The barrels are made from plates of iron, of suitable form and size, called _scalps_ or barrel plates. These scalps are a little more than two feet long, and about three inches wide. The barrel when completed, is about three feet six inches long, the additional length being gained by the elongating of the scalp under the hammer during the process of welding. The scalps are heated, and then rolled up over an iron rod, and the edges being lapped are welded together, so as to form a tube of the requisite dimensions--the solid rod serving to preserve the cavity within of the proper form. This welding of the barrels is performed at a building among the Middle Water Shops. A range of tilt hammers extend up and down the room, with forges in the centre of the room, one opposite to each hammer, for heating the iron. The tilt hammers are driven by immense water-wheels, placed beneath the building--there being an arrangement of machinery by which each hammer may be connected with its moving power, or disconnected from it, at any moment, at the pleasure of the workman. Underneath the hammer is an anvil. This anvil contains a die, the upper surface of which, as well as the under surface of a similar die inserted in the hammer, is formed with a semi-cylindrical groove, so that when the two surfaces come together a complete cylindrical cavity is formed, which is of the proper size to receive the barrel that is to be forged. The workman heats a small portion of his work in his forge, and then standing directly before the hammer, he places the barrel in its bed upon the anvil, and sets his hammer in motion, turning the barrel round and round continually under the blows. Only a small portion of the seam is closed at one heat, _eleven_ heats being required to complete the work. To effect by this operation a perfect junction of the iron, in the overlapping portions, so that the substance of iron shall be continuous and homogeneous throughout, the same at the junction as in every other part, without any, the least, flaw, or seam, or crevice, open or concealed, requires not only great experience and skill, but also most unremitting and constant attention during the performance of the work. Should there be any such flaw, however deeply it may be concealed, and however completely all indications of it may be smoothed over and covered up by a superficial finishing, it is sure to be exposed at last, to the mortification and loss of the workman, in the form of a great gaping rent, which is brought out from it under the inexorable severity of the test to which the work has finally to be subjected. [Illustration: THE WELDING ROOM.] RESPONSIBILITY OF THE WORKMEN.