Page:Popular Science Monthly Volume 6.djvu/747

Rh state of the atmosphere, the different degrees of moisture in the powder-meal, its varying bulk and elasticity, and other minor causes. The only practicable method of securing an approximately uniform density is to test the product of various pressings and then mix them, so as to reduce the whole to the average density required, and this is the constant practice at Waltham.

We have alluded incidentally to the decrease of pressure in the gun as the density of the powder increases. With the immense guns constructed in recent years, it is important to reduce the strain on the metal as far as possible, as this is the only way in which the gun can be safely fired. But it must be remembered that, by seeking to accomplish this by indefinitely increasing the density of the powder, we would at the same time decrease the velocity of the shot, or, in other words, its useful effect. Artillerists have, therefore, had recourse to the expedient of using a powder, each grain of which is a lump of press-cake. The effect of this is to make it burn slower than grain-powder; for these lumps, when ignited at the surface, burn, as it were, in concentric layers, until the whole is consumed; and by this means the explosion, though to all appearance instantaneous, is in reality much more gradual than that which follows the ignition of smaller grains. In other words, the explosion of the charge in a heavy gun is made to be less of the character of a violent blow on the sides of the bore and the base of the shot, and more like a gradual shove given to the shot with a corresponding pressure on the gun.

The first form proposed for cannon-powder for heavy guns was that invented in America by Dr. Doremus, in which the whole charge was made into a solid disk, the size of the bore of the gun. This, however, was found to give very unsatisfactory results. Then the Russian Government adopted a powder compressed into large hexagonal prisms, and in Belgium another powder was made in the form of cylindrical pellets. This was adopted by our Government, and an immense sum was spent on erecting machinery at Waltham for its manufacture, but the pellet-powder has since been superseded by a simpler form, much easier to make, and giving better results; and the pellet machinery has been altered, and, we believe, is now used in the manufacture of gun-cotton.

The pebble-powder now in use consists of cubes of compressed gunpowder, with sides about four-fifths of an inch square. These pebbles are made by passing press-cake of that thickness between two pairs of rollers, armed with sharp-cutting edges. The first pair of rollers, by means of these edges, cuts the cake into several small bars, about four-fifths of an inch square at the ends, and these bars, on passing between the second pair of rollers, are divided into cubes or pebbles. After having been rolled in a hollow cylinder, or reel, to round off the sharp edges and get rid of the dust, the pebbles are carried to the drying-house to be freed from the moisture they contain.