(164.) 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 are fusible at temperatures sufficiently low for this purpose. An alloy composed 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 ought not to be exclusively relied on at the ordinary working pressure of the boiler. The fusible plug ought to be capable of more than resisting the pressure; but if it be so, its point of fusion would be one at which the steam would have a pressure of at least two atmospheres above its working pressure. The plug would therefore be capable of being fused only as soon as the steam would acquire a pressure of 30 lbs. per inch above its regular working pressure. When a boiler ceases to be worked, and the furnace has been extinguished, the space within it appropriated to steam [Pg281] will be left a vacuum by the condensation of the steam with which it was previously filled. The external pressure of the atmosphere acting on the boiler would, under such circumstances, have a tendency to crush it inwards. To prevent this, a safety-valve is provided, opening inwards, and balanced by a weight sufficient to keep it closed until it be relieved from the pressure of the steam below. A large aperture closed by a flange secured with screws, represented at O in _fig._ 71., called the _man-hole_, is provided to admit persons into the boiler for the purpose of cleaning or repairing its interior. (165.) The manner in which the governor regulates the supply of steam from the boiler to the cylinder, proportioning the quantity to the work to be done, and thereby sustaining a uniform motion, has been already explained (p. 125.). Since then the _consumption_ of steam in the engine is subject to variation, owing to the various quantities of work it may have to perform, it is evident that the _production_ of steam in the boiler should be subject to a proportional variation. For otherwise, one of two effects would ensue: the boiler would either fail to supply the engine with steam, or steam would accumulate in the boiler from being produced in too great abundance, and would escape at the safety-valve, and thus be wasted.