THE STEAM ENGINE. 91 



and high pressure engines. In the former, steam of 

 ten to thirty pounds effective pressure is used ; in the 

 latter, the pressure often reaches, and sometimes ex- 

 ceeds, seventy-five pounds. To measure the pressure, 

 the steam guage described in paragraph 215 would have 

 to be five or six feet long. It is on account of this in- 

 convenient length, that other guages are often substi- 

 tuted. 



Whatismeant 219 ' E V effective pressure, is meant the 

 by e/ective surplus over and above that which is neces- 

 pre sary to counterbalance the pressure of the 



atmosphere, or that of the uncondensed steam, on the 

 opposite side of the piston. 

 , . , 220. SAFETY-VALVE. The safety-valve 



Hixplam ana 



illustrate the is a contrivance, by means of which the 

 P thTsafetij steam finds vent through a hole in the 

 valve. boiler, whenever its force becomes too great 



for safety. A piece of metal shaped 

 somewhat like a decanter stopper, fits 

 into the hole above mentioned, and is 

 loaded by a weight, which can be made , 

 greater or less at pleasure. As long as the steam has 

 not too great pressure, the stopper continues in its 

 place, and the boiler is as tight as if it had no such 

 opening. When this pressure is exceeded, the valve is 

 lifted, and stearn escapes. The stopper, being loaded, 

 falls back again, as soon as the pressure is relieved. 



221. THE STEAM ENGINE. The power 



Explain the . . , . . 



principle of applied in the steam engine is the elastic 



ttetteamen- force of steam. The figure represents a 



cylinder and close fitting piston, and tubes 



