568 MACHINES, WORK, AND ENERGY 



and down in the cylinder. When the valve, D, was opened, 

 steam rushed into the cylinder as the piston moved up and 

 the pump-rod descended. The valve, D, was now closed and 

 the valve, F, opened. This permitted a spray of cold water 

 from the tank, G, to enter the cylinder. This spray of cold 

 water condensed the steam in the cylinder, producing a vacuum 

 in the cylinder beneath the piston. The air pressure upon 

 the upper surface of the piston then forced the piston down 

 to the bottom of the cylinder. This raised the pump-rod 

 and plunger. This was the working stroke. The water 

 spray was then cut off and the water and condensed steam 

 drained off into the reservoir, R, which had to be placed 

 about 30 ft. below the cylinder. Do you see why? (Review 

 air pressure (Art. 210) and pumps (Art., 559 to 561).) The 

 valve, Z>, was again opened, the spray of water again admitted 

 and a second stroke was completed. 



679. Humphrey Potter's Invention. At first the valves, 

 D, and jF, were operated by hand. It was an easy task and a 

 boy did the work. Only about six or eight strokes were usually 

 made each minute. It is recorded that an ingenious boy, 

 Humphrey Potter, in 1713, tiring of this task, contrived a 

 system of levers and strings fastened to the moving beam 

 in such a manner as to operate the valves automatically 

 (Fig. 355). This boy's invention doubled the amount of 

 work which the engine could do, for the valves were now 

 opened and shut exactly at the right moment. With this 

 improvement the Newcomen engine made 15 or 16 strokes 

 each minute. But at its best this engine was extremely waste- 

 ful of fuel. It required from 35 to 50 Ib. of coal to do a 

 horse-power-hour of work, some eight to ten times as much as 

 is required by steam engines today (see Art. 656). 



680. Watt's Improvement. It remained for James Watt, 

 a Scotchman, to perfect the steam engine. About 1774, Watt 

 so perfected the steam engine that it became practically the 

 engine of today. He discovered the chief source of energy 

 loss in Newcomen's engine and overcame it. He saw that 



