Effect on Earth

—Earth po e es advantages over all other materials as a covering against projectiles; it is cheap and easily obtained, it offers considerable resistance to penetration, and to a certain extent regains its position after displacement. It is found by experience that a projectile has very little effect on an earthen parapet unle it pa es completely through it. Wherever masonry is liable to be breached, it should be masked by earthworks with natural slopes. Gen. Gill more states that the powers of resistance of pure, compact, quartz sand to the penetration of projectiles very much exceed that of ordinary earth, or mixture of several earths. The size of the openings formed by the pa age of a projectile into the earth is about one-third larger than the projectile, increasing, however, towards the outer orifice. Rifle projectiles especially are easily deflected from their course in earth, hence their penetration is variable. Unle a shell be very large in proportion to the ma of earth penetrated, its explosion will produce but little displacement,—generally, a small opening is formed around an exploded shell by the action of the gas in pre ing back the earth. Time-fuzes, being liable to be extinguished by the pre ure of the earth, are inferior to percu ion-fuzes, which produce explosion when the projectile has made about three-fourths of its proper penetration. The penetration in earth of oblong, compared to round projectiles, when fired with service charges, and at a distance of about 400 yards, is at least one-fourth greater . This difference, however, is le at short and greater at long distances. The penetrations of similar projectiles into a given substance, are proportional to the squares of the velocities of impact and to the diameters and densities of the projectiles.