Sir William Thomson early in this month wrote to the "Times," pointing out the great advance which this invention had made in the practical and economical storage of energy. His letter was answered by Professor Osborne Reynolds, who, with the intention of preventing the public from being astonished at the storing of so much energy as one million foot-pounds in apparatus occupying a cubic foot of space and weighing about seventy-two pounds, proceeded—somewhat irrelevantly as we think—to discuss the energy contained in a pound of coal, and also to complicate the now inevitable controversy by referring to a totally different problem, the transmission of energy by electrical means. The controversy thus started has gone on. Sir William Thomson, Professor Osborne Reynolds, Professor Ayrton, and Professor Tyndall, taking part in it.
The question, as far as the public are concerned, is a purely commercial one. As yet, of course, the data of the cost of the battery and its durability are not yet ascertained; but, in any future discussion on the subject, the question of convenience, as well as that of absolute expense, wall have to be taken into consideration. At present we know that, at some expense, probably not too great, we can utilize a source of energy of feeble power for many purposes by allowing it to act for a long time, collecting its energy, and using it quickly, and that the loss in the process wall be but small; and that, further, if it be desired to use the electric light temporarily, it can be produced conveniently, if not economically, by the use of M. Faure's invention. Sir William Thomson in his first letter points out many practical uses for the new invention; we may supplement them by pointing out how the new secondary battery may be applied conveniently for many purposes. Three ordinary Daniell's cells will charge an element of the new battery easily, so that, if there be plenty of time for preparation, we can, by the aid of Faure's batteries, use this cleanly apparatus, which gives off no noxious fumes and needs but little attention, for all the purposes for which, up to the present time, we were obliged to employ the costly and troublesome Grove's or Bunsen's batteries, which contain violent caustic poisons, and give off irritating and unwholesome fumes.
The whole discussion about the mechanical value of coal seems to us mistaken; neither Sir William Thomson nor any other physicist proposes to use the new battery universally, and, at present, our cheapest way of charging it is by the use of a dynamo-electric machine, driven by a steam-or gas-engine—i. e., by making use of the mechanical power of coal and the oxygen of the air; setting aside, of course, the exceptional cases where water-power is to be obtained. Sir William Thomson himself gave, we think, the coup de grâce to any attempt at comparing the relative values of transmitting electric currents through conductors from the source of energy to a distant station where energy is wanted, and conveying energy by exciting Faure's