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forward again by two power winches and tackles attached to hold- fasts beside the line. The gun recoils in a cradle with ordinary hydro- pneumatic gear; the upper carriage traverses 2 each way. Traverse beyond this limit is given by a curved siding.

This gun has rear trunnions and the hydro-pneumatic balance- spring already described. The advantage of the mounting is that the gun can be fired from the ordinary permanent way without any prep- aration; no extra rails are required, there is no platform, and the downward stress of recoil is taken on the truck-springs without the necessity for jacking them up. (H. A. B.)

(VI.) RAILWAY PIVOT AND FIXED MOUNTINGS

German designers during the war showed a marked preference for railway mountings which could be utilized both on the rails and in a prepared emplacement. Since the war, the develop- ment of mountings of this class has been taken up in the United States. The British did not favour mountings of this class. In the French service the pioneer mounting of the rail-and-plat- form type was a 24O-mm. equipment on Tournier carriage with double recoil gear. In this the truck, on arrival by normal- gauge railway, received under its ends two light railway bogies, on which it was brought to a prepared firing platform with pivot. When in position it was jacked up bodily, with ordinary and light-railway wheels still attached and lowered over the pivot. The gun travelled separately and was mounted after the carriage had been got into position. A much more powerful mounting, which, though it gave only a limited traverse, must be regarded as a true pivot-mounting, was the 45-cal. 34O-mm. (i3|-in.) gun mounted on St. Chamond truck. This, like the preceding, was fired only from a prepared platform, but it travelled complete and the platform was prepared (for a traverse of 10) on the railway track itself. It will be seen from the illustration that the travelling truck was brought over a pivot prepared in the line and jacked up. The long bogies were run clear and the main girder then rested in its centre on the pivot and at its ends, by jacks, on two platforms prepared in the line. The rear plat- form was connected with the pivot seating by girders, and the rails between them were removed and a recoil pit dug. The weight of the equipment (bogies included) was 166 tons, and the range of elevation from 15 to 42. Unlike these, the German and American mountings now to be described were all capable of being used as indifferently rolling (or sliding) mountings and as immobile pivot-equipments.

German Railway Mountings. The railway mounting of the German n-in. gun was a good specimen of the German pattern intended to fire either from the truck or from a platform. The gun was a 4O-cal. naval gun weighing 45 tons; it threw a shelj weighing 630 lb., M.V. 2,740 f.s., to a range of 30,800 yd. It is possible that a streamline shell with supercharge, ranging about 35,000 yd., was also used. The gun had ordinary recoil gear, with an air recuperator working at 85 atmospheres, allowing 38 in. recoil. It had rear trunnions and counterweight as already described. The mounting had electric power gear throughout, enabling a rate of fire of 2 rounds a minute to be attained, at any rate, with platform mounting.

In the railway mounting, the weight was taken by jacking up the springs so that the axle boxes (20 in all) transmitted the strain to the rails, and recoil so far as not absorbed by the buffer and recuperator, was taken up by scotches behind each wheel. A total traverse of 2j could be given.

The platform gave an arc of fire of 180 degrees. It consisted of 2 transverse caissons 24 ft. long, on which were laid 3 longitudinal caissons 36 ft. long. These were laid, bolted together, and filled with 45 tons of earth. The central longitudinal caisson carried the steel top plate of the platform with roller ring; and the main girder of the railway mounting, with the gun, was lowered on to the pivot, and traversed on rollers which travelled round the roller ring. The plat- form was laid, and the gun mounted on it, by means of 3 temporary lines of rail and a gantry, as described above, or by means of screw- jacks by which it could be raised to remove the bogie trucks, and then lowered on to the roller ring. In the later equipments auxiliary machinery was fitted, driven by the electric power plant used in working the gun. The work of laying the platform, not includ- ing the temporary railway, took from 4 to 7 days, and the gun could be mounted in one day.

The German sSo-mm. (i$-in.) Gun was used in the field in 3 ways as a railway gun pure and simple, as a railway gun with fixed pivot, and as an immobile gun in a semi-permanent emplacement. The gun itself was a 15-in. naval gun 45 cals. long and weighed 77! tons. It threw a comparatively light false-cap shell weighing 785 lb. with a huster of 69 lb. and 3 driving bands, the design of the pro- jectile being generally similar to that of the 75-mile gun which

bombarded Paris (see below). In this case the maximum range was 28 miles. The gun had a ring cradle with buffers and recuperator admitting of a 5O-in. recoil. It had a counterweight over the rear of the ring cradle to lighten the work of elevating.

As a railway gun it was mounted on a truck of the usual type, but an intermediate carriage between the cradle and the girders enabled 1 of traverse to be given either way. The position of the trunnions was such that in firing from the open track no more than 17 eleva- tion was possible, representing a range of less than 27,000 yd. only one-half of the range which could be obtained with the gun emplaced. In the firing position, the weight was taken and the shock not absorbed by the cradle transmitted by the axle boxes of the bogies, as in the I i-in. mounting above described. The total of axles in this case was 18. The gun was layed for direction by movement along a curved siding of i8o-in. radius. The total weight was 270 tons. This was the largest railway mounting used in the war.

When the railway mounting was used with an emplacement, the latter was provided by a number of caissons 7 ft. deep, bolted together so as to form a ring 40 ft. in diam., leaving a central pit recoil. The caissons were laid on a foundation of 3i-m. iron plates resting on I-girders. On top of the caissons was the platform plate, with a roller ring about 36 ft. in diam. ; the main girder of the railway mounting was supported by rollers on this ring. The gun was let down to the platform by jacks permanently fixed in the main girder, and no gantry was required. This mounting allowed of 55 elevation ; the range was 52,000 yd., and the rate of fire was one round in 3 minutes. The total weight of the gun and platform, besides the concrete filling of the caissons, was 270 tons. The platform took 3 weeks to build, not including the line of rails leading to it. Under favourable circumstances the gun could be mounted in one day.

German 38o-mm. gun, on railway mounting.

The fixed mounting was of a type peculiar to this equipment known as the " bridge mount." It can be used only with a gun which, like this one, has a low recoil-energy due to the abnormally light weight of the shell compared to the gun. The bridge mounting consisted of a quadrant-shaped pit spanned by a bridge formed by a pair of massive girders between which the gun was mounted on its cradle trunnions. The bridge was pivoted at the front end, and the rear end traversed on rollers round the quadrant arc at the bark of the pit, giving a total traverse of 90 degrees. The cradle trunnions were I ft. 6 in. in diam. and rested in trunnion bearings on top of the girders, 1 1 ft. 6 in. from the front ends. The girders were 35 ft. long, 6 ft. 6 in. deep, and 6 ft. apart. The pivot at the front of the bridge was 7 ft. in diam. and 6 ft. 3 in. high, carried by a cast-steel base bolted down with 3-in. bolts to a steel platform 20 ft. square. The sides of the square were formed by steel box girders 4 ft. broad and 6 ft. deep, produced towards the back of the pit. This was formed by a quadrant-shaped steel caisson to which the ends of the box girders were bolted; the caisson was 3 ft. 6 in. from front to rear and 7 ft. deep. It carried the roller quadrant, 32 ft. in radius, along which the rollers supporting the rear end of the bridge rolled. The pit was 10 ft. deep. The gun was mounted by means of a gantry carried by 2 trucks on 2 temporary lines of rail, one on either side of the pit.

The German 75-mile Gun (called " Big Bertha "). In March 1918 the Germans bombarded Paris from a distance of 75 m. The tactical aspect of the question of fire at this and at longer ranges is discussed under ARTILLERY, this article being concerned with the gun itself. The loo-m. gun, so called, was not a new invention ; designs for simi- lar weapons had been prepared by the British and French Govern- ments some years before the war, but had not been carried out, as the advantages were not considered sufficient to warrant the ex- pense. A design, due to Col. Maitland-Adclison, is shown.

The principle on which the gun depends is that if a shell be fired so as to pass through the dense layer of air next to the earth's surface and to emerge into the very thin atmosphere which exists at a height of 10 m., its flight will be practically unimpeded until it descends again, and therefore its range will be very considerably increased. The German shell in question rose to a height of 24 m. during its flight, and more than 50 m. of its trajectory lay in air of a density less than one-tenth of that at the earth's surface. To obtain this result the shell must be fired at an angle of 55 to the horizontal, and