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of being dismantled and split up into loads to conform to the necessary conditions. For railway transport, the railway mount- ings must be of the standard gauge l and conform to the railway load-gauge, which imposes limits as to width, height and shape of load. Any mounting must be able to travel on a curve of 150 feet radius, which governs the total length. The total weight is not limited as such, but it is governed by the span over which it is distributed; it must be supported on such a number of axles that the maximum load per axle is not more than 19 tons; and the total load divided by the length of the wheel- base over the buffers is not more than 2j tons per foot run.

Practically all modern mobile carriages at present have as their basic structure a gun carriage consisting of a pair of wheels, an axle-tree, a trail and a limber consisting of a box mounted on an axle and two wheels, provided at the rear with a hook for connexion to the trail-eye, and at the front with the draught fittings. This method of transporting mobile artillery is universally employed both with horse-drawn and tractor- drawn carriages which can be travelled as entities. At the present day, however, caterpillar mountings are in use or under experiment in many countries.

Regarded from the firing aspect, the requirements of a field carriage demand as extended a traverse and elevation as pos- sible, consistent with the requisite mobility. The main general designs may be given as follows:

Three-Point Support Carriages. In the usual form of carriage, the firing system, resting on the ground by its two wheels and the point of the trail (which is secured by a tract spade embedded in the ground), forms a three-point support. To preserve the side stability of this type, the force of recoil must be nearly coincident with the centre-line of the trail; consequently only a relatively small angle of traverse 4 either way is allowable. The amount of elevation possible is affected by the form of the trail.

Four-Point Support Carriages. This is a modification of the three- point support, consisting in the use of a split trail, the halves of which can be splayed outwards to form two struts, instead of one, in the firing position. In effect, this type has two trails and gives the advantage of an extended traverse 20 each way without change of position. The amount of elevation obtainable is controlled by the balance of the gun and its height above the ground.

Temporarily Fixed Mountings. When a road-mobile carriage cannot be travelled as an entity, there is no need to retain the wheels as part of the firing support ; consequently such carriages must have embodied in the design some arrangement to allow of large traverse at least 30 each way in order to obviate subsequent alignment of the basic structure. The basic structure consists in a platform temporarily secured to the ground with the superstructure mounted above it in such a manner as to be pivoted. The transporting carriages are designed to facilitate mounting and dismounting. Ele- vation is affected by the shape of the superstructure.

Railway Mountings. These are super-heavy carriages travelled as entities but restricted to the use of a permanent way.

Factors in Design of a Gun. The main object in the design of a gun is to ensure sufficient strength at all parts of the gun to withstand firing stresses, with a considerable margin of reserve strength to provide accidental abnormal pressures or deliberate increase of pressure that may subsequently be found necessary.

In the design two important principles must be observed (i) the tube in actual contact with the powder gases must be relieved of all longitudinal stresses; and (2) the work must be distributed over all the various parts of which the gun is built up. The pressure developed when the gun is fired acts in all directions; therefore the force acting upon the face of the breech- screw is equal to that acting upon the base of the projectile. In order to give effect to (i) above, it is always arranged that the breech-screw shall not gear direct into the inner " A "-tube, but into one of the outer parts of the gun; the only longitudinal stress, then, to which the inner tube is subjected is that caused by the projectile passing along the bore. As regards (2) above, the longitudinal stress is transmitted from the breech-screw through the various intermediate parts to the jacket or outer part of the gun (fig. i).

1 During the war period numerous equipments were designed for employment on narrow-gauge lines, notably by the French. But these were not railway mountings in the true sense, as they did not both travel and fire from the line. Their transport to the nar- row-gauge track might be either by road or by rail.

In the case of wire-wound guns, requirements of longitudinal and " girder " strength present certain difficulties, because the wire coils do not give any strength in the direction of the axis of the gun. The practice is to make the parts outside the wire take the whole of the longitudinal stress, while they take little part in resisting radial stresses. " Girder " strength, or stiffness, is provided by the inner tubes as well as the outer parts; but in all long guns, more especially when wire-wound, there is a tendency for the unsupported part to sag.

The radial stresses are taken by the inner tubes and the wire in wire-wound guns. Since these stresses are not equal at all points in the bore, it is necessary to arrange the wire-winding accordingly. The firing stresses are greatest in the chamber, and as the projectile travels along the bore they gradually decrease. With a heavy gun, the chamber-pressure is about 18 to 20 tons per square inch, while the muzzle-pressure is only from 5 to 7 tons. The greatest strength, therefore, is required over the chamber; the number of layers of wire is greatest at that part, and gradually decreases along the gun towards the muzzle. In addition to regulating the amount of wire, the tension at which the various layers are to be wound must be calculated; these winding tensions are so arranged that, when the gun is fired, the whole of the wire is, theoretically, equally stressed. In all designs the circumferential strength is calculated with a margin beyond any anticipated stresses; this "factor of safety" is usually between 1-5 and 2.

(a) Breech Mechanism Frame; (b) Breech Ring: (c) Shrunk Collar; (d) Jacket; (e) Wire; (I) "A' 1 Tube; (g) Breech Bush; (h) Inner "A" Tube; -x (i) Obturator Seating.

FIG. i. Breech end of B.L. 15-in. gun.

The longitudinal stresses are taken by the outer parts of the gun, viz. the jacket and " B "-tube, being transmitted to the former from the breech-screw through the breech-bush, shrunk collar and breech-ring. In order to provide for stretching of- the inner " A "-tube and to prevent the formation of " steel- choke " in the bore, there must not be external shoulders, or steps, on this tube, except those at the rear end and immediately in front of the chamber which are provided to prevent bodily forward movement of the tube.

Further, the " girder " strength of the gun must prevent it from bending under its own weight when supported on its mounting, which normally affords support for only a small por- tion of the length near the breech end. For this reason the inner and outer tubes must be of sufficient thickness. Practically all tubes are of greater thickness than is theoretically necessary.

Rifling. Exhaustive trials at various times have been carried out for comparing various systems of rifling for high-velocity guns.

The general considerations are : the greater the number of grooves and the slower the twist the narrower can the lands be, owing to the reduced pressure on each driving edge of the grooves; shallow grooves weaken the bore of the gun less than deep grooves, but in a worn gun the latter allow the power of rotating a projectile to last longer ; the broader the grooves in comparison to the lands, the greater the unengraved portions of the driving-band, and consequently the greater the resistance to shearing the band, with less danger of the band wearing smooth; the edges of the grooves should be normal to the surface of the bore so that the rotational pressure may be ap- plied by the driving-edge at right angles to the radius of the pro- jectile; the form of grooves should be as simple as possible, and the lower angles rounded to avoid any tendency for cracks to develop.

The form of groove in favour in Great Britain is a slight modifica- tion of the P.P.S. groove, so that the number of grooves per inch of