THE GROWTH OF THE STEAM-ENGINE. 



1 3i 



24. In the atmospheric engine as first designed, the slow process 

 of condensation by the application of the condensing water to the 

 exterior of the cylinder to produce the vacuum caused the strokes of 

 the engine to take place at very long intervals. An improvement 

 was, however, soon effected, which immensely increased this rapidity 

 of condensation. A jet of water was thrown directly into the cylin- 

 der, thus effecting for the Newcomen engine just what Desaguliers 

 had previously done for the Sa- 



very engine. As thus improved, 

 the Newcomen engine is shown 

 in Fig. 11. 



Here d is the boiler. Steam 

 passes from it through the cock </, 

 and up into the cylinder a, equi- 

 librating the pressure of the at- 

 mosphere, and allowing the heavy 

 pump-rod k to fall, and, by its 

 greater weight, acting through the 

 beam i ?', to raise the piston s to 

 the position shown. 



The cock d being shut, f is 

 then opened, and a jet of water 

 from the reservoir g enters the 

 cylinder, producing a vacuum by 

 the condensation of the steam. 

 The pressure of the air above the piston now forces it down, again 

 raising the pump-rods, and thus the engine works on indefinitely. 



The pipe h is used for the purpose of keeping the upper side of 

 the piston covered with water, to prevent air-leaks a device of New- 

 comen. 



Two gauge-cocks, c, c, and a safety-valve, JV, are represented in 

 the figure, but it will be noticed that the latter is quite different from 

 the now usual form. Here, the pressure used was hardly greater than 

 that of the atmosphere, and the weight of the valve itself was ordina- 

 rily sufficient to keep it down. The rod m was intended to carry a 

 counter-weight when needed. 



The condensing water, together with the water of condensation, 

 flows off through the open pipe j>>. Newcomen's first engine made six 

 or eight strokes a minute ; the later and improved engines made ten 

 or twelve. 



25. The steam-engine has now assumed a form that somewhat re- 

 sembles the modern machine. 



An important defect still existed in the necessity of keeping an 

 attendant by the engine to open and shut the cocks. A bright boy, 

 however, Humphrey Potter, to whom was assigned this duty on a 

 Newcomen engine in 1713, contrived what he called a scoggan a 



Fig. 11. Newcomen's Engine, a. d. 1705. 



