The view below represents the interior of one of the great boiler rooms where the boilers are put together by riveting the plates to each other at their edges. Some men stand inside, holding heavy sledges against the heads of the rivets, while others on the outside, with other sledges, beat down the part of the iron which protrudes, so as to form another head to each rivet, on the outside. This process can be seen distinctly in the boiler nearest to the observer in the view below. The planks which are seen crossing each other in the open end, are temporary braces, put in to preserve the cylindrical form of the mass, to prevent the iron from bending itself by its own weight, before the iron heads are put in. [Illustration: RIVETING THE BOILERS.] Sometimes operations must be performed upon the sides of the boiler requiring the force of machinery. To effect this purpose, shafts carried by the central engine to which we have already alluded, are attached to the walls in various parts of the room, as seen in the engraving. Connected with these shafts are various drilling and boring machines, which can at any time be set in motion, or put to rest, by being thrown in or out of gear. One of these machines is seen on the right of the boiler above referred to, and another in the left-hand corner of the room quite in the back-ground. Near the fore-ground, on the left, is seen a forge, where any small mass of iron may be heated, as occasion may require. The semi-cylindrical piece which lies in the centre of the room, toward the fore-ground, is part of a locomotive boiler, and is of course much smaller in size than the others, though it is constructed in the same manner with the large boilers used for sea-going ships. The process of riveting, as will be seen by the engraving, is the same. One man holds up against the under side of the plate a support for the rivet, while two men with hammers form a head above--striking alternately upon the iron which protrudes. From the boiler we proceed to the cylinder, which is in fact the _heart_ of the engine,--the seat and centre of its power. It is to the cylinder that the steam, quietly generated in the boiler, comes to exercise its energy, by driving, alternately up and down, the ponderous piston. The cylinder must be strong so as to resist the vast expansive force which is exercised within it. It must be stiff, so as to preserve in all circumstances its exact form. It must be substantial, so as to allow of being turned and polished on its interior surface with mathematical precision, in order that the piston in ascending and descending, may glide smoothly up and down, without looseness, and at the same time without friction. To answer these conditions it is necessary that it should be formed of cast iron.