SOME COMMON MECHANICAL MOTORS 571 



681. How Watt's Engine Worked. In Watt's engine the 

 steam entered through the pipe (D, Fig. 356). Just as the 

 piston reached the top of the cylinder the valves, C and F, 

 were opened and the valve, E, closed. The opening of the 

 valve, C, permitted the steam to flow into the cylinder above 

 the piston forcing it downward. The opening of the valve, 

 F, permitted the steam in the cylinder below the piston 

 to escape into the vessel, H, where it was condensed by the 

 surrounding cold water. The condensation of this steam 

 tended to produce a vacuum in the lower portion of the 

 cylinder. The steam also entered the jacket surrounding 

 the cylinder; thus the cylinder was always kept hot. Just 

 as the piston reached the bottom of the cylinder the valves, 

 C and F, closed and the valve, E, opened. The steam could 

 then no longer enter from the pipe, D, but it could flow through 

 the pipe, X, from the upper portion of the cylinder into the 

 lower portion. The pressure was now the same on both sides 

 of the piston and the weight of the pump-rod pulled that 

 end of the beam down and so raised the piston again to the 

 top of the cylinder. The valves were operated by the pins, 

 M, M, My on the rod N. The water formed by the con- 

 densation of steam was forced by the pump, /, into the 

 hot well, K. The pump, A, raised this warm water from 

 the hot well and forced it into the boiler again through the 

 pipe, B. 



682. Watt's Double Acting Engine. While the engine 

 just described was by far the most economical and effective 

 engine which had ever been made, still, Watt was not satisfied. 

 Live steam, i.e., steam under full pressure, entered only 

 one end of the cylinder and actually did work only while 

 forcing the piston downward. About 10 years later, 1784, 

 Watt invented a DOUBLE ACTING ENGINE, i.e., one in which the 

 steam under full pressure entered first one end of the cylinder 

 and then the other. In this way the piston was forced 

 first to one end of the cylinder and then the other end by the 

 live steam. 



