senting an average correction) is put on to the sight system. The
correction is effected by arranging the sight, when setting for eleva-
tion, to move in a plane inclined slightly to the vertical and inter-
secting the vertical plane in a line parallel to the axis of the gun;
thus the greater the elevation on the sight, the greater the auto-
matic deflection imparted. This method of compensation allows
of a convenient combination with the reciprocating sight, by mount-
ing a transverse bubble in such a manner as to ensure that, when
central, the body of the sight is tilted to the left at the required
angle ; but the open, telescopic or panoramic sights are mounted on
the rocking portion so as to have true verticality when the bubble
is cross-levelled. Thus the cross-levelling gear neutralizes any want
of level, and permits drift to be corrected for by movement of the
sight in a tilted plane when setting for elevation; but the range-
scale must be graduated to register elevation in a vertical plane.

In the system of independent line of sight in use with certain light
field carriages the two parts of which the quadrant angle is com-
posed the angle of sight and the tangent angle are applied to
the gun independently and by different men. This system necessi-
tates an intermediate component carrying the sight, and two ele-
vating systems, the lower of which elevates the intermediate com-
ponent, sight, cradle and gun for angle of sight, and the upper ele-
vates the cradle and gun for tangent angle. Thus, conjointly, the
two elevating gears give the quadrant angle. A sight clinometer is
fitted to the sight to register the angle of sight for indirect laying,
whilst a range-indicator, operated by the upper elevating gear,
registers the tangent angle. Once the sight has been aligned on the
target, the tangent angle may be altered to correct the range with-
out disturbing the layer for direction. Such a system is useful when
firing at moving targets, the direction and elevation having to be
altered simultaneously, but it has a disadvantage in that it is diffi-
cult to apply the reciprocating principle when the gun has to be
moved independently of the_ sight. Drift is usually corrected for
by inclining the trunnion axis. The latest method of securing this
principle, though not truly " independent," combines the recipro-
cating sight with an automatic drift correction.





The rocking-bar sight is mounted on a pivot which is always
parallel to the gun axis, and thus can be rocked laterally under the
control of cross-levelling gear to maintain verticality, which is indi-
cated by a transverse bubble. An acorn-pillar, or front sight, and
notched leaf, or rear sight, are fixed at the extremities of a sight-bar
which is mounted on a vertical pivot on top of a rocking-bar; the
rocking-bar is pivoted transversely at the front of a sight-carrier at-
tached to a support on the cradle. The sight is set for elevation by
means of a worm and arc-pinion drive in mesh with an arc on the
rocking-bar, the range or tangent-angle being registered on a drum
fixed to the pinion-spindle. The pivot of the rocking-bar is inclined
so as to give an approximate drift correction at all ranges. To give
deflection, the rear of the sight-bar is fixed to a nut mounted on an
endless screw carried at the extremity of the rocking-bar; the nut
traverses across a degree scale fixed to the rocking-bar. Bearings
for a telescope are fixed to the sight-bar.

A sight clinometer is usually fitted to the rocking-bar so that, if
necessary, the sight can be set first for angle of sight and subse-
quently for tangent angle. In effect, the clinometer is an adjust-
able spirit-level. A bubble is mounted in a sliding segment having
worm-teeth cut on a curved base; the segment is moved relatively
to a carrier by a screw mounted in the latter and in mesh with the
teeth. Degrees of elevation or depression are registered on the face,
while the screw is fitted with micrometer head to read minutes.



The employment of muzzle-velocity correctors for use in con-
junction with the range drum is becoming general. The corrector
(not to be confused with the " corrector " employed for varying the
height of burst of time fuzes) also provides means for determining
the muzzle velocity from firing at a known range, allowances being
made for standard conditions; sights so fitted are known as cali-
brating sights. The corrector, which may be attached to the oscil-
lating bracket or to some part of the range-gear in independent-line
sights, fits over the face of the range-drum and is graduated in rang-
ing muzzle velocities; a knife-edge reader, hinged at one end, has
the other end attached to a nut mounted on a screw in the corrector
and engraved with an arrow for reading the muzzle-velocity scale.
The knife-edge reads the yard-scale graduations while fine adjust-
ment along the muzzle- velocity scale is ensured by the nut-and-screw
arrangement. The yard-scale is engraved upon a spiral designed in
conjunction with the muzzle-velocity scale to give the correct tan-
gent angle for the muzzle velocity used.




FIG. 2



With certain carriages it is inconvenient to mount the sight on
either the trunnion or the cradle. In such cases the sight is mounted
upon a pivot on the intermediate carriage, the sight-carrier being
constrained, by means of parallel link-motion gear, to make the
same angular movement as the gun. For the sight to be effective,
the parallelogram must be correct, and adjustments may be required
for the lengths of the link and the arm. In certain designs the inde-
pendent line of sight is obtained without the use of an intermediate
component, and combined independent-line and reciprocating sights
are also in use.

The automatic sight is discussed and an example illustrated at
25.64; here, therefore, it is sufficient to recall its basic principle,
which is, that for a gun mounted on a level platform at a fixed
height above mean sea-level, there is only one angle of depression of
sight and one quadrant angle for any given range. Thus, if the sight
be pivoted transversely to the cradle and made with an arm pro-
jecting downwards from its front end, there is, for a given quadrant
angle, one position only for the lower extremity of this arm; if the
arm be caused to engage with a suitably cut cam fixed to the inter-
mediate carriage, any quadrant angle on the gun will impart the
correct sight-depression to the sight.

A distinctive feature in connexion with all anti-aircraft sights is
the necessity for an automatic reduction in the tangent angle for a
given range as the angle of sight and quadrant angle increase. Con-
sequent on the varying range, height and speed of aerial targets,
large corrections have to be applied to compensate for the vertical
and lateral angular movements of the target ; these corrections inter-



SIGHTS



485



act to complicate further the design of an efficient sighting system.
No description of such a sight, therefore, is possible within the
limits of this article. For reasonable accuracy of fire, the mounting
must be level. Anti-aircraft mountings are generally fitted with
training arcs; in some cases a second set of indicators is fitted to
enable horizontal ranges and quadrant angles to be used. A sight-
ing system which also embodies the principle of the independent
line of sight has been tried in the British service.

The panoramic sight is used witji field ordnance for indirect lay-
ing for direction, from a position out of view of the target. The
layer is enabled to take advantage of any auxiliary aiming-mark
and to lay without exposing himself. The sight may be used also
for direct laying.

A typical panoramic sight is shown in fig. I. It is a prismatic
telescope mounted in a vertical tube having a rotating hood at the
upper, and a horizontal tube at the lower end; the hood is
mounted on a horizontal graduated dial-plate with which it may be

revolved through a complete
circle, by worm-gearing; the
worm-spindle is mounted in an
eccentric to enable the worm to
be thrown out of gear for quick
setting; on the periphery of the
dial-plate are graduations read
by a pointer fixed to the vertical
tube; while the ends of the
worm-spindle are fitted with
micrometer-heads to read smaller
divisions. A prism mounted
within the hood can be rotated
about a horizontal axis by means
of a worm-spindle in mesh with
a toothed arc on the prism
holder; this permits of a wide
selection of aiming-points in the
vertical plane. The optical ar-
rangements are shown in fig. 2.
They consist in a reflecting prism
mounted within a holder in the
hood; in the vertical tube, an
erecting prism controlled by
differential gear to move at half
the rate of the dial-plate and
hood ; a fixed object-glass below
the erecting prism ; and a reflect-
ing prism below the object-glass
to reflect light along the hori-
zontal tube; in the horizontal
tube, a glass diaphragm en-
graved with horizontal and ver-
tical lines, and in rear of this,
eye-lenses to give a magnified
image. The rays of light from
the object enter through a plain
glass window and are reflected
downwards by the prism in the
hood; they pass through the
erecting prism, the purpose of
which is to give an erect image,
whatever the rotation of the
hood, and are focused by the
object-glass; they then pass to
the lower prism and are reflected
along the horizontal tube; the
eye-piece magnifies the image




FIG. 3



which is seen on the cross-lines of the diaphragm, the latter being
illuminated by a plain window let into the horizontal tube.

Other forms of dial sight are, the simple graduated and flexible
dial fitted with an open sighting-bar; and the collimator, which
exists in several forms. In principle an eye-lens and ground-glass
window are mounted at the ends of a tube, an arrow or cross on the
window being at the principal focus of the lens. With one eye applied
near the lens, the cross will be seen, but there is no vision beyond the
cross due to the ground-glass; the other eye sees the aiming-point.
The gun is traversed until the cross appears superimposed on the
aiming-point. Both eyes are used, and are viewing approximately
parallel rays. Unlike ordinary open sights, there are not three points
at varying distances to be brought simultaneously into alignment,
but the eyes view two points under practically the same conditions.

Sights for Rifles and Machine-Guns. The improved ballistics of
rifles generally, on the one hand, and the shortening of the sight
radius due to shortening of the barrel, on the other, have made the
question of better sights a very urgent one. Attempts to retain the
old sight radius by bringing the rear sight closer to the eye resulted
in the discovery that it was impossible effectively to use an open
sight nearer than twelve to fourteen inches from the eye (owing
to the inability to focus on rear sight, front sight, and target at the
same time) and led to the reintroduction of the aperture or peep
sight, in itself an old invention. In this the rear sight is placed close
to the eye, and no attempt is made to focus on the aperture. The



eye looks through the aperture and instinctively centres the front
sight. Any blur around the aperture will not impair the accuracy
as it is equal on all sides of the opening. If the eye is now focused
on the target, the front sight is at sufficient distance to afford very
good definition. Another advantage is that the accuracy of the
sight is not affected by slight changes of light.
_ Aperture sights are of two general types: the disc aperture con-
sisting of a large disc, which shuts off all view of the target except
that seen through a peep hole of from -03 to -06 in. in diameter;
and the Lyman type which has a thin rim with peep hole about -oio
in. in diameter. Disc sights, which are used principally for target
shooting, are unsuitable on account of their limited field for moving-
targets, or where quick aiming is required.

In the Lyman type the entire target and its surroundings may be
seen while aiming; it may be used in poor lights where open sights
cannot be used, and moving objects may be quickly aimed at. The
sights on the British 1914 rifle (U.S. Rifle Model of 1917), and the
Browning automatic rifle are of this type (fig. 3). The aperture (a)
is o- 1 in. in diameter, and is made through a disc of about twice the
diameter of the aperture. The battle-sight 1 is vertical and in use
when the leaf sight is flat and vice versa ; the sizes of aperture and
ring are the same in both. This sight was found very satisfactory
during the World War. It has no lateral adjustment, service expe-
rience having shown that the sights were satisfactory without this
refinement. An aperture o- 1 in. in diameter is thought by many to be
too large, -070 in. to -080 in. being the size most favoured. The U.S.
1903 Rifle (Springfield), and the now abandoned Ross rifle are the
only other military rifles with an aperture sight. In the case of the
Springfield, which has also sight notches of the usual type on the
slide, it is found that, although the aperture is very small (0^05 in.),
and too far from the eye, it is always preferred to the V. for fine
shooting. The superiority of the aperture sight over the open sight
is in fact now generally admitted. It is almost universally used by
rifle experts, and the delay in its general adoption for military use is
due to the impracticability of effectively and cheaply applying it to
rifles now in existence.

Optical rifle fore sights designed for use with aperture rear sights
have recently been placed on the market. These sights consist of a
dioptric convex lens of magnification of about 2j diameters, with a
spot or ring in the centre for use as a bead. The lens when viewed
3 or 4 ft. from the eye gives an upright magnified image of distant
objects. An aperture rear sight, preferably of the disc type, with an
aperture of 0-03 to 0^04 in. diameter, or a lens rear sight should be
used. Better definition as well as magnifying power is claimed for
these sights.

Luminous sights for night use have been experimented with and
used to a limited extent on machine-guns and rifles for military pur-
poses. These sights, as a rule, consist simply of metal sheaths which
slip over the front sight and rear sight and have suitable containers
for the luminous material used. For periscope sights see RIFLES AND
LIGHT MACHINE-GUNS.

Telescope sights are often applied to sporting rifles and military
rifles used for special purposes. These sights consist of a telescope
with a reticule for aiming which takes the place of a front sight.
The magnification is usually from 2-5 to 6 diameters; those from 2-5
to 3 power are considered the best for general purposes as they give a
larger field and can be used in poor light. Adjustments for range
are made either by moving the crosshairs only, as in most German
rifle telescopes, or by means of holding brackets with adjusting
screws which give both vertical and lateral adjustment; the latter
type being more accurate. The advantages claimed for telescopic
rifle sights are that they permit more accurate aim by magnifying
the errors of holding the rifle, also that they allow objects to be seen
more distinctly, particularly in a poor light. The latter is probably
their greatest advantage ; their superiority to a good aperture sight
not being as great as is popularly supposed.

The Vickers and Browning machine-gun sight embodies the prin-
cipal small-arms-sight developments of recent years. The slide, which
is made to travel obliquely in the leaf to compensate for drift, has
a revolving disc with apertures of various sizes which may be used
for different ranges and light conditions. A fine adjustment for ele-
vation and a windage adjustment are provided, also an open battle-
sight. The fore sight is a blade, open or hooded.

The Lewis gun has a simple tangent aperture sight without drift
or windage adjustments, but with a vertical adjusting screw for
fine adjustments in elevation. The sights used for direct fire on
Continental European machine-guns are all of the open type and
are usually without correction for either drift or windage.

Tubular sights, which consist simply of a straight piece of tubing,
are used to some extent on ta_nk machine-guns, as these sights only
require a very small opening in the armour plates for their effective
use. The latest tendency, however, is to provide tank machine-
guns with a telescopic sight of special construction.

Anti-aircraft machine-gun sights, which compensate for the drift
and trajectory of the bullet and offset the speed and direction in

1 A battle-sight is one which requires no adjustment for distance,
or rather is permanently set to a certain range, inside which the
bullet in its flight never rises more than a man's height above the
line of sight.



486



SIGNAL SERVICE, ARMY



which an aeroplane is travelling, were brought into use during the
World War. One of these is illustrated in fig. 4, showing a design
used by the British and U.S. armies. This consists of a slide having
an aperture 0-25 to 0-5 in. in diameter and fitting on the regular
back sight leaf. The aperture (6) is set at a given height for a stand-
ard range. The front sight has two concentric elliptical ovals
which are mounted as a leaf sight to the barrel or casing of the gun.
The shapes of the ellipses are proportional to the actual horizontal
sections of a cone of fire from the gun at a given height and eleva-
tion. The design of the outer oval (a) is based on an assumed angle
of gun elevation of 50, height of target about 1,000 ft., and a speed
of plane of 100 m. per hour; that of the inner oval, on an assumed
angle of gun elevation of 15, a height of 250 ft., and a speed of 120 m.
per hour. The middle figure shows the correct firing position with
the hostile plane central as viewed through the rear and front sights;
that on the right shows a plane at close range flying at an angle
with the gun position and perpendicular to the axis of the bore; and
that on the left shows the target at a greater range and at an angle
with the gun. (H. O.'L.)






FIG 4

The Panoramic Machine-Gun Sight. The use of machine-guns for
indirect fire during the World War was followed by the demand for
instruments for obtaining data and laying the guns. This demand
led to the production of many devices, some of which were well
fitted for their purpose, and some of which were make-shifts. But,
as the war progressed, it became apparent that indirect fire was be-
coming increasingly important, and an attempt was made to produce
one high-grade device that would replace as many instruments as
possible. This effort, in the case of the United States, resulted in the
production by the Ordnance Department of the panoramic machine-
gun sight, described below. At the same time, all Browning machine-
guns were fitted with a bracket on the left side which was designed
to carry the panoramic sight and also to furnish on its upper sur-
face a flat space for applying the clinometer.




The illustration (fig. 5) shows the panoramic sight in place in its
bracket on the Browning machine-gun. The sight consists of a X6
prism telescope capable of movement in elevation and azimuth
(360). The movements of the sight are controlled by means of
graduated knobs. The lower knob is for movement in azimuth (deflec-
tion), and a quick release is furnished to enable large changes to be
made quickly. The upper knob controls the vertical movement of
the sight. This knob has two sets of graduations. One of these is
for ranges, and the second, which is on a ring-sliding friction tight
on the knob, is for vertical angles, above or below the horizontal
(angle of site). The telescope is provided with a spirit-level which
is parallel with the axis of collimation. This instrument is used
on the same principles as the panoramic sight of artillery, to measure
angle of site and to lay the gun on a visible or invisible target. The
telescope has, besides the usual cross-lines, graduations showing
both vertical and horizontal angles, and also a vertical scale resem-
bling an inverted sight leaf, which is employed for the correction of
fire when the strike of the shots can be observed. The sight can also
be used separately from the gun (in combination with a compass and
a tripod of non-magnetic metal) for the same operations as those
performed by our artillery director. (J. S. HA.)



SIGNAL SERVICE, ARMY (see 25.71). In the ten years that
elapsed between the Russo-Japanese War of 1904-5 and the
outbreak of the World War in 1914, evolution in military
signalling was rapid, both as regards organization and as regards
instruments employed. The old principle of deliberate crudity
of means, based on the idea that no refined instruments could be
relied upon to survive the rough conditions of war employment,
was giving way to a sense of the great possibilities opened up by
modern science as applied to army signalling, while, in tactics,
the ever-increasing tendency towards articulation of forces and
distribution in depth was forcing the problems of liaison more and
more into the foreground. But such evolution as there was in the
period 1904-14 was naturally slight indeed compared with that
which took place during the World War, in which stabilized
conditions of warfare and the concentration of the scientific
talents of all belligerent countries upon war needs produced
results, both upon the army signal system and its instruments,
that amounted to a revolution in the practice of war and,
moreover, must affect profoundly the methods of intercourse
between civilized nations in peace.

In the following article an account is given of the organization,
working principles, and instruments of the signal service of the
British army in the World War, and of some of the more notable
features of signal practice in other armies.

(1) Definition and Duties. The signal (intercommunication)
service of a modern army is responsible for the maintenance of
efficient intercommunication between all branches, departments,
formations, and units of the army. Intercommunication within
units, other than signal units, is usually provided by means of
regimental signallers, but the officers of the signal service exercise
supervisory control here also. The signal service bears the same
relation to the army of which it forms a part as does the nervous
system to the human body. Its principal duties are:

(a) The transmission of information from the front to unit com-
manders and to the headquarters of formations.

(b) The transmission of orders from commanders to their sub-
ordinates.

(c) The maintenance of efficient liaison l>et\veen infantry and
other arms (such as artillery, air force, tanks, etc.) and between
neighbouring formations.

For the efficient working of an army, means of intercom-
munication must be swift, certain, and, under the circumstances
of modern war, varied. The system must be essentially simple
and standardized to the greatest possible extent, yet capable of
considerable expansion at short notice, and of modification to
meet the most diverse conditions of warfare. Organization and
working schemes must be elastic, and types of signal instruments
must be devised to cope with all special sets of conditions that
have been experienced or can be foreseen.

(2) British Army System. Until the application of electricity
to the long-distance transmission of messages, the intercommu-
nication of armies was carried out mainly by means of visual
appliances or by the use of message carriers. Liaison officers
and orderlies have been used from the very earliest times;
the arrow was frequently employed in mediaeval times for the
transmission of information into and out of besieged towns; the
pigeon was used with success, notably in the wars of the Nether-
lands against Spain; permanent lines of semaphore communica-
tion (masts with movable arms) were employed both by the
French and the British during the Napoleonic Wars. Later, the
invention of the Morse code and the adaptation of the semaphore
principle to field signals led to the general employment of flags,
lamps and, later, of the heliograph. The field telegraph made its
appearance in the middle of the igth century, and in the last
years of the century the field telephone came into use. Lastly,
in the Russo-Japanese War of 1904-5 wireless telegraphy came
on the scene.

In 1911 the British " Signal Service " was constituted as a
distinct branch of the Royal Engineers. At about the same time,