254 



WELLS'S NATURAL PHILOSOPHY. 



A steam-engine of this character is called a condensing steam-engino, bo- 

 cause the steam whicli has been employed in raising or depressing the piston 

 ia condensed, after it has accomplished its object, leaving a vacuum above or 

 below the piston. It is also called a low-pressure engine, because, on ac- 

 count of the vacuum which is produced alternately above and below the 

 piston, the steam, ia acting, does not expend any force in overcoming the 

 pressure of the atmosphere. Steam, therefore, may be used under such condi- 

 tions of low expansive force, or, as it is technically called, of " low-pressure." 



The practical construction of the 

 piston and cylinder, and the ar- ^IG- 213. 



rangement of connecting pipes by (^ 



which the steam is admitted alter- 

 nately above and below the piston, 

 is fully shown in Fig. 213. The 

 valves, which are of various forms, 

 are connected bj^ lovers with tho 

 machinery, in such a way as to 

 open and close with great ac- 

 curacy at exactly the proper mo- 

 ment. 



■iiTT, .• -u- i, 5'^8. In some 



vVnat IS a nign- 



pressure eu- engines, the appa- 

 ^°® ' ratus for condens- 



ing the steam alternately above 

 or below the piston, is dispensed 

 with, and the steam, after it has 

 moved the piston from one end of 

 the cylinder to the other, is al- 

 lowed to escape, by the opening of 

 a valve, directly into the air. To 

 accomplish this, it is evident that 

 the steam must have an elastic 

 force greater than the pressure of 

 the atmosphere, or it could not 

 expand and drive out the waste 

 eteam on the otlier side of the piston, in opposition to the pressure of the air. 

 An engine of this character is accordingly termed a "high-pressure" engine. 



High-pressure engines are generally worked with a pressure of from fifty 

 to sixty pounds per square inch of the piston ; of this pressure, at least fifteea 

 pounds must be expended in overcoming the pressure of the atmosphere, and 

 the surplus only can be applied to drive machinery. 



One of the most familiar examples of a high pressure engine is the loco- 

 motive used on railroads. The steam which has been employed in forcing the 

 piston in one direction is, by the return movement of the piston, forced out of 

 the cylinder into the smoke-pipe, and escapes into the open au- with irregular 

 puffs. 



