i 
- 
on the Steam Engine. : 331 
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 engineerin 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 prevestits the boiler ee at any time, being too strongly 
heated. 
A deficiency in the supile of water to the boilers is er i the most 
auses of the explosion of steam engines. By this means 
arts of the boiler, when the flame plays on them, become 
heated ‘even toredness: - 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 whiclt the boiler now is —_— to 
— and a violent explosion i is the inevitable consequence. 
It is avemarkable 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 ened: but of little eldstic 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 foamy, 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 inferiaedy heated vapor, flashes into highly elastic steam, 
and the safety valve not affording a sufficient vent for the discharge of 
leam, the boiler is rent.”* 
: Arago on the Explor of Ledieis: 
=. ra 
MN 
