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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, the adoption of the buzzer telephone as a standard army instrument and the employment of the motor cyclist as a message carrier placed two new methods of intercommunication at the disposal of the signal officer. The outbreak of the World War in