92 HEAT. 



through which steam maybe admitted at pleasure, either 

 above or below. When the valve in the 

 lower tube is opened, the steam under pres- 

 sure in the boiler, expands and enters the cyl- 

 inder, lifting the piston. If the steam is next 

 admitted above, it drives the piston back 

 again, and the latter may thus be kept in con- 

 stant motion, and made to move wheels, 

 shafts, or other machinery. It is only necessary, that 

 whenever steam enters, that which is on the other side 

 of the piston shall find its way out, into the air. Valves 

 are provided for this purpose, which are opened and 

 closed, at the right time, by the machinery which the 

 piston itself moves. 



222. HIGH PRESSURE ENGINE. The en- 



Whatisahigh 



pressure en- gine, here described, is called the high pres- 

 sure engine. The steam which moves it, 

 must evidently have elastic force greater than that of 

 the atmosphere, or it cannot expand and drive out the 

 waste steam, in opposition to the elastic force of the 

 air. Steam of much higher pressure is used in such 

 engines, than in those to be next described, and hence 

 their name. 



223. Low PRESSURE ENGINE. The 

 same fi S ure wil1 answer to illustrate the 

 \ ow pressure engine. The difference is, 



sure engne. , ., . 



that the steam which has been used is 

 not driven out. but disposed of, on the spot, by con- 

 verting it into water. The advantage of this will 

 be readily perceived. Suppose the space above the 

 piston to be full of steam. A jet of water is made to 



