the mass of the steam,—dry steam having no more power to carry away the excess of heat than so much air. After that, when the water rises again, the first wave or wallop that strikes the overheated plate absorbs the excess of heat, and its conversion into steam of higher pressure than that already existing is so sudden that it may be regarded as instantaneous. It is to be remembered that for every pound of water raised one degree, or heat to that amount absorbed in generating steam, a force of seven hundred and seventy-two pounds is created. In this case a new or additional force is created, which, acting in all directions from one point, first takes effect on the line which joins that point with the nearest opposite point in the wall of the boiler. If it is not like smiting with the edge of a ponderous battle-axe, it is at least as dangerous as a cannon ball shot along that line. If the local heat so suddenly absorbed be but enough to raise ten pounds of water ten degrees, it is equivalent to the force acquired by seventy-seven thousand two hundred pounds falling through a foot, or of a cannon-ball of one hundred pounds flying at the rate of more than a mile per second. If by any miracle the boiler should stand this shock or series of shocks, the pressure becomes equalized, and the overheated plate having parted with its excess of heat, safety is restored. But if cohesion is anywhere overcome by the sudden blow, the wild horses stampede in all directions. The boiler, minus the water and boiler-head perhaps, goes through ceiling, roof, and brick walls, as if they were cobwebs, and, surrounded with fragments of men and things, is seen descending like a comet through the neighboring air.
To get rid of this liability to have a Thor-hammer or thunderbolt generated in the stomach of a steam-engine, at any moment when the vigilance of the engineer happens to be at fault, something is going to be done. No safety-valve or fusible plug is adequate. The boiler cannot be all safety-valve. The trouble is, the hammer is not more likely to strike the first of its terrible series of blows on the valve than anywhere else. A safety-valve, in good order, is a sovereign precaution against the excess of an equally distributed strain, but it is not an adequate protection against a shock or unequal strain. The old-fashioned gaugecocks, which are by no means to be dispensed with, reveal the state of the water in the boiler to the watchful engineer about as surely as the stethoscope reveals to the doctor the condition of his patient's lungs. A surer and more convenient indication is the tubular glass gauge, on the fountain principle, which in its best form is both trustworthy and durable. No well-informed proprietor suffers his boiler to be without one; but it is not a cure for carelessness. It is only a window for the vigilant eye to look through, not the eye itself. Steam-boilers will have to be constructed so that when the subsidence of the water fails to check itself by enlarging the supply, it shall, before the point of danger is reached, infallibly check the combustion, let off the steam, and blow a whistle or ring a bell, which the proprietor may, if he pleases, regard as the official death-knell of the careless engineer. Human vigilance must not be superseded, but fortified,—as in the case of the watchman watched by the tell-tale clock. The steam-creature must be so constituted as to refuse to work itself down to the zone where alone unequal strains are possible; it must cry out in horror and strike work. Mechanically the solution of the problem is easy, and the enhancement in cost of construction will be nothing, compared to the risk of loss from these explosions. With this guard against the deficiency of water, steam-power will become the safest, as it is the most manageable, of all forces that have hitherto been subsidized by the civilized man.
But there is one more improvement worth mentioning. We do great injustice to our steam-slaves by the slovenly and unphilosophical way in which we feed them. We take no hints from animal economy or the laws of dietetics.
