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the maintenance of a free expansion chamber as the tips of the vanes, and not the base of the bomb, rest on the curved shoulders of the chamber proper. The bomb is of steel, thin-walled, and its body is built up from three pieces by autogenous welding, the cruciform vanes being secured to the body in the same way after being riveted

Fia.'S

to each other. A delay-action fuze only is used. With this, the pow- erful bomb, weighing 192 lb., of which 101 Ib. are high explosives,

will demolish thirty linear feet of trench work, break down all but

heavily protected dugouts, and make a crater in compact clayey

earth 10 ft. deep and 30 ft. across. The piece is rear-trunnioned, the trunnions

being firmly held in their seatings in the base of ,the carriage by a locking device. The carriage 'consists oT a bottom-plate which contains the

pivot-seating and the trunnion-seatings, and two

side-brackets which are formed at their top edges

as arcs for giving elevation. These arcs are

,toothed, and into them gear small pinions on a

^cross-shaft secured to a collar on the piece. Be-

, low this cross-shaft, and similarly secured to the

ollar, is another cross-shaft, the ends of which

,;ngage in slots in the side brackets of the car-

jriage and carry the clamping arrangements by

.vliich the piece is secured rigidly to the car-

'iage when the arc-and-pinion gear have brought

j t to the desired elevation. The elevation limits

ire 54 (with certain precautions 45) for maxi-

num range, 75 for minimum. On its underside he bottom-plate of the carriage has an ingen- ous arrangement of locking sectors which, when he carriage is placed over the pivot on the

.>aseplate_ and given a partial turn, engage in a

!ocking-ring on the face of the baseplate. (The ing originally admitted of all-round traverse,

.mt this was later restricted to one of 18 each ide of the mid-line, so as to avoid slantwise strain n the platform.) The baseplate is almost square nd has on its upper side the locking-ring and livot above mentioned. From its underside six ertical flanges project downwards. The plat- jrm is made up of five heavy square baulks, on-shod and fitted with distance pieces so that, s they lie in position in the shallow, levelled ia. 9

it, the four inner flanges of the baseplate can nter between them, while the two outer flanges fit oyer the uter baulks. Latches on both ends of each baulk engage with pro- acting tongues on the baseplate. Thus baseplate and baulks Jgether form a steady unit in firing. The unit is further steadied gainst the horizontal component of the thrust by being wedged, ack and front, against end-boards placed against the walls of the . The piece is laid for direction by an auxiliary aiming point, ny suitable dial-sight being adaptable to the mounting-means of ne or another form of dial-sight, a clamp on the bottom-plate of ie carriage binding the latter in the desired position of traverse.

All the parts are equipped with sockets for lighting-handspikes, except the piece itself which is carried by two staves passed through the two parts of carrying rings seen in the drawing. But movement in the trenches is as a rule by means of iron barrows pushed by hand. One barrow takes the mortar upright, another the carriage, a third the baseplate and accessories, while the baulks and end-boards are

, carried by hand.

The weights of the short 24O-mm. are as follows : mortar 485 lb., carriage 425 lb., baseplate 510 lb., platform baulks 132 lb. each. Total weight in action (exclusive of wedges, steadying pickets, etc.) 2,080 lb. The loaded weight of the heaviest barrow (baseplate, etc.)

i is 943 lb. The maximum range of the ig2-lb. bomb with a propellant charge of I lb. 9! oz., is 1,125 yd. at 45 elevation and 1,045 at 553o'. The maximum pressure in the bore is slightly less than I ton per sq. in. (150 kgm. per sq. cm.). The lengthened 24O-mm. (breech-loading and percussion-fired) ranged with a 179-lb. bomb to 2,265 yd. (m.v. 476 f.s., maximum pressure 3,700 lb. per sq. in.),

I using a charge of 2 lb. 13 oz., but it required a heavier and more

< elaborate platform. A still more powerful weapon of the same class was the 34O-mm. (i3'4-in.), which required a concrete bed and a light-railway track for supply. This carried a 43O-lb. shell (high

explosive burster 205* lb.) to a distance of 2,250 yards.

Pneumatic guns, as possessing a high-pressure reservoir and a low- pressure gun chamber, should also be included amongst the low- pressure class of trench ordnance. Owing to their silence and invisi- bility in action, they possessed marked advantages over the earlier forms of trench-mortar using explosive propellants. But their low power, their complication and liability to get out of order, and as regards some models, their entire dependence on a special form of supply (air or carbon-dioxide bottles) disappointed expectation ; and of many ingenious designs put forward in the different belliger- ent countries, one only was consistently employed in the field. This was the French Brandt, classed as a light trench-mortar, which could obtain its pressure either from a bottle or from a motor-car pump. Its first model, which had a tripod mounting, weighed 48^ lb. for the gun, 35^ lb. for the tripod, and 705 lb. for the box of accessories and pumps. A later model, firing to a fixed angle only, was much lighter. It threw a li-lb. bomb, or rather grenade, very accurately to a distance of 250 yd. but, although much greater ranges were obtainable, the accuracy fell off rapidly beyond that figure. (C. F. A.)

III. Rifled Trench-Mortars.

The Germans were the first to produce an officially designed model of trench-mortar (Minenwerfer) , and these weapons were used in the earliest sieges of the war. Although this gave German designers a long start in the competition, it did not save the German army from passing through the same phase of crude improvisations, which the French and British armies had to traverse. Bored-out shell mounted on blocks, old bronze siege mortars, experimental pneumatic mortars, and various smooth-bore types all figured in the armament of the German trenches for the first years of the war, and it is not till 1916 that the standard types are found in very large numbers. Thereafter, with the sole exception of a minenwerfer copied in principle from the Batignolles 24O-mm., the standard types alone were used for general battle and trench service. The older types, so far as they survived, were practically reserved for throwing gas-bombs and other special projectiles. All the standard types of light, medium and heavy minenwerfer are rifled, and are derived from the pre-war service weapon. Modifications during the war were few, and did not affect the main elements of the design. They were principally two the lengthening of the barrel in all three classes, and the adaptation of the light type to a direct-fire carriage, which enabled it to be used with conspicuous success as an " accompanying gun_" in the semi-open warfare of 1918. The models selected for description here are the original model of " medium," the new or 1916 model of " heavy " and the light type with and with- out direct-fire carriage. Rifled trench-mortars were not used in any of the Entente armies.

The characteristic of the German rifled trench-mortar in all forms is that, unlike the stick-bomb weapons and the low-pressure weapons which are radical departures from ordinary artillery prac- tice, they are designed essentially as siege howitzers of reduced weight and power. Recoil-gear is provided, but of a simpler kind than the intricate combinations of elements necessary in a howitzer of high velocity and recoil energy; similarly, to ease the strain of dis- charge, by reducing the power necessary to propel the projectile, the driving band of the shell is " engraved " in advance. But the arrangement of piece, cradle and buffer, and the form of the shell, is in accordance with the conventional artillery practice.

The "old" model medium trench-mortar (fig. 10) is a short, thin- walled howitzer of 17-cm. (6-6g-in.) calibre, rifled with 6 shallow grooves of uniform twist, I turn in 265 calibres. The length of the rifling is 3-3 calibres. It is muzzle-loading and has percussion-firing gear placed axially on the breech. It is mounted in a ring-cradle which carries, above and below, a combined buffer and spring recuperator of simple type. The piece slides back and forth in the cradle and has the pistons attached to horns above and below the breech. The cradle is centre-trunnioned and the trunnions rest in seatings in a top-carriage of the usual form. The left trunnion car-