All About Engines

5o All About Engines the equilibrium valve opened to render the pressure above and below equal; next the weight on the pump rod pulled up the piston; and finally the condenser valve opened and permitted the steam to press the piston down again. The condensers used were of the “ surface ” type —that is to say, the steam to be condensed was drawn by the pump through copper tubes immersed in cold water. When larger engines came to be made these were replaced by “jet” condensers, in which the steam was drawn into a vessel where it came into direct contact with a stream of cold water. The advantage of a surface condenser lies in the fact that the cooling water and the condensed steam do not mix. If pure water is scarce, as on board ship, so that the condensed steam is required to be re- turned to the boilers, water which would be impossible for that purpose can be used for the condensers. Watt soon began to apply the principle of ex- pansion in his engines. He tried it on one made for the Soho works in 1776, and definitely adopted it on an engine made to pump water at Shadwell in 1778. It is possible that he was led to adopt it less on theoretical grounds than in order to reduce the force acting on the piston towards the end of its stroke. At the same time he was aware that by cut- ting off at half stroke he obtained nearly twice the power from a pound of steam—a fact with which he had been familiar since 1769. As the pressure used in his engines was only about 10 lb. on the square inch above that of the atmosphere, he secured