batteries at the one place and conveying them to another, when he wrote in one of his letters of "Professor Reynolds's disappointment with M. Faure's practical realization of electric storage, because it does not provide a method of porterage superior to conduction through a wire." This is "like being disappointed with an invention of improvements in water-cans and water-reservoirs because the best that can be done in the way of movable water-cans and fixed water-reservoirs will never let the water-carrier supersede water-pipes wherever water pipes can be laid." If we may venture to extend the great electrician's metaphor, it is like finding fault with the Great Eastern Railway Company's service of sea-water brought to London in cans, on the ground that it is just possible to obtain sea-water by a large main laid down to the coast, and that such a scheme is now under consideration. Another valuable property of the new battery is pointed out by Sir William Thomson. If it were to be used either at a fixed station to work an electric railway, such as the firm of Siemens have already brought into practical use, or to be carried on an ordinary carriage to drive it, the energy developed by the vehicle in running down-hill would be stored up ready to be used for its propulsion when it again reached a level or an ascending incline.
In the course of the correspondence Professor Ayrton has again mentioned the experiments which he and Professor Perry are carrying out with the view of using coal or coal-gas instead of zinc in a primary battery. Should he succeed in doing so, we should obtain a source of energy about ten times cheaper in working than the best-known steam engine, and M. Faure's invention may very likely be the means of making it a commercial success; for, should Messrs. Ayrton and Perry, or any other physicist, succeed in making a coal or coal-gas battery giving a good proportion of the theoretical energy of the coal or gas, should it have a high internal resistance, it would be difficult to use it in practice; but, by the aid of Faure's batteries, in cases where work was only wanted to be done for a few hours a day, as in the case of electric lighting, the comparatively feeble current of the primary battery might be collected and stored for fifteen or sixteen hours, and then allowed to run out again in the eight or nine hours for which the source of energy is practically wanted.
The subject of this new secondary battery is one of great scientific importance. As the writer of a leader in the "Times" points out, it is by no means unlikely that a similar piece of apparatus may be made of some metal, and its appropriate salt, which shall be cheaper and lighter than one of M. Faure's form of similar powers; at all events, the invention and its results are pretty sure to turn the attention of inventors and investigators toward batteries both secondary and primary—a branch of inquiry which has for so many years been quite thrown aside in favor of endeavors to improve the dynamo-machine. Now, a primary battery is theoretically the most economical artificial source