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wheels. The latter were fixed to the railway wheels when the vehicle left the railway for the road. The power vehicle also had railway wheels, and, with the carrier, formed a small independent railway train. The carrier could be included in an ordinary train if desired, but not the power vehicle which, in that case, had to be loaded on a truck.

The American Christie motor overcomes the difficulty of combin- ing a road vehicle with a cross-country vehicle in another way. This motor has rubber-tired road wheels, four on each side. It can travel on the road at a fast pace on these wheels, or rather on the fore and hind wheels, the centre pairs being raised clear of the ground. To cross country a caterpillar track is put on round the four wheels on each side, the centre pairs of wheels being lowered to take their share of the weight. To put on the tracks, one-half of each track is laid out on the ground and the vehicle run on to them; the top halves of the tracks, which hang from the framework, are dropped on to the wheels and the ends pinned together so that the tracks are continuous. It is stated that this takes 15 min., and it is expected that the time will be reduced to 10 min. by improvements in the gear.

Motor Carriers. A tractor is usually expected to carry the gun- detachment and a " first supply " of ammunition, besides pulling the gun. But the platform carrier is a separate type, performing the duties of an ordinary lorry, and also capable of carrying heavy weights across country. When a carrier is used for transporting a gun it is fitted with a ramp to enable the gun to be run up on to the platform and run down when it has to come into action. Some car- riers have power winches for hauling the gun up. Light guns can, on emergency, be fired from the platform in a fore-and-aft direction, but not across it, since a modern field gun measures 10 ft. 6 in. from the lowest point of the wheels to the spade. The advantages of the carrier over the tractor are that the gun is not damaged by fast travelling on the road, and that in very deep boggy ground it is sometimes possible to carry a gun on caterpillar tracks, but im- possible to drag it on its wheels. The principal disadvantage of the carrier is that the gun takes up so much space on the platform that there is no room for the men and the ammunition, and an additional vehicle is required to carry them. The tractor is therefore generally preferred to the carrier, as being a more economical form of transport.

The British tried a caterpillar platform-carrier fitted to take a 6o-pdr. or 6-in. howitzer. It weighed 35 tons with the gun and 24 tons empty. It was too heavy to cross bridges, and was discarded as an artillery vehicle ; it was afterwards used to carry up stores over the crater-field on the Somme.

The largest road carriers used were the Austrian 8-wheel-drive howitzer wagons, driven by electricity from a separate carriage. These are described below. They weighed 35 tons loaded, which is the limit of weight even for bridges on the great " national " roads. This method of propulsion reduces the weight of the carrier, as the engine is transferred to a separate vehicle; the Austrian carriers weighed 15 tons and carried 20 tons. Since, however, they are quite incapable of moving across country, the caterpillar type of carrier is now preferred to them, especially as this type also can be lightened by driving by electricity from an accompanying power vehicle.

The Automobile Gun-carriage. This is quite distinct from the tractor and the carrier. It is a motor vehicle of the caterpillar type with the upper gun-carriage built into it, so that the gun and vehicle are inseparable. Several of these mountings, both of the ordinary and Christie pattern, are described with the guns for which they are designed. They admit of elevation up to about 40 being given to the gun, but not of traversing; it ; the gun is layed for direction by turning the whole mounting by means of the caterpillar tracks. In the heavier mountings, a small auxiliary motor is fitted for this purpose. A special pattern is the St. Cha-nond automobile gun-carriage de- scribed below, which has an electric drive, the power being supplied by a motor and dynamo in a trailer which follows the gun.

The automobile gun-carriage is mechanically efficient and the objections to it are tactical rather than technical. It is too big; it is conspicuous and difficult to conceal; it offers too large a target; and it requires an immense anount of labour and material to make a covered emplacement for it. While the gun is in action the ex- pensive and vulnerable motor is exposed to fire, whereas it would be much better employed behind the line bringing up ammunition. On this account most artillerists prefer the tractor to the automobile carriage. Moreover, as regards field artillery, the administrative point of view decides the matter. No nation can afford to keep up in peace-time the number of military motors required for the field artillery of a national army; it must depend on civilian motors. And of these the only ones capable of moving across country are the agricultural tractors. These are the machines that will have to be developed so as to suit both civil and military requirements, and they are essentially different from the special type built for an auto- mobile gun-carriage.

Armoured Motors. Automobile gun-carriages can be fitted with ordinary bullet-proof shields, but they cannot well be armoured so as to resist direct hits from artillery. A field gun of pre-war pattern, at a range of about 3,000 yd., will pierce a ij-in. armour plate with ordinary shell, or a 2-in. plate with armour-piercing shell. A modern high-velocity field gun, firing armour-piercing shell, will pierce about 3 in. of armour at the same range. But a 3-in. plate weighs 1,150 Ib. per sq. yd. If the automobile carriage were armoured in front only,

it would require some 8 sq. yd. of armour, weighing over 4 tons, and would then be proof only against field guns firing from the front at ranges greater than 3,000 yd.

Transport of Artillery Motors. The present types of artillery caterpillar tractors, carriers and automobile carriages are not suited to long journeys by road. The vibration caused by the ribbed steel tracks shakes them to pieces. Future motors will no doubt be better built, with spring-centred rollers and probably with smooth tracks. But all motors should be designed so that they can be transported by rail for long journeys. The practical limit of weight is the 4p-ton commercial truck used on European railways; the limita- tions of height and breadth comply with the local loading gauge.

For road transport the extreme width ought not to exceed 8 ft., but some of the automobile mountings are 9 ft. wide. The principal limitation is that of weight, and is generally taken at 25 tons for main roads and 20 tons for by-roads, though many country bridges will carry only 15 tons. On the other hand, the bridges on the great main roads of the continent of Europe are mostly equal to 35 tons; much depends on the length of the wheel-base or the tread of the caterpillar tracks. Practically, a 1 5-ton tractor was the largest size commonly used in the war, though the Germans used some 2O-ton F.W.D. Daimler tractors, and the Austrians used 8-whee!ed car- riers up to a gross weight of 35 tons.

All road motors are liable to be stopped by the destruction of bridges, though it is anticipated that military motors will in future be built so that they can cross fords, and some will even be made to float. These are described under BRIDGING, MILITARY.

Field-Gun and Howitzer Equipments. The British i8-pdr. (see 20.221), owing to its heavy shell, proved to be admirably suited to trench-warfare conditions, and remained the standard British field gun throughout the war. It underwent several improvements. In 1915 the spring-case was filled with oil, so that the running-up springs worked in an oil-bath; this reduced the vibration in running up and made the carriage steadier. In 1916 the springs were replaced by an air recuperator fitted into the old spring-case; this change was made on account of the number of springs crushed during rapid fire and the difficulty of replacing them. In 1918 a new barrel was issued, which had an improved Welin screw breech-mechanism, with the actuating lever pivoted horizontally instead of vertically, so as to work with a downward instead of a lateral pull.

FIG. 8. British i8-pdr. gun and carriage, 1918.

The carriage issued with this gun (fig. 8) is a reversion to the nor- mal type, with a trough cradle under the gun in place of the ring cradle which was a characteristic feature of the original l8-pdr. equipment. The lateral guide-ribs on the old gun had always been a source of trouble, as the unsymmetrical section of the gun caused it to bend after continued firing. In the 1918 gun these are done away with and replaced by guide blocks which slide on the cradle. The cradle is extended to the rear beyond the breech, so as to give good support when the gun is in the recoil position. It contains the hy- draulic buffer of ordinary variable-recoil construction and the hydro- pneumatic running-up gear now called the " recuperator." The lower carriage has a box trail instead of the old tubular trail, with an opening in it which allows the breech to be depressed to give 30 (later patterns, 37) of elevation, thus obtaining a range of 9,000 yd. with ordinary shell or nearly 11,000 yd. with streamline shell.

The carriage traverses on the axle as in the French equipments. The shield is fixed to the axle, and there is a small inner shield fixed to the cradle which closes the port in the main shield in which the gun traverses. The upper part of the shield is made to fold over. The weight of the whole equipment is not materially greater than that of the previous model, and the gun in action weighs 26 cwt.

Only one i8-pdr. battery of the above pattern was actually used in the war. The equipment did well, but the latest developments in field-artillery design, which are discussed below, made it undesirable to continue the production of this type, and a new gun and carriage, adapted either to horse or motor transport, was designed in 1920-1.

The French field gun is the 75-mm. (2-95-in.) Q.F. gun, well known as the soixante-quinze (fig. 9). It fires a i6-lb. shrapnel with M.V. 1,740 f.s., and an n-68-lb. H.E. shell, M.V.