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through a gun-shield, thin wall, thin parapet, etc., so as to produce splinter effect on troops immediately behind it. When an instan- taneous fuze acts properly the effect, even on soft ground, is to form a saucer-shaped crater not more than 6 in. deep, in which no splinters are to be found. With flat-trajectory guns the splinters fly forward and sideways, and no reverse effect on troops behind cover can be expected; with howitzers fired so as to give angles of descent of 30 to 45 degrees a few splinters from the base come back, but the re- verse effect is slight; with howitzers fired at angles of elevation be- tween 45 and 65 degrees, giving angles of descent of 55 to 75 degrees, the effect is almost equal in all directions.

The size and weight of the splinters are of great importance. Owing to their irregular shape small splinters do not fly very far. The object of the ammunition designer is to get as many effective splinters as possible. As the result of experiments it is considered that the best man-killing weight is 25 grammes (0-88 oz.), though splinters as small as 10 grammes (0-35 oz.) may be effective close to the point of burst. It is not always possible to realize this ideal ; the French field gun H.E. shell, weighing 11-68 lb., gives only 50 effective splinters, averaging 100 grammes (3-52 oz.). The German 1914 field-gun shell, weighing 15 lb. gave 135 splinters averaging 1-65 ounces. A more recent projectile, that of the French 7'7-pr. trench gun, gives 90 splinters of about 1-2 ounces. Theoretically it is possible to design a shell so as to produce any required fragmen- tation. A violent H.E. burster tears a soft metal shell to minute fragments, while a mild burster in a hard steel shell merely breaks it into a few large pieces; the designer has to adjust the violence of the burster to the hardness and " shock-test " strength of the steel so as to produce the desired number of fragments, as uniform in weight as possible. But in practice the problem is a difficult one, as the stresses to which the shell is subjected in the gun, and the shape and balance desirable for ballistics, have to be taken into account. However, manufacturers produce a fair approximation; thus Krupp claimed to get 20 splinters of 25 grammes and over per kilogramme of field-gun shell, or about 9 per pound.

The French field-gun shell is effective over an area of 25 sq. metres only, but with large calibres much better effect is obtained. Roughly, a 6-in. howitzer shell, weighing 100 Ibs., clears an area of 300 sq. yards.

During the war considerable success was obtained in firing thick- walled H.E. shell from flat-trajectory guns with percussion fuzes giving a slight delay action, so as to burst in ricochet in the air from 20 to 30 ft. above the ground. Ricochet fire is applicable only when the angle of impact is so small, and the ground so hard, that the shell has no tendency to bury itself. With an angle of opening of about 1 20 degrees the downward and lateral effect is good, and the forward effect is appreciable, though far inferior to that of time shrapnel. This method was evolved by the French before the war; the ricochet effect is styled the " coup de hache." It was used also for wire-cutting.

Mine Shell. With howitzers above 6 in., mine shell, not man- killing shell, are usually employed. With medium and heavy guns mine shell are not used except at long ranges, where the angle of descent is steep enough to ensure deep penetration, and even so, for the reason above given, they have to have fairly thick walls and lose correspondingly in explosive capacity. Heavy high-velocity guns therefore usually fire only shell of the thick-walled type, bursting on graze and producing effect by the action of heavy splinters. There is now, however, a tendency to employ reduced as well as full charges with guns, in order to save wear, and with these it will be possible to use efficient mine shell.

The burster of a mine shell is of such a nature as to do as much work as possible in displacing earth. A very violent explosive of the fulminate type, even if it could be used, would be less effective than T.N.T. or amatol (see AMMUNITION) because its action is too local, and much of the force of the explosion would be wasted on pulverizing the earth at the point of explosion instead of shifting it. The fuze has to be made with a delay suitable to the ballistics and to the nature of the ground ; if the shell penetrates too deeply it forms a " globe of compression " or hollow chamber beneath the surface, while if it does not penetrate deeply enough much of the energy is wasted on the air. The mine shell of the German 15-cm. (s-g-in.) howitzer was effective ; it penetrated to a depth of 3 to 4 metres, at which depth its burster of 18 lb. of picric acid gave good mine effect.

Effect on Tanks. Fire from all natures of guns and howitzers is effective on unarmoured or lightly armoured tanks. The best pro- jectile is H.E. with normal or instantaneous fuze, as the delay action fuze is liable to cause the shell to pass right through and out again before it bursts. In one instance during the war a German tank protected by 3O-mm. (i-i8-in.) hard steel armour was fired on by British l8-prs. firing ordinary thick-walled H.E. shell at ranges of 3,000 to 4,000 yards. It was disabled, and on examination it was found that several of the shell which had struck it had just failed to penetrate. The effect obtainable with armour-piercing shell is described below.

Effect on Armoured Forts. The penetration of armour is dis- cussed under Armour-Piercing Shell. But this is not the only means of reducing an armoured fort. The Germans obtained effect on the Belgian forts in 1914 chiefly by " undermining " fire. The mine shell of their superheavy howitzers, with delay-action fuzes, buried them- selves almost under the foundations of the cupolas, and either blew

the latter up and out of their seatings, or racked the whole concrete mass so severely that the machinery was put out of action. Or, again, they ruined and blocked up the vaulted passages and so prevented access to the cupola chambers.

In future constructions it has been proposed to guard against this method of undermining by extending and strengthening the apron surrounding the cupola. The ground for 50 metres round it is to be protected by one or more layers of blocks of hard cast-iron, one metre cube, each weighing 8 tons. These are to be sandwiched between layers of concrete, and are expected to burst all shell on the surface. This method, if applied, will probably be effective against under- mining. There remains the possibility of damaging the projecting muzzles of the guns with heavy splinters from thick-walled shells, and of penetrating the armour with armour-piercing shell from high- velocity guns. An attack with gas is also possible.

Effect on Field ^Entrenchments. Gun emplacements and dugouts require a " bursting course " of rails or stones at or near the surface of the earthen roof, otherwise the emplacements are easily penetrated. Even a field-howitzer shell with delay action will go through 5 ft. of earth and blow in a timber roof beneath it. Double roofs of rails with an air-space of 8 to 12 in. between them are used when possible, and such a roof, with 5 ft. of earth and a bursting course on top of it, is fairly safe against a single hit from a 6-in. howitzer shell. For pro- tection against 8-in. and heavier natures it is necessary to burrow 20 to 30 ft. underground.

Concussion and Asphyxiation Effects of H.E. A mine shell of 6-in. calibre and upwards does great damage when burst inside a building, dugout, or other confined space, by the force of its blast, irrespec- tive of splinter effect. The same effect, to a lesser extent, is produced by thick-walled shell. Walls of buildings are blown out, and men in the room in which the burst takes place are killed by the concussion. The idea that poisonous gases are produced by the detonation of high explosives is a ir.istake; only small quantities of carbonic acid gas and carbonic oxide are generated, and these, except in a deep dugout, are quickly dissipated by the inrush of air which succeeds the blast. However, it was found at Liege and Antwerp that the deeply sunk ammunition rooms below the armoured forts were rendered uninhabitable by the fumes from mine shell filled with picric acid which penetrated them or burst underground near them. These were presumably acrid fumes due to the incomplete detona- tion of the bursting charges.

High-Explosive Shell uith Time Fuzes. These were originally introduced for the German field gun, in order to attack troops in deep trenches. The method used was to burst the shell exactly over the trench, and to obtain effect by splinters striking downwards. But even under peace conditions of precision of fire it was found that an average of four shells were required to hit one man, and under war conditions the expenditure of ammunition was out of all propor- tion to the effect produced. In the war, the Germans occasionally used time H.E. shell for annoyance, on account of their supposed moral effect on troops in trenches. Their use ceased altogether wheti time and percussion fuzes for H.E. shell were withdrawn. A H.E. shell has little or no incendiary effect.

Smoke Shell. These are used to form a screen between pur troops and the enemy. In barrage fire time shrapnel produce quite enough smoke to make an opaque screen ; but when a barrage is formed with H.E. shell, these are mixed with 25 % of smoke shell to make a good screen. This method was frequently used by the French, either be- cause of shortage of field shrapnel, or because the barrage was car- ried beyond shrapnel range.

Incendiary Shell. During the retreat the Germans used these shell to set fire to villages which they had evacuated. Some were of the ancient " carcass " type, filled with black powder and grease, and spouting flame from holes in the walls; others were filled with petro- leum. The British introduced thermit shell, filled with aluminium dust and iron oxide, which, when ignited, form molten iron. The filling consisted of short cylinders of thermit, like firework stars, and the shell was preferably burst in air like a shrapnel, so as to blow the stars out forwards. These were used to a small extent in the Allied advance towards the end of the war, and gave good results.

Illuminating Shell.- These contain firework stars, which are ignited by a time fuze at such a height as to give them time to burn out before reaching the ground. They are fired principally from field howitzers.

Armour-Piercing Shell. These may have to be used against armoured tanks, and turrets and cupolas of land defences.

The effect of field gun armour-piercing shell against tanks depends not only on the thickness of the armour but on the manner in which it is supported, and the angle at which it is struck. The following table of penetration for the British i8-pr. gun used in the war may be taken as a guide. It is assumed in the table that the armour is efficiently supported and that it is struck at an angle not exceeding 30 degrees to the normal. The formula is :

t =

vd

2500 4v

where t is the thickness of hard-faced armour in inches, v is the striking velocity in f.s., and d is the calibre of the gun in inches.