THE GROWTH OF THE STEAM-ENGINE. 



539 



diameter. Steam is carried at GO pounds, and is expanded nine times. 

 The boilers are ten in number, cylindrical in form, and with cylindri- 

 cal flues ; they are 13 feet in diameter, 10-| feet long, with shells of 

 iron H 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 8-| 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 IT Z are coupled at an angle of ninety degrees, 

 only two cylinders, A H, 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 pecu- 

 liarity being that the steam exhausted by the one cylinder, A, is used 

 again in the second and larger one, B. In this combination, the ex- 

 pansion 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 The revolution by which the screw has superseded the paddle- 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 ai*e driven by the horizontal high-pressure en- 

 gine usually, as in the days of Oliver Evans, and retain their peculi- 

 arities of construction. Some of the Mississippi steamboats (Fig. 61) 



