It does not at all follow from this, however, that the energies of liquefied air may not be utilized with enormous advantage. It is not always the cheapest form of power-transformer that is the best for all purposes, as the use of the electrical storage battery shows. And so it is quite within the possibilities that a multitude of uses may be found for the employment of liquid air as a motive power, in which its condensed form, its transportability or other properties will give it precedence over steam or electricity. It has been suggested, for example, that liquefied gas would seem to afford the motive power par excellence for the flying-machine, once that elusive vehicle is well in harness, since one of the greatest problems here is to reduce the weight of the motor apparatus. In a less degree the same problem enters into the calculations of ships, particularly ships of war; and with them also it may come to pass that a store of liquid air (or other gas) may come to take the place of a far heavier store of coal. It is even within the possibilities that the explosive powers of the same liquid may take the place of the great magazines of powder now carried on war-ships; for, under certain conditions, the liquefied gas will expand with explosive suddenness and violence, an "explosion" being in any case only a very sudden expansion of a confined gas. The use of the compressed air in the dynamite guns, as demonstrated in the Cuban campaign, is a step in this direction. And, indeed, the use of compressed air in many commercial fields already competing with steam and electricity is a step towards the use of air still further compressed, and cooled, meantime, to a condition of liquidity. The enormous advantages of the air actually liquefied, and so for the moment quiescent, over the air merely compressed, and hence requiring a powerful retort to hold it, are patent at a glance. But, on the other hand, the difficulty of keeping it liquid is a disadvantage that is equally patent. How the balance will be struck between these contending advantages and disadvantages it remains for the practical engineering inventors of the future--the near future, probably--to demonstrate.