" accompaniment." The tactical employment of trench ordnance, whether in its designed r61e or as accompanying artillery, is dealt with under ARTILLERY: the present article is concerned with the weapons themselves.
The German trench-mortars handled not by artillerymen but by engineers in their capacity as the technicians of the siege trenches were used with great effect in the attack of certain of the Liege forts, and a little later at Maubeuge and Antwerp. In this they were carrying out the special duties for which they were designed, and at that stage of the war it was not foreseen that weapons of this class would be required for the equipment of a trench-front hundreds of miles in length. Consequently very few were available when the western front became stabilized, and the demand was made simultaneously from all quarters for some short-range trench weapon capable of curved fire, by which alone the covered enemy could be attacked from covered posi- tions. For the time being, it was impossible for any belligerent to do more than provide stop-gaps in the form either of mechani- cal throwers (see BOMBTHROWERS) or small mortars mounted on wooden beds, which were in fact either artillery shell sawn off at the neck and bored with a vent or clsecohorn and other mortars of the smooth-bore era, unearthed from arsenals or museums. But while the troops made shift with these, inventors and de- signers were producing experimental models which, when ap- proved, were brought out in enormous numbers, with little difficulty from the point of view of manufacture since the designs were deliberately kept simple so as to be serviceable in the rough- est conditions.
Setting aside, for the present, direct-fire trench-guns, of which various types came into use for combating machine-gun cm- placements, and, later, tanks trench ordnance for curved fire is classified either according to weight, as light, medium or heavy, or according to the principle of design, as high-pressure or low-pressure. The latter basis of classification is taken here; and it is desirable at once to indicate the specific differences which mark off trench ordnance from normal artillery weapons, applying equally to high-pressure and to low-pressure types.
The most important difference is in range. Although by the end of the World War some remarkable results had been ob- tained by progressive experiments in the direction of increased range, the trench-mortar remained essentially a weapon of less than 2,000 yd. effective range. In the earlier models save a few the limit was about 500 yd., while many were incapable of exceeding 250. Even this last range sufficed in some cases, owing to the short distances separating the opposed front lines, and during the period of position-warfare most of the work of light and medium trench-mortars was done at ranges of 400 to 600 yards. The main motive for increasing range was not the desire to reach a more distant target from the front line, but the desire to emplace the trench-mortar at such a distance behind one's own front-line troops that premature explosions specially liable to occur with improvised ordnance t and ammunition would not affect them. An almost equally cogent motive was the desire to avoid drawing the enemy's reprisal-fire, time after time, upon the same body of friendly infantry. Another motive was the desirability of withdrawing the trench-mortar itself from the effects of fire directed upon the front line, and preventing too close observation of its position by the enemy.
The next point is weight. For use in or near the front trenches the trench-mortar must be portable, either as a single unit (as in the case of light mortars) or in parts which severally do not exceed certain limits fixed by experience, either in point of weight or in point of dimensions. For the heavier trench-mortars, having greater range, the limits imposed on those which have to be taken close up to the front line are somewhat relaxed; indeed, the very heaviest, used only in very favourable conditions, is de- signed for a concrete bed and light-railway transport.
A third point is simplicity, or better, in the French phrase, " rusticity." The trench-mortar is by hypothesis subjected to the worst conditions of exposure and handling. From birth it lacks the mechanical finish of standard artillery; it is required in great numbers at a time when all engineering shops for high-
class ordnance works are fully occupied and take the pick of the available metal. Then, on service, it is handled in the main by a hastily trained personnel, not all of whom possess the gunner's respect for his gun or the mechanic's pride in his machine. More- over, the medium and heavy models are in most cases taken down and assembled frequently, and the parts are transported by hand through winding, muddy trenches, or across broken country in the dark. Save perhaps a detachable sight (which is carried by a non-commissioned officer or gunlayer), none of the customary adjuncts of modern ordnance is admissible.
Lastly, the ammunition, even of rifled trench-mortars, differs considerably from that of standard artillery. In most cases itsi design is special to the particular design of mortar, and there is no intcrchangcability such as that of 6-in. gun and howitzer ammunition. But in all cases, not being exposed to the bore pressures which standard artillery ammunition must endure, it has much thinner walls and a far greater explosive content.
These specific differences will be best realized by comparing a standard heavy trench-mortar with a heavy howitzer of the ordinary kind. The trench-mortar here taken is the Batignolle; 24-mm. used by the French, British and Italian armies; the; howitzer the German 2i-cm., 1910 model, with a somewhat thick-i walled shell.
Trench-mortar Howitzer .
Shell
kgm.
89
Weight of gun exclu- sive of mount- ing
kgm.
220 2,625
Pro-
pellant Charge
Sinn.
720
3.200
Burst- ing Charge
kgm.
45
%of Burst- er to weight
of
filled shell
o /o
56
9'7
Range
in Metres
1,040 9,100
Here the trench-mortar shows roughly one-fifth the propel- lant charge and one-ninth the range, but one-twelfth the weight of gun and six times the shell "efficiency" of the howitzer.
All trench-mortars fire at super-angles; that is, they use eleva- tions of about 47 for their longest range and highest elevations up to 75 or more for the shorter ranges. This enables them to be used from deep and narrow trenches.
The design may now be considered under the three headings of high-pressure smooth-bore, low-pressure smooth-bore, and rilled.
I. High-Pressure Smooth-Bore Trench- Mortars.
The prototype is the Krupp bomb-gun of 1912. This, and all the types presently to be described have " stick-bombs." The stick-bomb is a large-bodied explosive container sometimes spheri- cal ("football " bomb), as in the case of the Krupp and the British 2-in. T.M., sometimes formed; to overcome air resistance, as in the Dume'zil types. In either case, attached to the base of the pro- jectile is a rod which fits closely into the bore of the mortar like the rod of a rifle-grenade and^ may or may not be fitted with a gas-check disc at its end. The gun itself is either a solidly forged small cannon or a long stout tube, trunnioned at its breech end, and its mounting is supported on a wooden bed. It is fired either by friction tube or percussion lock like an ordinary piece of ordnance, or by the primitive device of lighting a length of match placed in a vent or touchhole. As the base or gas-check of the rod is in con- tact with the bag containing the propellant charge, the pressures developed in the bore are comparable to those in a regular howitzer, and it is necessary not only for the gun to be stout but for the seat- ing of the rod in the bomb base to be solid and cup-shaped, lest (as sometimes happened) the shock of discharge should drive the rod into the bomb before the inertia of the latter has been overcome. All stick-bombs, owing to the air-drag of the stick, and in the Duin<5- zils and some others to the steadying effect of external vanes, fly nose first, and can be used with ordinary percussion or instantaneous fuzes provided the arming resistance of these is adjusted. Of this class, the most widely used was the Dumdzil; designed by General Dum6zil, in substantially its final form, as early as June 1915, and subsequently used by the French, Italian, Russian (in a modified form) and American armies, though declined by the British. The " Dume'zil No. 2," here described was the standard medium trench- mortar of the French army throughout the last three years of the war. It is simple and easily transported and assembled by unskilled personnel.
The mortar (fig. l) is a stout cannon of forged steel (i) with simple vent-firing and rear trunnions. The maximum pressure with the heaviest bomb is 16 tons to the sq. in. The trunnions (2) are massive and bored through transversely to take a carrying bar or
