Perhaps some of my readers may have visited those great iron-works in which huge masses of iron are rolled into plates of greater or less thickness, or are cut up into strips as easily as if they were butter. The mechanism is in its principle simple enough. The cylindrical rollers are placed nearly in contact, and forced towards each other by mechanical means, such as levers, screws, or springs, or all three combined. These cylinders revolve in opposite directions, and, if any object be placed between them, they draw it through them, and present it on the other side in a flattened condition. [Illustration: JAWS OF SKATE.] [Illustration: CRUSHING-MILL AND ROLLER.] Many years ago, one of my schoolfellows, who had been brought up entirely under the care of some maiden ladies, was visiting a workshop, and must needs put his finger between two revolving rollers. Of course the hand was drawn between them, and simply squeezed flat. The machine was instantly stopped, and the hand extricated; and the strange thing was, that the crushed and shapeless hand afterwards recovered its full power, though not its shape, and was able to touch the keys of the piano. The whole process of the Rolling-mill is singularly interesting, whether it be used for large or small objects. Supposing that the grooved rollers of the illustration were cut across so as to present a number of points, it is evident that anything which got between them would be bitten to pieces, each piece being of a tolerably uniform shape. This plan is now adopted in the granulation of gunpowder. After the future powder has emerged from the hydraulic press in the form called “press-cake,” it was formerly broken to bits with wooden or copper mallets, and then placed in a very peculiar kind of sieve. This was shaped like an ordinary sieve, but the bottom was made of cowhide, pierced with innumerable holes. A round pebble was placed in the sieve, and, when the latter was violently shaken backwards and forwards, the powder was driven through the holes by the pressure of the stone, and was afterwards separated into its various degrees of fineness. I have only twice seen this process, and confess to have been in a very nervous state on both occasions. The sieve is whirled about with enormous velocity, and the pebble flies round as if it were a thing alive. Let but a broken needle or a fragment of stone get into the sieve, or even let the stone itself break asunder, and there will be an instantaneous explosion, which will hurl the house, the machinery, and the workmen into unknown regions. Now, however, the mode of granulating powder is radically altered. There is a series of double cylinders, such as shown in the illustration, and each of them has the ridges cut into teeth in regular order. Thus the first set of rollers or cylinders merely bites the press-cake into convenient pieces, though seldom of the same weight.