converted into force, and hence vapor is converted into watery globules mechanically suspended on clouds, which, by their friction, sweep the electrical ether into excessive condensation in the great Leyden-jar arrangement of the sky. Whatever it may be that gives relief to this condensation, the relief itself consists in motion, either translatory or vibratory, of the electrical ether or ethers. As this motion, if it be such, often takes place through gases, liquids, and solids, without any sensible mechanical effect, and at other times is contemporary with phenomena of intense heat, we may, till otherwise informed, suppose, that, whenever it produces a mechanical effect, it is by so impinging on the calorific ether as to produce the motion of heat, which is instantly thereafter converted into mechanical force. It is not so much the greatness of the amount of this mechanical force which gives it its peculiar destructiveness, as the inequality of its strain; not so much the quantity of matter projected, as the velocity of the blow. One may have his brains blown out by a bullet of air as well as one of lead, if the air only blows hard enough and to one point. Whatever its material, the edge of the thunder-axe is almost infinitely sharp, and its blow is as destructive as it is timeless. But it is always heat, not electrical discharge, which only sometimes causes heat, that strikes the blow.
Now in the case of a steam-boiler, when the water, having been reduced too low, is allowed suddenly to foam up on the overheated crown-sheet of the furnace, there must be just that sudden or instantaneous conversion of heat into force which may take place when the current of the electrical discharge passes through the gnarled fibres of an oak. The boiler and the oak are blown to shivers in equally quick time. The only difference seems to be, that in one case electricity stood immediately, in point of time, behind the heat, and in the other it stood away back beyond the crocodiles, playing its rôle more genially in the growth of the monster forests whose remains we are now digging from the bowels of the earth as coal. In the normal action of a steam-boiler, the steam-generating surfaces being all under water, however unequally the fire may act in different localities, the water, by its rapid circulation, if not by its heat-absorbing power, diffuses the heat and constantly equalizes the strain resulting from its conversion into mechanical force. The increase of pressure takes place gradually and evenly, and may easily be kept far within safe limits. It is quite otherwise when the conductivity of the boiler-plate is not aided and controlled by the distributiveness of the water, as it is not whenever the plate is in contact with the fire on one side without being also in contact with the water on the other. Everybody knows that boilers explode under such circumstances, but everybody does not know why.
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
