b_ and 30_c_. The edges of the two wedges which
come together are divided into scales of equal parts of convenient
magnitude. Each scale begins with zero; not at the extreme point of
the wedge outside, but at a point, which, allowing for the thickness
of the glass sides, is opposite the point of the wedge of liquid
inside. It will be observed in figures 30_b_ and 30_c_ that the sum
of any two adjacent numbers, on the respective scales, over the whole
overlapping portion of the wedges, is the same. Thus in figure 30_b_ it
is 11, and in figure 30_c_ it is 7. These figures measure the relative
thickness of the liquid layers in the two respective settings of the
apparatus. Suppose the image is just obliterated, when looking with
the unaided eye, at the setting shown in figure 30_b_, and when using
the glass at the setting shown in figure 30_c_. This would mean that
the illuminating power of the glass is seven-elevenths. In using the
apparatus, a focusing cloth, used by all photographers, is useful in
excluding stray light.

_Field._--Maintaining the head and eyes as motionless as possible, the
field of vision of the unaided eye or the range within which objects
can be perceived by the unaided eye varies according to direction.

De Schweinitz gives the following limits: Outward, 90°; outward and
upward, 70°; upward, 50°; upward and inward, 55°; inward, 60°; inward
and downward, 55°; downward, 72°; downward and outward, 85°.

It may be safely said that the field or "visual angle" of the unaided
eye for _distinct_ vision is at least 45° in all directions.

The "visual angle" or "field" of a field glass is always smaller, no
field glass having yet been designed which could equal the field of the
unaided eye.

The field of a telescope or field glass can best be determined by the
use of a transit or other instrument used in measuring horizontal
angles. The glass is placed upon the telescope of the transit in such
a way that the axes of collimation of the transit and the telescope or
field glass are parallel. The extreme limits of the field of view are
marked and the horizontal angle between the markers noted on the limb
of the transit.

_Definition._--One of the chief qualities of the eye is its power of
defining outlines and details distinctly. Relative characteristics in
this respect may be determined in various ways. Thus the distance at
which printed matter can be read, or the details of a distant object
distinguished, will give a fair measure of the defining power of the
eye; but a better method is to express the definition of sight by
angular measurement--that is, by the determination of the smallest
visual angle giving clear results. Experience teaches that this angle
of the normal eye (with good light and favorable color conditions) is
about 40´´, and it is therefore possible to determine the smallest
object which can just be seen, well defined, at an arbitrary distance.
For instance, at a distance of 15 feet an object can be seen which
is one-twentieth of an inch high or broad; at 30 feet distance,
consequently, the object must be twice the size (one-tenth of an inch)
to be seen, and so on relatively, within limits, as distance increases.
But as the distance becomes greater sharpness of vision is impaired
materially by the interposing atmosphere, while it is also affected by
color contrasts and conditions of illumination. It therefore follows
that at considerable distances objects which subtend a visual angle
of 40´´ are no longer clearly defined but become so only as the angle
approaches 60´´, 120´´, 180´´, or more.

The most important and essential quality of a telescope or field glass
is definition, i. e., the sharpness, clearness, and the purity of the
images seen through it. To obtain good definition it is necessary
that spherical and chromatic aberration be overcome, that the polish
of the lenses be as perfect as possible, that the cement possess no
inequalities, and that the lenses be well focused, that there be
no dampness in the interior of the tubes, and, generally, that the
instrument be without optical defect.

Faults in this direction are discovered at once by examination of
definition, whereas in determining the other constants they are less
noticeable. In comparing the definition of any two instruments it is
ordinarily necessary only to scan distant objects and observe to what
extent details may be distinguished.

The following test may also be used: Focus on printed matter at a
distance just beyond that at which perfect clearness is given and
gradually approach until the letters are distinctly defined. The
instrument with which the print can be read at the greatest distance
has the best definition.

To express definition as an absolute measure, use instead of printed
matter, a white sheet of paper upon which a series of heavy lines are
drawn at intervals equivalent to their thickness. Focus upon this and
gradually approach from a point where the impression of a uniform gray
field ceases and the black lines and white intervals begin to appear
distinct and defined.

Let the distance thus found be 20 yards and the thickness of the lines
and intervals between them one-tenth inch. The circumference of a
circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by
3.1416 or 45,240 tenth inches; but a circumference equals 360° or (360
by 60 by 60) 1,296,000´´.

If, therefore, 45,240 tenths inches correspond to 1,296,000´´, then
1 tenth inch equals 1,296,000 divided by 45,240, or 28.6´´. The
definition is therefore 28.6´´, or practically half a minute.

The capabilities of glasses, including telescopes, in a general way,
lie between the following limits:

(1) Power between 2 and 1,000.

(2) Light may be 0.01 to 200 times that of the unaided eye.

(3) Field measures in most favorable case, 10°; in the most
unfavorable, .01°.

(4) Definition varies between 40´´ and 0.1´´.

Thus, as a maximum, an object may be seen by means of a telescope,
magnified 1,000 times, 200 times brighter and 400 times sharper than
with the naked eye.

If these advantages could be fully utilized for military purposes the
use of glasses would be extraordinary, a power of 1,000 practically
effecting the same purpose as the approach of the observed object to
one-thousandth of the distance. A hostile command 10 miles distant
could be seen theoretically as well as if they were only 53 feet away,
and the slightest movement of each single man would become visible.
Of course no such wonderful effect is physically practicable, and the
limiting conditions increase greatly in proportion as either one or the
other of the qualities, power, field, etc., is especially sought.

While astronomers require only that the telescope be made as capable
and perfect as possible in an optical point of view, making all other
conditions subordinate to this one, the military, to whom the glass is
simply an accessory, make other conditions of the first importance. The
glass must have suitable form, small volume, little weight, and that it
may be used without support, mounted or dismounted, and the image must
appear as looked at by the naked eye--that is, not inverted.

The capability of the instrument, however, is thereby much limited;
great powers give plain images only with relatively long tubes; glasses
must be held the steadier the more they magnify; and with increasing
power all vibrations become more troublesome and render minute
observations very difficult or impossible. The additional lenses in
terrestrial telescopes somewhat decrease power and affect also light
and definition. It is clear therefore that expectations of achieving
great power should not be entertained, the function of field glasses
being to bring out and define objects which to the naked eye appear
indistinct and doubtful.

The distinctness with which anything can be seen through the telescope
depends, primarily, upon the number of straight lines of light which
are collected by it from every point of the object.

Telescopes, the object glasses being equal in size, diminish light as
a general rule in proportion as their magnifying power is great. The
most powerful glasses are therefore to be used for minute observations
on the clearest days or when there is a strong light upon the observed
object. When the light is fading or there is a little light upon the
observed object the clearer view will be had with glasses of large
field and low magnifying power.


FIELD GLASSES AND TELESCOPES ISSUED BY THE SIGNAL CORPS.

The Signal Corps issues four standard field glasses, viz, Type A, Type
B, Type C, Type D.

Field glasses issued by the Signal Corps are not supplied for the
personal use of an officer and will not be used in lieu of the
officer's personal field glass prescribed by paragraph 97, General
Orders, 169, War Department, 1907 (Par. 1, G. O. 16, War Dept., 1910).

Under paragraph 1582, Army Regulations, as amended by paragraph I,
General Orders, No. 207, War Department, October 16, 1909, the Signal
Corps will sell field glasses to officers of the army for their
personal use.

Application for the purchase of field glasses should be addressed to
the Chief Signal Officer of the Army, Washington, D. C., inclosing
post-office money order or check on the Treasurer or Assistant
Treasurer of the United States for the amount, payable to the
Disbursing Officer, Signal Corps, and Signal Corps Form No. 240
accomplished in duplicate.

The Government does not pay transportation charges for the shipment of
articles sold to officers. Field glasses are sent from the Signal Corps
General Supply Depot, Fort Wood, New York Harbor, by express, charges
collect, unless purchase request is accompanied by funds so that field
glasses may be sent by registered mail. Forwarding by registered mail
is somewhat cheaper than by express, and the amount of postage required
is 40 cents for Type D glass, 46 cents for Types A and B, and 74 cents
for Type C. Express charges depend upon the distance from New York.

The Signal Corps has purchased many samples of field glasses from
various manufacturers with a view of testing their suitability for the
military service. These samples may be examined by officers of the army
at the signal office in Washington. Among these samples there are many
excellent glasses especially suitable for the military service, but the
higher grades are too expensive for general issue to line organizations
in large quantities. Officers desiring an especially fine field glass
should inspect the samples referred to; these, however, are not for
sale by the Government, but information will be supplied concerning
dealers and cost.

No advice or fixed rule can be stated as to what constitutes the most
suitable characteristics of a field glass. No single field glass can
furnish maximum results under all conditions on account of varying
conditions of the atmosphere.

A high-power glass is unsuitable for use at night, hazy atmosphere,
or for use of a mounted man where the glass can not be rested against
a firm support. A low-power glass with large object lens to permit as
much light as possible is a necessary condition for use at night. The
double power glass which is issued as a part of the visual signaling
outfits was designed for the military service as a compromise for
conflicting conditions.

A brief description of the field glasses issued by the Signal Corps,
together with the cost of the same, is given below.

Type A:

This glass is the current result of the efforts of the Signal Corps
to provide a field glass that will meet the greatest variety of
conditions, and insure efficient service to the greatest number of
military observers. It is really two glasses in one--a _day_ glass of
medium power, and a _night_ glass of low power.

[Illustration: FIG. 31.--Type A. Showing the field glass and case with
sling cord, shoulder straps, belt loops, and compass.]

It is to be clearly understood that while this glass is considered
superior for moderate ranges, it does not replace, under special
conditions, for long ranges, either the porro prism glass or the
telescope.

When held as shown in figure 32 with the tubes drawn out about 1 inch
to secure proper focus, the glass has a power of about 5.6 diameters,
and a field of about 5.4 degrees.

[Illustration: FIG. 32--Signal Corps field glass, Type A.]

If the glass is turned into the position shown in figure 33, the small
plus lenses, just in front of the eye pieces, drop automatically into
position and reduce the power to 3.8 diameters, and increase the field
to 8.3 degrees. This position requires a different adjustment, the
tubes being drawn out about one-third of an inch to get the proper
focus. It will be observed in the illustrations that the rear bar of
the frame is not only lettered to indicate which power is being used,
but the bar itself is shaped with a hump on one side, and hollowed on
the other. When the hump is up, the low power is in use. This is to
facilitate adjustment in the dark.

The action of the small automatic lenses is free and positive. _Neither
the eyepieces nor the sections containing the small lenses should be
unscrewed, except in case of necessity, and then not by unskilled
hands._

[Illustration: FIG. 33.--Signal Corps field glass, Type A.]

The frame, of aluminum and brass, is composite, to give lightness and
strength; and while it is constructed to withstand the rough handling
of field service, no field glass is proof against careless or wanton
treatment. The tubes are covered with tan leather, and a round sling
cord, braided from four strands of pliable tan leather, is fastened by
snaps to eyes in the frame.

The case is of tan calfskin, provided with shoulder strap, and has an
efficient small compass set into the cover. Two loops are sewed to the
back of the case so that it may be worn on a belt.

The glass, complete with case, cord, and straps, weighs 21.5 ounces.

Two of these glasses are issued to each company of infantry and coast
artillery, Philippine Scouts, and Signal Corps, and to each troop of
cavalry for use in instruction in visual signaling. Below is a brief
description of the type A glass.

Magnification, 3½ and 5½ diameters; Galilean type; object
lens, 1½ inches; tan leather finish; tan leather carrying case
with compass; weight of glass, complete, with case, cord, and strap,
25 ounces. At a distance of 1,000 yards the field of view includes
a diameter of 123 yards for the 3½ power, and 73 yards for the
5½ power. Length of glass closed, 4 inches. This glass is issued
as a part of the visual signaling kit to each company of infantry,
coast artillery, and Philippine Scouts, troop of cavalry, machine-gun
platoon, and Signal Corps field company. Price, $12.15.

The latest issue of this glass known as the Type A, model 1910,
includes provision for interpupillary adjustment, the two barrels being
hinged to accommodate the glass to the distance between the pupils of
the eye. The price of the model 1910 glass is $14.75.

Type B:

This field glass is similar in appearance and construction to the
Type A glass, and is issued to the field artillery organizations upon
requisition. The following is a brief description:

Magnification, 4½ and 6½ diameters; Galilean type; object lens,
1¾ inches; interpupillary adjustment; tan leather finish; tan
leather carrying case with compass; weight of glass, complete, with
case, cord, and straps, 26 ounces; length of glass closed, 4½
inches. At a distance of 1,000 yards the field of view includes a
diameter of 90 yards for the 4½ power, and 60 yards for the 6½
power. This glass is issued as a part of the fire-control equipment to
field artillery. Price, $17.50.

Type C:

The type C is a high power glass of the porro prism type and is issued
only to certain organizations of the field artillery, Signal Corps, and
to all machine-gun platoons.

_Description._--Magnification, 10 diameters; prismatic type; object
lens, 1¾ inches; interpupillary adjustment; tan leather finish;
sunshade; tan leather carrying case; weight of glass, complete, with
case, cord, and straps, 46 ounces; length of glass closed, 7¾
inches. At a distance of 1,000 yards the field of view includes a
diameter of 80 yards. This glass is issued to reconnaissance officers
of field artillery. Price, $39.90.

Type D: Purchase has been made for delivery in the near future of a
supply of a new type of high power prismatic field glass for sale and
issue. This new type of glass, to be known as type D, is considerably
smaller than the type C glass, as is shown by figure 34. The glass in a
tan-colored carrying case weighs 15 ounces, the field glass without the
case weighing but 9 ounces. The magnification is 8 powers and the field
of view (with both eyes) 5° 40´. The estimated cost will be $27.


TELESCOPES ISSUED BY THE SIGNAL CORPS.

Type A: This glass complete consists of a 2-inch prism terrestrial
telescope, powers 18 and 24, with alt-azimuth, folding tripod, and
carrying case.

Type B: This telescope is a 19-27 power, 2-draw terrestrial telescope,
in leather carrying case with sling. The leather carrying case also
includes a holder which can be screwed into a tree, post, or other
stationary wooden object.


GENERAL SPECIFICATION NO. 263.

[Revised February 10, 1910.]

SERVICE FIELD GLASSES.

1. _Preliminary._--This specification covers the design and
construction of field glasses, types A and B, each having two powers as
hereinafter specified.

2. _Sample._--The bidder shall furnish with his proposal a sample of
the glass which he will supply, and award will be made after comparison
of the samples with models on file in the office of the Chief Signal
Officer. The maker will be allowed to examine the model glasses
in detail in the office of the Chief Signal Officer of the Army,
Washington, D. C.

3. _Inspection and test._--When the order under this specification is
complete, the contractor will notify the Chief Signal Officer of the
Army, who will cause an inspection to be made. It shall be the duty of
the contractor to remedy any defects pointed out by the inspector, and
the contractor will be held accountable for any imperfections which the
inspector may have overlooked.

[Illustration: FIG. 34.--Field glasses, Types C and D.]

The Chief Signal Officer of the Army reserves the right to inspect
any or all processes of manufacture, and unsatisfactory material will
be marked for rejection by the inspector before, during, or after
assembly, as occasion may arise.

Each glass will be tested for power, field, definition, and light. Any
glass which is not the equal of the sample and model in all respects
will be rejected. The properties above enumerated will be tested as
follows:

(_a_) Power: In testing for power the glass will be placed upon a
firm support about the height of the eye and directed upon a range
rod, accurately divided into divisions of 1 foot, with alternate
divisions colored red and white, respectively. The rod should be placed
approximately 100 feet from the glass in a good light and with strongly
contrasted background.

The rod is observed through the glass with one eye and at the same time
with the other eye unaided. An accurate comparison of the two images by
means of the rod scale determines the magnifying power of the glass.

(_b_) Field: The field will be determined by the use of a transit or
any other instrument adapted to the measurement of horizontal angles.
The glass will be placed upon the telescope of the transit in such
a way that the axes of collimation of the telescope and field glass
barrels are parallel. The extreme limits of the field of view of the
glass are marked in a convenient way and the horizontal angle of view
accurately measured with the transit.

(_c_) Definition: In determining the definition of the glass expressed
in units (seconds) a target will be provided with a number of lines
one-tenth inch thick with one-tenth inch spaces between them drawn on a
piece of heavy white paper.

At a certain distance this target will appear uniformly gray when
viewed through the glass.

The inspector will gradually approach the target, focusing the glass
until he reaches the most distant point from the target where the
uniform field ceases and the black and white intervals appear distinct
and defined.

Assume the distance thus found to be 20 yards and the thickness of the
lines and intervals between them one-tenth inch. The circumference of
a circle with a radius of 20 yards or 7,200 tenths inches is 14,400 by
3.1416, or 45,240 tenths inches; but a circumference equals 360°, or
(360 by 60 by 60) 1,296,000 seconds.

If, therefore, 45,240 tenths inches correspond to 1,296,000 seconds,
then one-tenth inch equals 1,296,000 divided by 45,240, or 28.6
seconds. The definition is therefore 28.6 seconds, or practically half
a minute.

The definition should be as follows:

    For 6.5 power glass    30 seconds.
    For 5.5 power glass    35 seconds.
    For 4.5 power glass    40 seconds.
    For 3.5 power glass    55 seconds.

(_d_) Light: The light of a field glass is expressed by a number which
is the ratio of the amount of light which reaches the eye through the
glass to the amount which enters the eye unaided. This comparison
will be reached by means of the absorption apparatus furnished by
the Signal Corps. This apparatus consists of two wedge-shaped vessels
made of brass with glass windows in the sides, and are filled with
a perfectly black liquid. The sky line is first viewed through the
apparatus with the naked eye and the instrument adjusted to limit of
visibility. The reading of the scale is then noted. The sky line is
again observed, using the glass, but in other respects as before, and
a second scale reading obtained. The ratio of these readings measure
the illuminating power of the glass which must conform to the standard
sample.

4. _Service field glass, type A._--(_a_) This glass shall conform in
general to the model, now on file in the office of the Chief Signal
Officer at Washington. The arrangement for changing automatically from
the low power to the high power, and vice versa, by the interposition
of the plus lens at the proper distance in front of the eyepiece, must
be strictly adhered to.

(_b_) The _low power_ shall be approximately 3½ diameters and the
_high power_ shall be approximately 5½ diameters. The figure of
merit given by multiplying the numbers of diameters power by the number
of degrees of field will be considered in the examination of samples,
along with the other properties of light, sharpness of definition, and
general excellence.

(_c_) _The tubes, frame, and metal fittings_ shall be of aluminum or
an aluminum alloy, with the exception that such metal parts as in the
opinion of the maker require greater strength may be made of brass.

Tubes shall be held firmly in the frame, single draw, the draw action
to be through a bearing surface of at least five-eighths of an inch of
best black felt, perfectly fitted so as to preserve perfect alignment.

The exterior metal parts, except where leather covered, must be given
the best and most durable, lusterless black finish. The tubes and
shades will be neatly covered with best quality tanned calfskin, the
leather to be sewed on, and the seams to lie flat next to the focusing
standard.

The interior of all parts to be painted a perfectly dead black.

The sunshades, when drawn out, shall project at least five-eighths of
an inch and not over 1 inch beyond the edge of the cell.

The focusing screw and standard should follow closely that of the
sample, except that the milled focusing disk should have a face as
nearly one-half inch wide as possible and the milling should be sharper.

In addition to the diaphragm upon which the automatic lens is mounted,
there shall be two diaphragms in each tube, so situated and so
proportioned as to cut off all stray light and all internal reflections.

The crossbar supporting the draw tubes should be shaped and engraved
exactly as found in the model.

(_d_) The lenses must be entirely free from mechanical defects, such as
specks, air bubbles, etc.; must be free from interior strain, and must
be ground from the best obtainable glass for the purpose, selected for
general transparency, as colorless as possible, perfectly ground and
polished, and accurately centered.

_The object lenses_ shall be composite, achromatic, and well corrected
for spherical aberration, with a clear aperture of at least 1½
inches, and not exceeding 1-5/8 inches. Bidders will state the number
and shape of the pieces used to make up this lens.

_The compound lenses_ may be either cemented together with Canada
balsam, or left uncemented, as the maker may deem best for durability
and optical performance, but if left uncemented the components shall
have a permanent mark to indicate their proper positions in the cell.

_The eyepieces_ shall consist of a single double concave lens having a
clear aperture of not less than three-eighths of an inch and not more
than one-half of an inch.

(_e_) The sling cord attached to eyes in the frame by means of brass
snaps with black burned finish shall be round and braided from four
strands of pliable tan leather, and shall have a diameter of at least
one-eighth of an inch and not over one-sixth of an inch.

(_f_) _The case and strap_ must be exactly like sample, and of No. 1
stock. Care must be taken to put in only compasses that are in perfect
condition. The strap buckle must be of brass. The glass, when closed,
must not exceed 4 inches in length, and the glass, case, cord, and
strap, complete, must not exceed 25 ounces in weight.

(_g_) The frame shall be constructed with jointed bars for
interpupillary adjustment.

5. _Service field glass, type B._--(_a_) The requirements of part 4,
service field glass, type A, of this specification, shall be followed
in the design and construction of the type B glass in so far as
applicable.

(_b_) Power: The lower power shall be approximately 4½ and the high
6½ diameters.

(_c_) Object lenses: These shall have a clear aperture of at least
1¾ inches diameter.

(_d_) Case: Case and carrying strap shall be furnished as required in
part 4 of this specification.

(_e_) This glass shall be constructed with jointed bars for
interpupillary adjustment.

(_f_) The sunshade, when drawn out, shall project not less than
three-eighths of an inch and not more than 1 inch beyond the edge of
the cell.

6. _Marking._--Glasses furnished under this specification shall be
marked on one barrel with the words "Signal Corps, U. S. Army," and on
the other barrel "Serial No. ----." Serial numbers will be furnished
with the order. If not furnished the contractor at the time the order
is placed, the Disbursing Officer of the Signal Corps should be called
upon for same, and the numbers and other marking placed on the glasses
prior to the delivery of the order.

                              JAMES ALLEN,
                                   _Brigadier-General,
                                      Chief Signal Officer of the Army_.

    SIGNAL OFFICE,
    _Electric and Telegraph Division_.

       *       *       *       *       *

Transcriber's Notes:

There is no figure 25 in this text. The numbering goes from
figure 24 to figure 26.

Page 23, "porportions" changed to "proportions" (in proper proportions)

Page 106, "engineeer" changed to "engineer" (a French engineer)

Page 126, opening bracket added to subtitle ([Revised February 10,
1910.])