Page:Popular Science Monthly Volume 12.djvu/150

138 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 improved is shown in Fig. 15, which represents the engine as patented in April, 1769. Watt's first engine was erected with the pecuniary aid of Dr. Roebuck, the lessor of a coal-mine on the estate of the Duke of Hamilton, at Kinneil, near Borrowstounness. This engine, which was put up at the mine, had a steam-cylinder eighteen inches in diameter.

In the figure, the steam passes from the boiler through the pipe d 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 f 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 beneath it, until, the pressure becoming equal above and below by the weight of the pump, it is rapidly drawn to the top of the cylinder, while the steam is displaced above, passing to the underside of the piston.

Now the valve e is closed, and c and f 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 is