and we shall of course compensate ourselves by a lower death-rate from diseases, many of which will no doubt in a hundred years' time have disappeared from the planet. If we need any motive power other than electricity, or if we need motive power of some other kind to produce electricity, no doubt the explosive recombination of oxygen and hydrogen, controlled by devices developed from existing gas-engines and petrol-engines, will be a starting-point: because coal will, probably before the complete exhaustion of the supply of it, have been found altogether too dirty and unhealthy a thing to use, at all events by way of combustion, though rumours are heard from time to time of new methods by which the stored energy of coal may be utilised directly, to the great economy of the material. [13] In all sorts of ways the early years of the century will be employing themselves in seeking out new sources of man's chief necessity--power: and a hundred years hence we shall have entered upon the full inheritance of them. But the obtaining of power is only one problem of the mechanician. Of almost equal, if not quite equal, importance is that of applying power at the place where it is needed, and the careful reader will not have overlooked the fact that while we have been discussing the use of electricity as a source of power we have already been anticipating, and perhaps anticipating a good deal. For, when we now speak of machinery and locomotive engines being "driven" by electricity, we are really only employing a sort of convenient periphrasis. All our electric machinery, all our electric railways, our "tuppeny" tubes and the horrible electric trams which make life almost intolerable in houses along many of the main roads out of London, are really driven by coal-burning steam engines. In a few places (especially in the Niagara valley) waterfall power is used. But whatever the real source of power, electricity is only a means, more or less convenient, of transmitting it. Even electric launches, and slow-going electric broughams driven by accumulators, only represent slightly more subtle examples of the electrical transmission of power. The ultimate source of power is always either a steam-engine or a waterfall. A few lecture-table toys and the like are the only existing examples of machinery in which the actual source of power is electricity. Even here, it may be objected, the actual source of power is not electricity, but chemical action in the battery. But no contrivance of man is an ultimate source of power. Even a steam-engine is only a device for utilising the stored solar energy of coal. Of course man can no more create power than he can create matter: the stock of each in the universe is a fixed quantity. All that we are able to do is to harness to our use a part of the cosmic store. When I speak of electricity becoming hereafter a "source" of power, I am merely distinguishing between its use as