266 HEAT. 



down. The water in the bulb, a^ is heated with a 

 spirit-lamp or sand-bath ; the rising steam forces up the 

 piston. [NTow, 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 j)iston. By thus alternately applying heat and 

 cold, it is driven up and down like the piston of a steam- 

 engine. The only difference is, the steam-engine is fur- 

 nished with apparatus so that this application of heat and 

 cold is performed by the machine itself The bulb repre- 

 sents the boiler, and the tube the cylinder ; but in the 

 steam-engine, the boiler is separate, and connected by a 

 pipe with the cylinder ; and instead of applying the cold 

 water directly to the cylinder, it is thrown into another 

 vessel, called the condenser, connected with the cylinder. 



When Newcomen, who made the first rude regularly 

 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 labor, 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 ol a common steam-engine may be 

 understood from the following figures, one representing 

 the boiler, and the other the working machinery. 



- The boiler, B (fig. 290), contains water in the lower 

 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 , whenever 

 the boiler is full enough, by means of the rising of the 

 float, S, and opened whenever the float sinks from low 

 water. M, barometer gauge, to show the pressure of the 

 steam ; w, weight on the lever, e b, for holding down the 

 Sdfety-valve : this lever being graduated like a steelyard, 



