AMERICAN INSTITUTE. 701 



liigliest pressure. The steam cylinder and valve boxes, are the 

 only parts of the engine exposed to the action of the steam; and 

 they have invariably been found secure and durable. They are 

 not exposed to any destructive agency, except the friction of the 

 piston and valves; and this being almost harmless, they remain 

 in a state of safety without deterioration. 



The boiler of the engine is the only organ exposed to mischief, 

 and with this alone explosions are found to occur. Who will then 

 have the temerity to assert that only high pressure boilers are 

 subject to danger, and that low pressure ones are secure. 



Every boiler may become surcharged with steam when the 

 quantity drawn off is less than the quantity generated, and when 

 safety valves do not perform their duty, therefore as similar safety 

 apparatus are used for both high and low pressure boilers, they 

 are both liable to similar interruptions in their working. If, then, 

 an over-filling of the boiler with steam is equally possible in both 

 high and low pressure engines, both are liable to danger from this 

 source. It will be found that as great a proportionate number of 

 low as high pressure boilers have exploded in England, America 

 and France. The high pressure has an advantage over the low 

 in that the sediment (which is a bad conductor of heat), when 

 the elasticity is great, seldom attaches itself firmly to the boiler, 

 but collects in a loose state and is easily removed. Explosions 

 occur from shortness of water in the boiler; when added, the 

 glowing metal being suddenly covered with water, generates 

 steam instantly and bursts. Both iron and copper generate, when 

 red hot, a large quantity of steam. Ten pounds of copper, heated 

 sufficiently to glow in the dark, convert, according to Adam Hall, 

 one pound of water into steam, which will occupy twenty-seven 

 cubic feet. According to Marestier four pounds of red hot iron 

 will convert one pound of water into steam. 



Professor Johnson, of Philadelphia, found that iron, at a white 

 heat, repelled the w^ater, and that nine pounds of iron, at a dull 

 red glow, scarcely visible by daylight, converted one pound of 

 water into steam. He also remarked that cast iron generated 

 more steam than hammered iron, in the propertion of nine to 

 eight and a quarter. Boilers should never be constructed of cast 



