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 fore that second alternative comes into operation—if ever it is to do so.

Let us consider what other possibilities seem to offer themselves. Let us revert to the ideal we have already laid down, and consider what hopes and obstacles to its attainment there seem to be. The abounding presence of numerous experimental motors to-day is so stimulating to the imagination, there are so many stimulated persons at work upon them, that it is difficult to believe the obvious impossibility of most of them—their convulsiveness, clumsiness, and, in many cases, exasperating trail of stench will not be rapidly fined away. Explosives as a motive power were first attempted by Huyghens and one or two others in the seventeenth century, and, just as with the turbine type of apparatus, it was probably the impetus given to the development of steam by the convenient collocation of coal and water and the need of an engine, that arrested the advance of this parallel enquiry until our own time. Explosive engines, in which gas and petroleum are employed, are now abundant, but for all that we can regard the explosive engine as still in its experimental stages. So far, research in explosives has been directed chiefly to the possibilities of higher and still higher explosives for use in war, the neglect of the mechanical application of this class of substance being largely due to the fact that chemists are not as a rule engineers, nor engineers chemists. But an easily portable substance, the decomposition of which would evolve energy, or—what is, from the practical point of view, much the same thing—an easily portable substance which could be decomposed electrically by wind or water power, and which would then recombine and supply force, either in intermittent thrusts at a piston, or as an electric current, would be infinitely more convenient for all locomotive purposes than the cumbersome bunkers and boilers required by steam. The presumption is altogether in favour of the possibility of such substances. Their advent will be the beginning of the end for steam traction on land and of the steam ship at sea: the end indeed of the Age of Coal and Steam. And even with regard to steam there may be a curious change of method before the end. It is beginning to appear that, after all, the piston and cylinder type of engine is, for locomotive purposes—on water at least, if not on land—by no means the most perfect. Another and fundamentally different type, the turbine type, in which the impulse of the steam spins a wheel instead of shoving a piston, would appear to be very much better than the adapted pumping engine, at any rate for the purposes of steam navigation. Hero of Alexandria describes an elementary form of such an engine, and the early experimenters of the seventeenth century tried and aban- I do not think that it is