on the Sieam Ens:ine, 331 



t; 



The corrosion effected by the chemical agents that are constantly 

 acting upon the material of the boiler, and the changes of strength 

 produced by sudden changes of temperature, render safety unattaina- 

 ble by any such single proof of strength in the boiler ; nor ought 'it to 

 be relied on, except for intervals of time of moderate duration. " A 

 safety valve," says M. Arago, " however well constructed, can never 

 warrant the engineer in neglecting to prove his boiler from time to 

 time, nor can it warrant him in not endeavoring to prevent by all the 

 means in his power abrupt changes in the elasticity of the steam, 

 and in not preventing the boiler from, at any time, being too strongly 

 heated. 



A deficiency in the supply of water to the boiler, is one of the most 

 common causes of the explosion 'of steam engines. By this means 

 the upper parts of the boiler, when the flame plays on them, become 

 heated even to redness. This greatly impairs the cohesion of the 

 metal, and proportionally weakens the boiler. The metallic surface 

 also, under such circumstances, rapidly corrodes, and is thus weak- 

 ened still farther. Meanwhile the steam, in contact with the upper 

 surface of the boiler, is becoming intensely hot, without acquiring a 

 proportionate density and elasticity. The moment, therefore, the 

 water below is, by any means, brought into contact with this heated 

 surface, and mixed with the hot steam, the latter instantly acquires 

 great density and a tremendous force, against which the safety valves 

 are incompetent to provide, and which the boiler now is unable to 

 resist, and a violent explosion is the inevitable consequence. 



It is a remarkable fact, and one unaccounted for, until it was ex- 

 plained by Mr. Perkins after his investigations into the properties of 

 steam- at very high temperatures, that boilers frequently burst at the 

 very moment of opening the safety valve. The water within the 

 boiler being low, and the steam which presses upon its surface very 

 much heated, but of litde elastic force, suppose the safety valve to be 

 opened : a copious discharge of steam takes place ; the water re- 

 lieved from the pressure upon its surface, rises up in foam, the action 

 being similar to that which takes place in a champaigne bottle on 

 drawing the cork ; the water thus thrown in small drops into the 

 midst of an intensely heated vapor, flashes into highly elastic steam, 

 and the safety valve not aflbrding a suflicient vent for the discharge of 

 the steam, the boiler is rent."* 



* Arago on the Explosion of Boilers,, 



