COLLAPS1ON OF BOILERS.
" Boilers constructed of plates of laminated copper, or iron, low pressure boilers particularly, are subject to accidents from collapsion." Some cases of this kind in American boats have been mentioned in the pre ceding pages; it not having been thought necessary to separate them from those strictly called "explosions," and which proceed from the expansive force of steam. " These boilers," says Arago [or their internal tubes], "are sometimes completely crushed; the sides bending to the pressure from without. The cities of Lyons and Etienne in France have been the theatres of several accidents of this kind. The inner cylinders of boilers having the furnace and flues within, give way occasionally, from their inability to resist the pressure of the steam in the circular space around them, and become flattened; this change of figure can not take place without the metal giving way, and hot water thus escaping produces dreadful havoc. Four collapses took place in Cornwall, of boilers of this form, as has been already mentioned, p. 447. " An en gineer" in commenting upon these accidentst says, " that as far as appearances after explosion are to be relied upon, they were such as to justify a suspicion, that the boilers were short of water;" and condemns their construction.
Arago mentions another cause of collapse. "At the time of lighting the fire under a boiler, the space within the boiler not occupied with water is filled with atmospheric air. This air mixed with steam passes by degrees into the engine fed by the boiler, and at last is completely expelled therefrom. If the ma chine be stopped, and the fire suffered to go down, the steam will be gradually condensed as the cooling proceeds, and after some time, the space which it occupied will be almost void. The boiler is then
pressed inwards by the pressure of' the atmosphere, without there being any interior force to counter balance the action. A sudden condensation would crush the boiler. To prevent such accidents, interior valves called air valves opening inwards were invented. They are kept in place by a spiral spring within the boiler, the strength of the spring being a little more than equal to the weight of the valve; or it is sus pended horizontally to the arm of a lever, placed on the outside of the boiler, so arranged that the valve exactly touches the interior face of the opening, which it is to close. With this arrangement, the elasticity of the steam within the boiler, can never become less than that of the atmosphere, without the immediate opening of the valve, which will admit the air into the boiler; thus when the engine is stopped, no vacuum will be formed within the boiler.t It seems that we cannot so safely conclude, that the same arrangement would prevent certainly the crushing of the boiler, for such accidents result from an abrupt diminution of elasticity in the steam. The gradual action of the valve might, to a certain degree, lessen the evil, but could no t prevent it. There is but one remedy against such accidents; to watch carefully the means of supplying the boiler, and to prevent the reservoir of steam within the boiler from being suddenly cooled, as would happen for example, if' a quantity of cold water should be thrown upon the exterior. It is important also when we use this kind of boiler, not to close the register doors until the fire is extinguished."§