242 HEAT. 



steam forces up the piston. Now immerse the bulb in 

 cold water or snow, and the steam is condensed again 

 into water, the tube is left vacant, and the pressure of 

 the atmosphere forces down the piston. By thus al- 

 ternately applying heat and cold, it is driven up and 

 down like the piston of a steam-engine. The only dif- 

 ference is, the steam-engine is furnished with appara- 

 tus so that this application of heat and cold is perform- 

 ed by the machine itself. The bulb represents the 

 boiler, and the tube the cylinder ; but in the steam- 

 engine the boiler is separate, and connected by a pipe 

 with the cyhnder ; and instead of applying the cold 

 water directly to the cylinder, it is thrown into an- 

 other vessel called the condenser, connected with the 

 cylinder. 



When Newcomen, who made the first rude regular- 

 ly-working engine, began to use it for pumping water, 

 he employed a boy to turn a stop-cock, connected with 

 the condenser, every time the piston made a stroke. 

 The boy, however, soon grew tired of this incessant la- 

 bor, and endeavored to find some contrivance for relief 

 This he effected by attaching a rod from the piston or 

 working-beam to the cock, which was turned by the 

 machine itself at every stroke. This was the origin 

 of the first self-acting engine. 



The different parts of a common steam-engine may 

 be understood from the following figures, one represent- 

 ing the boiler, and the other the working machinery. 



The boiler, B {Fig. 203), contains water in the low- 

 er part and steam in the upper ; F B is the fire ; v o is 

 the feed-pipe ; v, a valve, closed by the lever, b c a, 

 whenever the boiler is full enough, by means of the ris- 



