attained a degree of perfection which has enabled it to play a role of unparalleled importance in the late War. To it we owe the aeroplane, the tank, the submarine, the motor cycle and the possibility of a road motor service which has shewn itself capable of rivalling even railway transport.
Consider for a moment the requirements of the aeroplane. It is supported by inclined planes which must be driven through the air at such a velocity that the upward pressure on those planes is sufficient to lift the weight of the machine together with the engine, the petrol it requires, the passengers and any extra load such as bombs which it needs for its special work. All the power necessary for driving the planes against the air with sufficient speed for this purpose must be derived from the engines. The essential therefore of the engine of an aeroplane is extreme lightness together with economy of fuel as compared with the power produced. The improvements effected in the petrol engine during the war have brought the weight of the
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