axle. It rests, when assembled for firing, in trunnion seatings formed in the inner parts of two cheek-pieces (4), which are heavy castings with accurately fitting inner faces. The foreparts of these are moulded upward to form elevating arcs forming a collar (3) on
FIG. I
the gun slide, the gun being secured at the desired angle of eleva- tion by screwing home the clamping screws. The cheeks (4) carry- ing the gun rest in a trough (5), attached to the back of which is a curved plate. This curved plate butts against a similar curved plate (10), which is jammed firmly against the back wall of the trench or emplacement. The mortar can thus be laid for direction by sliding the back-plate of the trough (5) along this back-plate (10).
The bed, in which the system rests without being attached, consists of three planks (8, 8, 9). The system may be moved without being taken down, by means of wooden truck wheels attached to the axle (6) which is passed through to the sides of the trough, the cheeks and the bored trunnion. These accessories were, however, not much used in practice, as only 4 to 5 min- utes are required to bring up and re- assemble parts in a new position, ya- rious methods of indirect laying for line were employed, as well as direct lay- ing by periscope (excluding axle and wheels). The weights of the various parts are as follows: mortar, trun- nions and elevating collar 165 ib. ; two cheek pieces, 1435 Ib. each ; trough 132 Ib. ; wall back-plate 1 06 Ib.; three planks 128 Ib. Total weight in action 818 Ib.
The stick-bombs vary in weight from 35 Ib. (burster 13! Ib.) 1099 Ib. (bur- ster 50! Ib.). The earlier types, as in the 99-lb. bomb of fig. 2, had three vanes; later, as in the 4O-lb. bomb of fig. 3, six were fitted. Some types had considerably more elongated bodies than those illustrated. With a charge of 4 oz., the 35-lb. bomb ranges to 715 yd. at an angle of elevation of 45, the muzzle velocity being 263 f.s., and
nth a charge of 4! oz. a 68-lb. bomb, m.v. 220 f.s., ranges at the ame angle of elevation to 490 yards. (C. F. A.)
II. Low-Pressure Smooth-Bore Trench-Mortars.
Trench-mortars of this class are characterized by the fact that he pressure of the propellant gas is reduced by being allowed to xpand into an " expansion chamber " of one form or another before eginning to take effect on the projectile. This device enables a hm-walled gun-tube to be used (instead of a true cannon such as he Dumezil above described) with considerable saving of weight. n some types, the firing of the charge was arranged to take place
FIG. 2
in a separate chamber which communicated by a channel with the expansion chamber, i.e. the breech portion of the gun-tube in which the projectile is seated. Such were the Aasen, the Sutton- Armstrong, which was employed as a naval " bombthrower " (see ORDNANCE: section Naval), and the Temple. The last-named was distinguished by possessing a silencer attachment, which was con- sidered by some authorities as a desirable addition in the period of short ranges and very advanced emplacements.
Of more importance in the history of the World War are those types in which the strong-walled "combustion chamber" does not form a separate component, but is included either in the base of the gun- tube (as in the Batignolles type) or the base of the shell (as in the Stokes). In both of these, the " expansion chamber " is formed automatically by the fact that only certain projections from the shell- base, and not the base itself, rest on the bottom of the bore or chamber-shoulders when in the loaded position.
The principal representatives of the class are the French Batignolles heavy trench-mortar, used also by the British, Italians and Americans, and copied, with modifications, in certain German patterns, and the British light Stokes mortar, which, with its derivative the medium Stokes- Newtpn, were used by the British and American armies. The Stokes Mortar, in- vented by Sir Wilfred Stokes early in 1915, was considerably improved in the course of the war as the result of experience with the weapon itself and with trench-mortars generally. It was first used in battle at Loos, Sept. 25 1915, when a few 4-in. tubes on bipod mountings, with straw- board-bodied shell, improvised at short notice, were employed for firing smoke
FIG. 3
shell; but by the battle of the Spmme (July I 1916) both 3-in. and 4-in. mortars (the latter specialized for gas and smoke projectiles) had been issued in large numbers. Thereafter, to the last phase of the war, in which its lightness enabled it to do useful service as an " accompanying " piece, it satisfied all expectations. Each British infantry brigade of 4 or 3 battalions included a battery of 3-in. Stokes. Their principal characteristic was automatic ignition.
Although it is believed that the first record of automatic ignition in trench-mortars, where the projectile carried its own propelling charge and igniter attached to the base of the bomb, existed in an early patent taken out in Germany by Messrs. Krupp, no practical use seems to have been made of this, and the first application of the theory of dropping a bomb of this nature down the bore of a mor- tar, so as to fire the charge and eject the bomb upon impact with the base, was that put forward by Sir Wilfred Stokes. The bomb in its first service form (fig. 4) had a container screwed on to the base, and this took a 12-bore sporting cartridge, the percussion cap of which, after the bomb had slid rapidly down the bore, impinged upon a striker fixed in the base of the mortar. In the 6-in. medium mortar (Stokes-Newton), developed later in the war, the principle of the fixed striker was adhered to, but both the mounting and the projectile differed entirely in design.
The barrel of the 3-in. Stokes mortar (fig. 5) consists of a light seamless steel tube (A) drawn down at the base end, which is bumped into a semicircular form having a slight projec- tion in the centre of the axis of the bore. This projection is drilled and threaded to take the striker (H), which, when screwed home, projects into the bore ; the size of the combustion cham- ber is regulated by the extent to which the striker protrudes into the barrel or bore of the mortar. A cap (G) is screwed over the out- side of the projection to take the set-back of firing. This cap, which is made of steel, rests in a small depression in the base-plate (C), which is fixed in the ground approximately fac- ing the objective, and at a convenient angle, and has several such depressions placed in an arc to provide for changes of direction. The thin tube or barrel of the mortar has a smooth bore ; it is supported above the centre of gravity by a pair of folding legs (B), between the apex of which (when open) and the barrel are the elevating and traverse gears, the latter being attached to the barrel by means of a steel band (see
There is no attempt to take up or absorb mechanically the set- back on firing, or the recoil, as the low combustion pressure renders this unnecessary at the ranges used.
As already noted, the 3-in. Stokes was in the British service set apart for the firing of high-explosive bombs (although smoke-pro-
