No. 243a-Back of 6-in. face hardened nickel steel plate A-874, group 13, Carnegie Steel Co., showing the back bulges of 6-in. impacts Nos. 1, 2, 3 and 4, and 8-in. impact No. 5; the striking velocities being 1472 f-s., 1659 f.-s., 1975 f.-s., and 2110 f.-s. for the 6-in., and 1472 f.-s for the 8-in. N. P. G.. letter No. 314, April 13, 1894.

Impact No. 1 = 6; 100 lbs. at 1472 tt.-s. 1504 ft.-tons.

"

2=6"

1659

1975 42110

1472

3754

tunity to separate successively and flow in the direction of least resistance. Stress is applied to each individual particle, and the separation of a single pair means the separation of all.

One point more: until penetration occurs the cone of depression is intact; to resist the first penetrating effect therefore until the base of the cone is widespread is to greatly increase the resistance. The first effect of penetration in homogeneous plates is a distinct flow of metal. If that before the projectile refuses to become viscous under pressure and flow; if it lies there, for example, like granite, that may be pulverized but the particles of which will not slide over each other, an entirely new character is given the resistance of the surface of the plate. The point of the projectile must carry this intractible film before and around it. Every advance drags in more of the jagged fragments of the hardened surface, cutting and scoring ogival and plate as well, thus largely increasing the volume of metal which must be removed to permit the passage of the shot. There is a different thickness of this hard surface most suitable for every thickness of plate. Its object is to distribute the energy of the projectile; to prevent the flow of metal up and around it; to force down the plate surface and keep its resistance extended over as large an area as possible, as though bottled up and braced at the point of impact. Thus the absence of a front bulge must indicate that all of the resistance of the plate has radiated from the shot's head, and been widely distributed. The presence of a front bulge indicates a more local resistance, the displaced metal having simply eddied about the shot's head.

The idea has been expressed by some that the merest film of hardened surface is all that is necessary. This is incorrect. Service projectiles which have succeeded in piercing good face hardened plates, frequently have a considerable depth of the ogival surface fused and scraped off. As the head of the projectile enters there is opposed to each zone of the surface of the ogival the resistance of strata of varying hardness and elasticity, the largest at any one instant being that of the hardened surface. Should this be a mere film, its peculiar effect would be hardly appreciable, although the varying characteristics of the strata below would. permit the plate to concentrate its resistance and give a good account of itself.

No. 243a-Back of 6-in. face hardened nickel steel plate A-874, group 13, Carnegie Steel Co., showing the back bulges of 6-in. impacts Nos. 1, 2, 3 and 4, and 8-in. impact No. 5; the striking velocities being 1472 f.-S., 1659 f.-s., 1975 f.-s., and 2110 f.-s. for the 6-in., and 1472 f.-s for the 8-in. N. P. G.. letter No. 314, April 13, 1894.

It is found that when projectiles large in caliber compared with the thickness of a face hardened plate most nearly match it, the head enters until its bourrelet is clear of the intact hard surface when it breaks out a dome-shaped back bulge, to which its head remains welded, while the body and base are crushed and thrown to the front. In most cases, a few feet less velocity will make the difference between complete perforation and the bare clearance of the hard surface by the shoulder of the projectile. The crushing of the latter in such a case naturally takes place at the instant maximum resistance is offered; that is, when the crumbling, expanding zone of hard metal about the ogival is most depressed and strongly and extensively supported by the under layers.

The subject of caliber in the attack of face hardened plates is important. The question may be asked, Would a face hardened plate, barely capable of resisting a certain caliber of projectile, be unable to resist a larger caliber having no greater energy?

The area of zones in the ogivals of the projectiles exposed to like resistance in the two instances vary as the calibers, but the sections of the projectiles along the planes of greatest weakness vary as the squares of the calibers. The larger caliber would, for this reason alone, inflict more damage on the plate before being broken up. Also, as its energy consists in a much larger degree of indestructible, or at least less easily diverted mass, and less of velocity, it would produce an equal amount of damage upon the plate with a proportionately smaller loss of velocity than the smaller caliber. From this it is evident that of the two projectiles the lighter, with less resistance to crushing, has its velocity diminished much more suddenly and violently than the larger caliber with broader planes of resistance. It would therefore appear that when the target is superior to the gun, and as the vast majority of impacts in action will be oblique where this will be the case, the larger the caliber, even with the same energy, the greater the damage inflicted. The introduction of armor of increased resistance is certain therefore to bring about an increase in the calibers of the guns.

On the other hand, when plate and target are about evenly matched, the larger caliber must break away a greater volume of the plate before perforation, this volume varying as the squares of