half apart) accurately fitting sheets of hard rubber; the crushed powder is shoveled into the trough, and a powerful hydraulic press applied; a screw-press would be dangerous, as particles of powder might get into the thread of the screw, and be fired by the friction. When removed from the press, the powder is in slabs of close texture, not unlike slate in appearance. These slabs go next to the granulator, in which they pass through a series of rollers, separated by sieves; as the broken powder passes from one set of rollers to the succeeding one, the sieves are kept in constant agitation, the pieces which pass through them falling to the bottom of the machine, where they are collected. Each sort is then taken to a rotating reel of wire gauze, in which, as it revolves, the dust is removed. If the powder is to be glazed, the clean grains are placed in a slowly revolving drum, with a very small quantity of plumbago, or black-lead; if glazing is not required, as is the case with some sorts of powders, the same process is gone through, the black-lead being omitted; in this manner, the grains are rounded off, and rendered smooth.
The final step is the drying, which is done in a steam-heated house, the powder being spread upon shelves for this purpose. The finished product is placed in oak kegs fastened with copper hoops, and care is always enjoined to use no iron tool in opening them. These various operations are conducted in buildings situated at a distance from each other sufficiently great to prevent the explosion of one causing that of another; they are generally placed along the banks of some stream from which the requisite power for operating the machinery can readily be obtained.
The manner in which the powder burns is greatly affected by variations in its manufacture: the greater the size of the grains, the more slowly does the burning take place, as the combustion goes on upon the surface, particle by particle; the shape of the grain regulates the amount of space taken up by the charge, as also in some degree the amount of surface exposed to the action of the flame; the density (depending upon the amount of pressure applied to the press-cake) also affects the rapidity of the burning; the lower the density, the more quickly does the combustion take place.
A desire to reduce the strain upon the walls of the modern guns has led to many experimental trials of various sizes and shapes of grain; the principal credit for these ideas is unquestionably due to an American ordnance-officer, Rodman. Much care has been taken in this regard, and we now see much higher velocities given to projectiles than heretofore, with, at the same time, less strain upon the gun.
There are various other mixtures which are explosive in their character, but their use is prevented by various considerations, chiefly by the fact of their greater sensitiveness to friction or percussion, and their consequent greater danger, or by their corroding effect upon the metal of which the gun is constructed. A powder which might prove