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1200 sleepers are laid at half the usual interval, and extra long sleepers are used. Sleepers laid in good ballast offer sufficient resistance to displacement by stress acting in the general direction of the line of rails, but, as was found in FJanders, a track laid in wet clay is liable to shift, and has to be anchored by driving in stout pickets.

Guns on British " rolling " mountings, as opposed to the French " sliding " types, are fired from the springs. That is, the downward firing stress, reduced by the hydraulic buffer, is taken on the ordinary truck springs. Messrs. Krupp, in their heavier mountings, jacked up the gun-mounting at every axle, so that the stress of firing came directly on the axle-boxes and not on the springs. Messrs. Schneider use wooden baulks under the central part of the mounting, which are jacked down before firing so as to bear on the 3 pairs of rails. In some American mountings the gun-mounting is jacked up and then lowered on to wooden baulks inserted between it and the rails.

FIG. 40. French 155-mm. Schneider, on railway mounting.

On account of the limitations of height imposed by the loading gauge, all the heavier mountings have to be ot the general type illus- trated, namely, a pair of massive girders bent down in the centre nearly to rail level, with the ends supported on pairs of bogie trucks.

The lateral stability of a railway mounting supported on a line of rails of ordinary 4 ft. 8| in. gauge is small, and it would be impossible to fire a heavy gun across the line of rails without some special sup- port. Therefore all-round-fire mountings have lateral outriggers to take the firing stress. Modern improvements are in the direction of improving the efficiency of these outriggers.

All recent railway mountings allow of the gun being elevated to at least 40, and to ensure room to recoil at high elevations rear- trunnions are usual, the resulting forward preponderance being balanced by counterweights or by compressed-air gear. Thus the German I i-in. gun had a box on the rear end of the cradle which was loaded with 4-5 tons of iron pigs. This box was dismounted for travelling, as it was too high to pass through tunnels. This was a clumsy device and entailed heavy labour in shifting the pigs. British and American makers use the hydro-pneumatic balance spring described earlier in this article.

In order to reduce the effort required to elevate heavy guns relieved trunnions are usually fitted to the cradle, or to the gun when no cradle is used. (This device is, of course, not peculiar to railway mountings.) The trunnions which support the piece when firing are necessarily of large diam. causing considerable friction. They are therefore fitted with projecting secondary trunnions of small diam., supported by rollers resting on leaf springs. These springs are ad- justable by screws at their ends so that they take nearly the whole weight off the bearings of the main trunnions. This reduces the friction so that the piece can be easily elevated by hand gear.

In France there is an extensive network of metre-gauge railways built as feeders to the main lines, and these again connect with 6o-cm. tramways. The latter were often joined up to the system of trench railways. In order to utilize this network of railways to the full, many French mountings are built with additional narrow-gauge bogies. These can be lowered and the main bogies raised so that the mounting can travel on its narrow-gauge wheels, but no attempt has ever been made to fire from them. They are used to convey the mounting to a prepared platform on to which it is lowered.

The heavier French, German and American railway mountings are provided with an electric power plant for running the gun-truck into position on the curved siding, hoisting the shell, ramming home, elevating, and closing the breech. The British mountings have usually power winches, run by small petrol engines.

Ammunition supply to a gun on railway mounting is by means of a trolley, running from the ammunition truck to the gun along a small tramway laid on top of the intervening trucks. For small calibres such as the 6-in. a long trough is used. The shell is hoisted by a crane and loaded by a power rammer; the gun is usually depressed to the horizontal position for loading. In all-round-fire equipments the loading platform traverses with the gun, so that the gun has not to be traversed to the fore-and-aft position after every shot.

In the French mountings which slide along the rails on recoil, the ammunition truck has to be uncoupled from the gun truck during firing. Ammunition has then to be brought up by a trolley running on a light railway laid by the side of the main track.

Roving railway guns were often used in Flanders. These were field guns or naval 12-pdr. mounted on trucks with all-round fire, and were run out by night to positions in the open whence they could enfilade German trenches and advanced works. They used to fire 50 rounds of rapid fire and withdraw before they could be located. But this was annoyance rather than serious warfare.

A heavy railway gun requires an attendant train, including ammunition trucks, workshop, trucks with materials and tools for building sidings and emplacements, and trollies (preferably motor- driven) for bringing up ammunition at night. The train is usually kept well back or camouflaged or both. But the real difficulty lies in concealing the specially laid sidings, and the places where they leave the main line.

German Methods. The Germans used few if any railway mount- ings till 1916, although Messrs. Krupp had a 12-in. gun on truck mounting in 1913. They brought up heavy naval guns by rail and mounted them on permanent concrete platforms with central pivots; later they used steel caisson platforms bolted together on the spot and weighted with earth. The gun was brought up on a 4O-ton commercial truck, or two such trucks for the heavier pieces. To mount it two temporary lines of rail were laid, one on each side of the platform and a third line up to the centre of the platform. A horizontal crane or gantry, consisting of a girder capable of carrying 40 to 100 tons according to the size of the gun, was supported at the ends on trucks run on to the two lines, so that the gantry was over the platform. The gun-truck was run up on the third line, the gun lifted, the truck removed, and the gun lowered on to its platform.

Later on, the Germans developed a new system which enabled the gun to be fired either from a railway mounting or from a plat- form. The railway mounting was of the ordinary type, consisting of two heavy girders bearing the gun-carriage, supported at their ends on trucks, and bent down in the centre so as nearly to touch the rails. The mounting was run on to a siding built across the plat- form, jacked up, the trucks removed, and the gun-mounting lowered on to the central pivot of the platform. The siding was then taken up and all traces of it were concealed.

In 1916 the Germans, impressed by the mobility of the French railway guns on the Somme, began to use travelling railway mount- ings more freely. However, they never developed a satisfactory traversing mounting, and their guns were nearly all on non-traversing rolling or sliding mountings layed by means of curved sidings. To save time in running the gun-truck forward after recoil, they used a scotch clamped to the rail, behind each wheel. By Nov. 1917 they had 9-45-in. guns of 30 and 40 cals., and n-in. guns, long 9-45*8, and, finally, 15-in. guns on truck mounts. In 1918 they had long 9'45-in. guns of 45 cals., and by July 1918 they had 15-in. guns on truck mounts.

A representative of the lighter nature of railway guns designed for all-round fire is the Schneider 155-mm. (6-i-in.) gun shown in fig. 40. This was a 28-cal. siege gun of only moderate power, but it proved

FIG. 41. British g-2-in. gun, on railway mounting.

very useful in the early days of the war. The railway mounting is of simple construction. The gun recoils up inclined guides and returns to the firing position by its own weight. The carriage is on a roller ring fixed to the platform of the truck, and gives all-round fire. The truck js supported when firing by screwing down the baulks under- neath it upon the rails; the outriggers seen in the plate are used when firing across the rails. This gun fires from the ordinary permanent way, no extra rails are required, and only a few minutes are spent in coming into action. Another example is the French ig-cm.

The British Q-2-in. Gun on Railway Mounting. The 9-2-in. gun formed the principal armament of the British coast defences. In 1914 a number of old guns of this calibre were sent to France, and these were afterwards replaced by Mark X. guns from coast de- fences, and later by Marks XIII. and XIV., which were more accurate at long ranges. The gun weighs 30 tons and fires a 38o-lb.