ARMOR-PIERCING PROJECTILES. Projectiles intended for practice at objects composed of wood, masonry, or earth are made of cast-iron; but since the introduction of iron for the defense of ships and for fortifications, a material possessing greater hardness than ordinary cast-iron is required to overcome the resistance opposed by thick wrought-iron plates. Both elongated and spherical projectiles for use against armor should be of the hardest and toughest material possible. The power of a projectile to stand up to its work and deliver rts full blow on the target depends on the shape as much as on the quality of the metal of which it is composed.
The flat-ended form of elongated projectiles possesses a peculiar advantage as regards the projectile, and another as concerns the plate. As to the projectiles, in direct impact the whole of the resistance of the target acts in lines parallel to the projectile's axis, which direction is the most favorable to the projectile retaining its mass and delivering its full blow on the target; and, again, if the target is to be punched by actual shearing, the flat head is the form best adapted to effect it. The flat head would probably be best in the case of direct firing against plates composed of hard iron, for it is easy to conceive of a hard material offering very great resistance to the forcing open of a pointed head, which might be punched by the clean shearing of a flat-headed projectile. The power given by rotation of keeping the same portion of a projectile presented to the front is of peculiar value in punching armor plates; it enables the head of the projectile to be made of any . desired form, while the power of reducing the calibre of projectile in pro portion to its weight, which is perhaps the principal advantage obtained by rifling, is also most important here, the depth of penetration being in inverse proportion to the circum ference. In shells, however, this stability of the axis of rotation tells more fully, for it enables every part of the projectile to be made of such proportions as will give the maximum power at the moment of impact. The walls of an elongated shell being chiefly subjected to a longitudinal strain, an interior hollow may be made without entailing the great weakness existing in spherical shells as compared with solid shot. Hence it follows that while smooth
bore shells have seldom or never been fired at armor, rifled shells have proved very suc cessful.
There are two causes which contribute to give shells peculiar power against iron plates. The first is that it is not necessary to weaken the head of a shell by making a fusehole in it, because no fuse is required, the heat generated on the impact of a projectile against the armor being sufficient to fire the bursting-charge. To such an extent is light as well as heat gen erated, that on firing at a target after dark a pale flash is seen to follow the impact. The second cause that operates to favor the action of shells is the fact that when the shell has penetrated to a depth of even a few inches before rupture occurs, the sides are supported by the armor around them, and the explosion, being confined at the sides, acts to the front with greatly increased force.
In a conical head the normal pressures throughout form a zone of compression acting as a wedge toward the body of the projectile, whose angle is the supplement of that of the cone of the head. This is better than that formed in the spherical head, because the angle is less acute, and because the apex of the wedge, instead of being a fixed point through out (the centre of the sphere) moves along the axis of the projectile as it enters deeper and deeper into the target. In the ogival head it will easily be seen how much superior is the action. In this the wedge is at the com mencement slightly acute, but then the resist ance acts on a small surface and is compara tively small, and the angle increases, till, at the junction of head and body, it becomes 180°, or a straight line, so that we then have the body of the projectile in much the same con dition as the flat-headed bolt driving before it an ogival wedge, which opens the armor by wedging rather than by clipping or punching. As the softer and more plastic natures of plate-iron have been found to hold their bolts the best, and stand the longest, and so have been universally adopted, the ogival has become obviously the correct form of head.