THE STEAM-ENGINE. 



511 



conducted into the feeding cistern) where the steam carried off by it is con- 

 densed, and heats the feeding water. 



The magnitude of the safety-valve should be such that, when open, steam 

 should be capable of passing through it as rapidly as it is generated in the 

 boiler. The superficial magnitude, therefore, of such valves must be propor- 

 tional to the evaporating power of the boiler. In low-pressure boilers the 

 steam is generally limited to five or six pounds' pressure per square inch, and 

 consequently the load over the safety-valve in pounds would be found by 

 multiplying the superficial magnitude of its smallest part by these numbers. 

 In boilers in which the steam is maintained at a higher pressure, it would be 

 inconvenient to place upon the safety-valve the necessary weight. In such 

 cases a lever is used, the shorter arm of which presses down the valve, and 

 the longer arm is held down by a weight capable of adjustment, so that the 

 pressure on the valve may be regulated at discretion. Two safety-valves 

 should be provided on all boilers, one of which should be locked up, so that 

 the persons in care of the engine should have no power to increase the load 

 upon it. In such case, however, it is necessary that a handle connected with 

 the valve should project outside the box containing it, so that it may always 

 be possible for the engineer to ascertain that the valve is not locked in its seat, 

 a circumstance which is liable to happen. 



Sometimes also two safety-valves are provided, one loaded a little heavier 

 than the other. The escape of steam from the lighter valve in this case gives 

 notice to the engine-man of the growing increase of pressure, and warns him 

 to check the production of steam. The lever by which the safety-valve is 

 held down is sometimes acted on by a spiral spring, capable of being so ad- 

 justed as to produce any required pressure on the valve. This arrangement is 

 adopted in locomotive engines, where steam of very high pressure is used ; and 

 in such cases also there are always provided two such valves, one of which 

 cannot be increased in its pressure. 



The pipe by which the boiler is fed with water will necessarily act as a 

 safety-valve, for when the pressure of the steam increases in an undue degree, 

 it will press the water in the boiler up through the feed-pipe, so as to dis- 

 charge it into the feed-cistern, a circumstance which would immediately give 

 notice of the internal state of the boiler. The steam-gauge already described, 

 fig. 60, would also act as a safety-valve ; for if the pressure of steam in the 

 boiler should be so augmented as to blow the mercury out of the steam-gauge, 

 the steam would then issue through the gauge, and the pressure of the boiler 

 be reduced, provided that the magnitude of the tube forming the steam-gauge 

 were sufficient for this purpose. 



In high-pressure boilers which are exposed to extreme temperatures and 



pressures, and which are therefore subject to danger of explosion, a plug of 



metal is sometimes inserted, which is capable of being fused at a temperature 



above which the boiler should not be permitted to be raised. If the pressure 



| of steam increase beyond the proper limit, the temperature of the water and 



steam will undergo a corresponding increase ; and if the metal of the plug be 



capable of being fused at such a temperature, the plug will fall out of the 



£ boiler, and the steam and water will issue from it. Various alloys of metal 



j are fusible at temperatures sufficiently low for this purpose. An alloy com- 



j! posed of one part of lead, three of tin, and five of bismuth, will fuse at the 



<| common temperature of boiling water ; and alloys of the same metals, in various 



\) proportions, will fuse at different temperatures from 200° to 400°. 



<| Although fusible plugs may be used, in addition to other means of insuring 



]> safety, they ou^ht not to be exclusively relied on at the ordinary working 



i[ pressure of the boiler. The fusible plug ought to be capable of more than re- 



