STEAM AND GASOLINE ENGINES 183 



was entering it. This made the thrust of the piston very power- 

 ful. He also incased the cylinder in a larger one in order to keep 

 steam in the space between them. This kept the inner cylinder 

 hot so the steam entering it would not condense in part and thus 

 lose its power. In the fourth place he attached the free end of the 

 piston rod to a heavy flywheel in order to make it whirl round. 

 The stroke of the piston is a back-and-forth stroke, and at each 

 end of the stroke there is a moment when it stands still and is 

 exerting no pressure to make the machine go. The momentum 

 of the revolving flywheel carries the piston past this dead point 

 and makes the engine run smoothly rather than jerkily. The 

 governor was the fifth major improvement that Watt devised. 

 When an engine is working, the load on it is necessarily a variable 

 one. Thus it is more work to lift the water in a mine pump than 

 it is merely to drop the pump plunger for the next stroke. The 

 engine thus tends to slow down when hard work is being done and 

 to race when the load is lessened. Watt's governor automatically 

 partially closed the valve on the steam inlet pipe when the engine 

 speeded up and opened it wider when it slowed down. The 

 method of operation will be described below. It is evident from 

 what has been said here that Watt was the real inventor of the 

 steam engine. He did so much more than his predecessors 

 toward making it a practical machine that he deserves the 

 lion's share of the credit. 



He not only largely created the steam engine, but he devised 

 the measure which we still use to express its work capacity. 

 Since the "fire engine" was taking the place of the horse as a 

 means of doing work, it was natural that its ability to work should 

 be expressed in horse-power. Watt concluded that a good horse 

 could draw 1,000 pounds up a hill 33 feet high in one minute and 

 so he adopted this as the unit of measure to indicate the power 

 of an engine. He rather overestimated the power of a horse, 

 but we use his horse-power today to measure the work capacity 

 of an engine. A fifty-horse-power engine is one that could raise 

 50,000 pounds 33 feet in one minute. 



