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270 the exhaust being passed away into the chimney, it is turned into the second cylinder and drives the piston in it before being released. The high-pressure cylinder is smaller in diameter than the low-pressure cylinder, which is of such diameter that the power from the low-pressure cylinder is practically equal to that of the smaller high-pressure cylinder. For starting purposes, or when special effort is required, the driver can turn high-pressure steam into both cylinders. The idea of the compound engine is economy of the steam consumption, as more work is got out of the steam before its final release. Linking up, or using the steam expansively in an ordinary or simple engine, is done with the same end in view.

The Serpollet Engine.—This engine is designed to use superheated steam, and is entirely different from the ordinary steam engines which we have just described.

The Serpollet engine is practically an adaptation of the internal combustion engine to fit it for using superheated steam. Fig. 26 is a side view of the engine, half of it being shown in section. It has four cylinders and a two-throw crank, but only two cylinders are shown, as the other two are immediately behind them. It is a single-acting engine, that is to say, each piston is forced towards the crank by the steam, but it is not forced back. Instead of slide valves, mushroom valves (see chapter on Petrol Engines) are used. shows one of these valves in section, the steam inlet valve. The exhaust valve to each cylinder is exactly the same as, but as it is behind it cannot be shown in the drawing. The exhaust steam passes through and out at 1. The valve is kept closed by a spiral spring, which encircles its stem, and  is opened and closed by the to-and-fro motion of  through,  being moved by the cam  on the cam-shaft , which is driven by a toothed wheel on cam-shaft , working into a similar toothed wheel on crank-shaft. The exhaust valve is similarly operated, and the engine is reversed by sliding another pair of cams under the rollers working the valves. A piston