tain-peak, it has often been a very slight force that has determined the path it has taken, and which of two villages miles apart was to be demolished by it. The fire-alarm system in our cities uses electricity as an initiatory motion: the hammers of the tower-bells are worked by the descent of heavy weights—wound up by manual power from time to time—and the store of energy contained in the elevated masses is instantly made available by an electric current from the central office simply freeing a detent and allowing the weight to fall. Many other recent inventions for working railway-signals, looms, etc., embody this principle. The magneto-electric machines, which are extensively employed in lighthouses and electro-plating factories, yield electricity from the mechanical motion of a steam-engine. A small permanent steel magnet is indispensable in the apparatus; it induces magnetism in soft-iron cores, and these again in others in an increasing series. The power may be gigantic, and the magnet, were it not inconvenient, might be as small as a cambric needle: yet without it neither electricity nor light can be had. A similar illustration occurs in the development of a current in a common galvanic battery: the two pieces of different metals used, as zinc and copper, when quite dry before being placed in the bath, if simply brought into contact for a moment show opposite electric polarities which, if given a mechanical expression, would be a very small amount indeed. This minute force, only to be detected by the most delicate means, is the necessary opening of the flood-gate of energy in a working battery. And when light is desired from a group of powerful cells, it is first requisite to use a small effort in bringing the poles together, and then separating them at a short distance apart. The brilliant arc of light, once across the chasm, can continue to span it; but its force, although so great, is unequal to leaping over it without help. These examples show the importance of knowing the fit initiatory forces in processes whereby one form of energy is sought to be converted into another. It is probably for lack of such knowledge that at present we waste so lamentable a quantity of heat, our commonest force, in changing it into the more desirable form of electricity, light, and mechanical motion. In exceptionally favorable circumstances, steam-engines of the best kind give but a fourth in work of the theoretical value of the heat applied, and in obtaining an electric current from an engine further loss occurs; while the production in a thermo-battery of a current from heat directly has never yet yielded as much as one-hundredth of the force employed.
A slight and well-aimed effort may not only free an immense magazine of energy, but also give it a particular direction and pilot it into one or. another of two entirely new seas. Hydrogen and oxygen gases in separate receivers might remain tranquil and unchanged for ages, and it depends upon the choice of the experimenter, when he wishes to render their energy available, whether their intense chemical force
