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THE POPULAR SCIENCE MONTHLY 



surface of the piston where its pressure was readily variable, and 



therefore more manageable than that of the atmosphere. It also, 



besides keeping the cylinder hot, 



could do comparatively little harm 



should it leak by the piston, as it 



might be condensed and readily 



disposed of. 



36. This completed the change 

 of the " atmospheric engine " of 

 Newcomen into the steam-engine 

 of James Watt. The engine as im- 

 proved is shown in Fig. 15, which 

 represents the engine as pat- 

 ented in April, 1769. Watt's lirst 

 engine was erected with the pecu- 

 niary aid of Dr. Roebuck, the les- 

 sor of a coal-mine on the estate of 

 the Duke of Hamilton, at Kinneil, 

 near Borrowstounness. This en- 

 gine, which was put up at the 

 mine, had a steam-cylinder eigh- 

 teen inches in diameter. 



In the figure, the steam passes 

 from the boiler through the pipe d 



Fig. 15. Watt's Pumping-Engine, a. d. 1769. 



and the valve c to the cylinder casing, or steam-jacket, Y Y, and 

 above the piston b, which it follows in its descent in the cylinder a, 

 the valve /being at this time open to allow the exhaust to pass into 

 the condenser h. 



The piston now being at the lower end of the cylinder, and the 

 pump-rods at the opposite end of the beam y thus raised, and the 

 pumps filled with water, the valves c and f close, while e opens, 

 allowing the steam which remains above the piston to flow be- 

 neath it, until, the pressure becoming equal above and below by 

 the weight of the pump, it is rapidly drawn to the top of the cylin- 

 der, while the steam is displaced above, passing to the underside of 

 the piston. 



Now the valve e is closed, and c and/ are again opened, and the 

 down-stroke is repeated as before. The water and air entering the 

 condenser are removed, at each stroke, by the air-pump i, which 

 communicates with the condenser by the passage s. The pump q 

 supplies condensing-water, and the pump A takes away a part of the 

 water of condensation, which is thrown by the air-pump into the 

 " hot well " k, and with it supplies the boiler. The valves are moved 

 by valve-gear very similar to Beighton's, by the pins m m in the 

 " plug-frame " or " tappet-rod " n n. 



The engine is mounted upon a substantial foundation, B B. F'y$> 



