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worked by chains connected with the beam, and athird, placed above, which received the hot waterraised by the others. In the engines now con-structed, only one air-pump is employed, and thisfully answers the intended purpose.
Another improvement introduced by Mr. Watt,consisted in surrounding the upper part of thecylinder with a cap, through a hole in the centreof which the piston rod worked air-tight. Theforce of steam was then substituted for that ofthe atmosphere, and at a pressure of more thanfifteen pounds on the square inch; so that whena vacuum was formed beneath the piston, steamof considerable impellent force was entering theupper end of the cylinder, by means of a pipeconnected with the boiler.
By thus substituting the force of highly elasticvapour, for the ordinary pressure of the atmos-phere, the upper and under side of the pistonwere preserved at the same temperature, and thesupply of steam being regulated by the width ofthe aperture, any given amount of force mightreadily be produced. In the atmospheric enginethis could not be effected, as the whole pressureof the atmosphere was made to act on the piston,the instant the vacuum was formed by the con-densation of the vapour beneath ; so that in theevent of a pump-rod breaking, by which the ele-vation of the water might be impeded, and thelabour of the engine taken off', the rapid descentof the piston would evidently cause the destruc-tion of the entire apparatus.