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pen·e·tra·tion of spher·i·cal pro·jec·tiles

Their penetration when of the same size, with different velocities or charges, is nearly as the squares of the velocities; when of different sizes the penetration will be proportionate to their diameters multiplied by the density, and inversely as the tenacity of the medium. The depth of penetration of a projectile fired from field-pieces at the distance of 500 or 600 yards, is from 41⁄2 to 6 feet in parapets recently constructed, and will traverse walls of ordinary construction; but a 12-pounder is necessary to make a breach in walls of good masonry and of 4 feet in thickness, and in this case the position of the battery must be favorable, and the operation a slow one. The depth of penetration of projectiles fired from the 41⁄2-inch siege-gun, is about the same as that of projectiles fired from the 30-pounder Parrott gun, namely, 12 feet. Sand, sandy earth mixed with gravel, small stones, chalk, and tufa, resist shot better than the productive earths. Shells may be considered as round shot of a lower specific gravity, and their penetrations are therefore proportionally less. A bank of earth, to afford a secure cover from heavy guns, will require a thickness from 18 to 24 feet. In guns below 18-pounders, if the number of the feet in thickness of the bank be made equal to the number of pounds in the weight of the shot by which it is to be assailed, the requisite protection will be obtained. Earth possesses advantages over every other material. It is easily obtained, regains its position after displacement, and the injury done to an earthen battery by day can be readily repaired at night. Where masonry is liable to be breached, it should be covered with earth. Wrought-iron plates 41⁄2 inches in thickness will withstand the effects of 32-pound shots, and of all inferior calibers at short ranges, as 400 yards. Plates of this thickness, however, are soon destroyed by 68-pound shots, and afford little protection from the elongated shots of the new rifled ordnance. To resist successfully the fall of heavy shells, buildings must be covered with arches of good masonry, not less than 3 feet thick, having bearings not greater than 25 feet, and these must be again protected by a covering of several feet of earth. Iron plates half an inch thick, oak planks 4 inches thick, or a 9-inch brick wall, are proof against musketry or canister at a range of 100 yards. Iron plates 1 inch thick, oak from 8 to 10 inches thick, a good wall a foot thick or a firm bank of earth 4 feet thick, will afford secure cover from grape-shot, from any but the largest guns at short ranges. The common musket will drive its bullet about a foot and a half into well-rammed earth, or it will penetrate from 6 to 10 half-inch elm boards placed at intervals of an inch. The penetration of the rifled musket is about twice that of the common musket. A rope matting or mantlet 31⁄2 inches thick is found to resist small-arm projectiles at all distances; it may therefore be employed as a screen against riflemen.

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