IT is inevitable that a book which professes to be 'All about Flying' should make some attempt, however brief and sketchy, to trace the outlines of what is sometimes known as the 'Power Unit' of the aeroplane—that is to say, the engine and propeller and all the apparatus that appertains to their proper working.

An aeroplane is as dependent upon its power unit as a motor-car upon its motor, or a railway train upon its locomotive. Of late years aeroplane design and aeroplane engines have developed side by side, each advance in the one making progress in the other possible and inevitable. In the beginning, as we have seen, it was the lack of a suitable engine alone that held back aviation from expectant generations.

The problem was to arrive at an engine sufficiently light in comparison to its horse-power. When in 1809 Cayley first conceived the idea of flying with an engine-driven machine he found that the steam-engine of his day, including fuel for an hour, would weigh 163 lbs. for a single horse-power. Fifty years later a French engineer, Giffard by name, attempted to turn a balloon into an airship by putting an engine into it; and the very lightest thing he could procure for his purpose was a 3 horse-power steam-engine weighing without its fuel a good 4 cwt. In the early nineties, when Maxim was making experiments with almost the first flying machine that ever lifted itself from the ground, he evolved a steam-engine which was considered a very miracle of lightness because it only weighed about 20 lbs. to every horse-power developed. ' It is good, but it is not good enough,' I remember hearing Maxim say. 1 What we want is a machine that shall yield a horse-power for the weight of a barn-door hen.'

It has come (the high-power Gnome motors develop a horse-power for only two pounds weight!)—but it is not a steam-engine. This form of engine is ruled out of the race for lightness because it must always be provided with a boiler. A steam-engine consists mainly of a hollow cylinder closed at each end, inside which a piston—a thick disc fitting closely to the inside walls—is made to travel up and down. The force which compels it to do so is the pressure of expanding steam, which is generated in a boiler and admitted under pressure through a valve at one end of the cylinder. The pressure of the expanding steam drives the piston down to the other end of the cylinder, where it meets another supply of 'live' steam, fresh from the boiler, admitted through another valve, which sends it back again. Up and down the piston travels ceaselessly, and by means of a connecting-rod is made to revolve a shaft; and so its to and fro motion is converted into rotary motion, which can be used to make wheels or paddles or propellers revolve. To insure that the one motion is turned smoothly into the other, so that the shaft revolves perfectly evenly all the time, it is provided with a heavy fly-wheel which, once set moving, has so much momentum of its own that it carries the shaft along, although there are times when it has a momentary tendency to stop.

This is substantially the form of engine which James Watt evolved more than a century ago when (according to legend) he watched the steam raising the lid of his mother's kettle, and realized that in that steam lay a mighty force that might be harnessed to the labours of the world. Ever since his day the steam-engine has been improved and rendered more and more efficient; but it is obvious that an apparatus where water has to be converted into steam in a boiler, heated by a furnace, can never be made a specially light one, and hence it came about that progress in flight was at a standstill until man lit upon a substitute to drive his pistons to and fro. And presently he found it in the expansive force of an explosion of gas and air. When paterfamilias wakes in the night, and his nose tells him that the new servant maid, raw from the country, has blown out the gas in the kitchen instead of turning it off, he knows better than to go down with a lighted candle to remedy the mischief. He is aware that a mixture of gas and air forms a highly inflammable compound, an explosion of which may be sufficient to blow out the windows and wreck the house. Half a century ago it occurred to ingenious man that this explosive force, which had often worked him woe, might also be harnessed to his needs ; and he invented the gas-engine, in which a series of small and properly regulated explosions are used, in the place of steam, to work a piston up and down a cylinder.

But yet more recently engineers, searching for efficient and convenient light engines to drive the motor-cars which were becoming the toys of the wealthy, hit upon the idea of making their explosive compound not of air and household gas, but of air and vapour of that highly volatile substance which we call petrol, and which is distilled from the mineral oil petroleum. In the petrol-driven, internal-combustion engine the volatile petrol is made to turn into vapour and mixed with air in the 'carburettor,' an outer chamber which leads into the cylinder. The mixture is drawn into the cylinder by a down stroke of the piston and there compressed by the piston moving back upon it. After this an electric spark ignites or fires it, and the force of the explosion drives the piston forward again. The compression of the mixture of vapour and air, before it is fired by the electric spark, is necessary to render the explosion powerful and efficient. Should it ignite before it has been fully compressed the result is a 'back-fire,' noisy but useless. On the return stroke after the explosion the piston forces out the burnt gases that remain, known as the 'exhaust,' through the exhaust valve. It is the forcing of them out through this valve that causes the familiar noise of the petrol-engine. In a motor-car the noise is deadened by a 'silencer'; but an aeroplane before the War was generally allowed to fly with 'open exhaust,' making as much sound as it would.