You see these shops, one by one, in care of an expert guide. You see the wooden patterns going to the blast furnaces at the foundries and to the sullen tappings of the trip-hammers. You leave the blacksmiths and stand for a moment--not long--under the terrific din of the boiler-makers. The boiler, the great trunk of the locomotive, is built of steel plate--plate that is the very pride of the rolling-mills. In some foreign lands, copper fire-boxes are demanded; but the real American locomotive has these also of steel. The steel plates are rolled to form the boiler itself, flanged by angle-workers into the square fire-box. Finally the boiler and the fire-box are riveted together, section by section--made as fast by steel thread as man's ingenuity can make them. Together they form a unit. Another unit is being formed in an adjacent shop, the solidly welded steel frame in which the boiler shall yet set, and to which truck and drivers will be firmly fastened. Forward on this frame will sit the cylinders; in another corner of this shop they are being made ready. Cast-iron still remains the best material for the cylinders and the steam-chests. These are cast in one piece and the rule holds good where there are two cylinders, as in the case of the compounds. The cylinders, and steam-chest for one side and half the "saddle" of the locomotive, upon which the forward end of the boiler rests, are nowadays generally made in a single casting. After that it is a simple enough matter to smooth down the outer surface, bore the cylinders to perfect surfacing, and line the steam-chests with a bushing that can be readily removed once it is worn out. The driving-wheels are an important detail of the construction of the locomotive. They are made in rough castings--of steel for fast passenger engines, and of iron for other forms of motive power--and are then made true in giant lathes. The steel tires are shrunk on the wheels, a work of astounding nicety; and in turn the wheels themselves are heated and shrunk upon the axles--of the best steel that man can forge. To place these wheels upon the axles is hair-line work. A 9-inch hub receives an axle just 8.973 inches--no more, no less--in diameter. It is keyed and then under the slight expansion of a gentle heat it is rammed upon the axle-end. It goes on to stay, and stay it must. From all these shops, a busy industrial railroad brings the different parts to the great and busy hall of the erecting-shop, a vast place of vast distances and filled always with the noisy clatter of great industry. Here the different parts, which have been carefully built by skilled artisans, are assembled into the finished whole. The cylinders and saddle-halves are placed and firmly riveted together. Into the collar of that saddle a giant overhead crane carefully sets the boiler and the fire-box. They are quickly riveted to the upper flange of the saddle: the locomotive is coming into a semblance of itself.