All About Engines

James Watt: The Man and his Work 45 deep—34 feet being the greatest height to which the pressure of the atmosphere will force a column of water into a vacuum. But all water contains a little dissolved air which is driven off with the steam; and this air, together with air which entered through imperfect joints, accumulated in the con- denser. The only way to remove it was by means of an air pump, and every “ condensing ” engine, therefore, must be furnished with this contrivance. The statements that the cylinder must be kept as hot as possible and the condenser as cold as possible, show that Watt recognised that the steam engine was a heat engine, that the power to work the engine came from the heat resident in the steam, and that steam was only a means of conveying heat from the fire to the cylinder. Any substance which expanded as it cooled could be made to serve the purpose ; but water was the most convenient and in many respects the most suitable. This fact is more particularly emphasised by his reference to using steam expansively. If steam is allowed to enter the cylinder during the whole length of the stroke, a whole cylinder full of steam at, roughly, the pressure in the boiler is used every time, and the temperature and pressure at the end of the stroke are the same as at the beginning. But if steam be cut off at, say, one quarter of the stroke, only one-fourth of the quantity of steam is used, and the piston is pushed forward for the remaining three- quarters, not by the pressure in the boiler, but by the expansive force of the steam itself. The tem-