A cylinder of plate-iron will withstand a gradually applied, evenly distributed, and constant pressure, one thousandth part of which, acting at one spot, as a blow, would rend its way through, or establish a crack. This slight rent, giving partial relief to the sudden but comparatively small force that causes it, would be nothing very serious in itself,--no more so than a rent produced by the hydraulic press,--if the whole force, equal, perhaps, to that of a thousand wild horses imprisoned within, did not take instant advantage of it to enlarge the breach and blow the whole structure to fragments, or, in other words, if it did not permit nearly the whole of the accumulated heat in the boiler to be at once converted into mechanical motion. For example, a boiler whose ordinary working pressure is one hundred pounds to the square inch, which may give an aggregate on the whole surface of five millions of pounds, would not give way, perhaps, if that pressure were gradually and evenly increased to thirty millions. But if the water is allowed to get so low that some part of the plate exposed to the fire is no longer covered with it, that part will directly become far hotter than the water or 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.