After the barrel is turned to nearly its proper size it is next to be ground, for the purpose of removing the marks left by the tool in turning, and of still further perfecting its form. For this operation immense grindstones, carried by machinery, are used, as seen in the engraving. These stones, when in use, are made to revolve with great rapidity--usually about _four hundred times in a minute_--and as a constant stream of water is kept pouring upon the part where the barrel is applied in the grinding, it is necessary to cover them entirely with a wooden case, as seen in the engraving, to catch and confine the water, which would otherwise be thrown with great force about the room. The direct action therefore of the stone upon the barrel in the process of grinding is concealed from view. [Illustration: GRINDING.] The workman has an iron rod with a sort of crank-like handle at the end of it, and this rod he inserts into the bore of the barrel which he has in hand. The rod fits into the barrel closely, and is held firmly by the friction, so that by means of the handle to the rod, the workman can turn the barrel round and round continually while he is grinding it, and thus bring the action of the stone to bear equally upon every part, and so finish the work in a true cylindrical form. One of these rods, with its handle, may be seen lying free upon the stand on the right of the picture. The workman is also provided with gauges which he applies frequently to the barrel at different points along its length, as the work goes on, in order to form it to the true size and to the proper taper. In the act of grinding he inserts the barrel into a small hole in the case, in front of the stone, and then presses it hard against the surface of the stone by means of the iron lever behind him. By leaning against this lever with greater or less exertion he can regulate the pressure of the barrel against the stone at pleasure. In order to increase his power over this lever he stands upon a plate of iron which is placed upon the floor beneath him, with projections cast upon it to hold his feet by their friction; the moment that he ceases to lean against the lever, the inner end of it is drawn back by the action of the weight seen hanging down by the side of it, and the barrel is immediately released. The workman _turns_ the barrel continually, during the process of grinding, by means of the handle, as seen in the drawing, and as the stone itself is revolving all the time with prodigious velocity, the work is very rapidly, and at the same time very smoothly and correctly performed. DANGER.