The first impact shall be located near the central portion of the plate, and the other two impacts shall be located as directed by the Bureau; no impact, however, to be nearer another impact or an edge of the plate than 3½ calibres of the projectile used.

On these three impacts no projectile or fragment thereof shall get entirely through the plate and backing, nor shall any through crack develop to an edge of the plate or to another impact.

From the above it is seen that a manufacturer supplying both armor-plate and shell to the Government is called upon to produce a shell with sufficient integrity to completely penetrate, and without breaking up, his armor-plate of sufficient thickness to resist that shell.

The capping of projectiles consists in placing over the point a cone or mass of metal of comparative softness. In the United States services soft steel is used for the purpose. Authorities disagree as to the exact function which the cap plays, some claiming it to act as a lubricating metal facilitating the passage of the projectile, others claim that it gives an initial shock to the armor-plate before the shell proper has struck it, which latter then strikes the plate in a state of molecular unrest, and, therefore, of impaired resisting power. Firing tests of shell at armor-plate at oblique angles have proven the capped shell superior, which would indicate that the cap in this instance at any rate is capable of securing a hold on the plate which the bare point of the shell cannot, in so much as uncapped shells glance off. At any rate capped projectiles are, on the whole, superior to the uncapped and the practice of capping is recommended as an additional advantage when used in conjunction with the improvements here-in-after described.

At a specified distance from the base of the shell a groove or band-score is turned for the rotation band. For projectiles under 7-inches calibre, pure copper is usually employed, but for larger calibre an alloy of 97½ per cent of pure copper and 2½ per cent of nickel is used and is annealed before banding. The rough bands are in a form of solid rings cut from drawn tubes or cylindrical castings, and must be carefully hammered into the score or preferably pressed in by hydraulic pressure and finally turned to proper size, shape, and finish.

Their use has been previously described and the improvements in armor-piercing shells hereinafter described are based upon a study of the stresses sustained by a projectile upon impact while rotating about its major axis at the high rotative velocity which the engaging of these bands with the rifling of the gun has imparted to the shell.

The following table compiled by the author gives the rotative velocities of various projectiles:

U.S. SEA-COAST LAND SERVICE GUNS

KRUPP GUNS

From the above table it will be noted that the R.P.M. are exceedingly high in some cases. Upon the impact of a shell with armor-plate the physical phenomena occur instantaneously and the resultant forces are so great that it is impossible to mechanically record their action. A study of the stresses in the shell can, however, be made on a theoretical basis.

In the first place, if the projectile were twenty calibres in length and of a material offering less resistance to torsional stress than steel and rotated at the high velocities indicated we would find that upon impact the torsion would be plainly evident as per the following:

Assume a projectile A of length twenty calibres, about to penetrate an armor-plate B of thickness sufficient to prevent complete penetration