THE STEAM-ENGINE. 



259 



Fig. 218. 





How does a 

 diminution of 

 water in boil- 

 ers often oc- 

 casion eiplo- 

 Bions ? 



it overcomes the resistance of 

 the weight, lifts the valve, and 

 allows the steam to escape. 

 "When sufficient steam has 

 escaped to diminish the pres- 

 sure, the valve falls back into 

 its place, and the boiler is as 

 tight as if it had no such opening. 



Fig. 218 represents the ordinary construction of the safety-valve. 



583. The explosion of steam-boilers, when the safety-val\-9 

 is in good condition and working order, is sometimes inex- 

 plicable ; but explosions often result from the engineer allow- 

 ing the water to become too low in the boilers. When this 

 occurs, the parts of the boiler which are not covered with 

 water, and are exposed to the fire, become highly overheated. If, in this 

 condition, a fresh supply of water is thrown into the boiler, it comes suddenly 

 into contact with an intensely -heated metal surface, and an immense amount 

 of steam, having great elastic force, is at once generated. In this case the 

 boiler may burst before the inertia of the safety-valve is overcome, and the 

 stronger the boiler the greater the explosion. 



What is a ^S"^- '^^^ degree of pressure which the steam exerts upon 

 Bteam-guage ? the interior of the boiler, and which is consequently avaQ- 

 able for working the engine, is indicated by means of an instrument called 

 the "steam" or "barometer-guage." It 

 consists simply of a bent tube, A, C, D, 

 E, Fig. 219, fitted into the boiler at one 

 end, and open to the air at the other. 

 The lower part of the bend of the tube 

 contains mercury, which, when the pres- 

 sure of steam in the boiler is equal to 

 that of the external atmosphere, will 

 stand at the same level, H R, in both legs 

 of the tube. "When the pressure of the 

 steam is greater than that of the atmos- 

 phere, the mercury is depressed in the 

 leg C D, and elevated in the leg D E. A 

 scale, G, is attached to the long arm of 

 the tube, and by observing the difference 

 of the levels of the mercury in the two 

 tubes, the pressure of the steam may 



be calculated. Thus, when the mercury is at the same level in both 

 legs, the pressure of the steam balances the pressure of the atmosphere, 

 and is therefore 15 pounds per square inch. If the mercury stands 30 

 inches higher in the long arm of the tube, then the pressure of the steam 

 is equal to that of two atmospheres, or is 30 pounds to the square inch, and 



Fig, 219. 



