diameter. Steam is carried at 60 pounds, and is expanded nine times. The boilers are ten in number, cylindrical in form, and with cylindrical flues; they are 13 feet in diameter, 1012 feet long, with shells of iron 1316 inch thick, and have 520 feet of grate-surface, 16,500 square feet of heating-surface, and 1,600 square feet of superheating-surface. The smoke-funnels, or stacks, are 812 feet in diameter and 70 feet high.
Fig. 60 shows a section of the simplest and the least costly form of compound-engine, as it is now built on the Clyde, in Great Britain, and in the United States, on the Delaware.
Here, the cranks Y Z are coupled at an angle of ninety degrees, only two cylinders, A B, being used, and an awkward distribution of pressure is avoided by having a considerable volume of steam-pipe, or by a steam-reservoir, O P, between the two cylinders.
The valves, y y, are set like those of an ordinary engine, the peculiarity being that the steam exhausted by the one cylinder, A, is used again in the second and larger one, B. In this combination, the expansion is generally carried to about six times, the pressure of steam in the boiler being usually between sixty and seventy-five pounds per square inch.
Fig. 61.—The Mississippi Steamboat, 1876.
105. The revolution by which the screw has superseded the paddle-wheel elsewhere, has not taken place in our shallow American rivers, where there is not depth enough for the screw.
In the West, boats are driven by the horizontal high-pressure engine usually, as in the days of Oliver Evans, and retain their peculiarities of construction. Some of the Mississippi steamboats (Fig. 61)
