Page:Steam and the steam engine- land, marine and locomotive (IA steamsteamengine00everrich).pdf/192

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STEAM. a long stroke, without being obliged to strengthen the other parts of the engine in proportion to the stroke. The slides are not ordinarily worked in the common manner by an eccentric, but by tappets on the air pump rod, or else on the plug rod. As the beam goes up carrying the pump rod, a tappet or projection on the rod strikes the extremity of a simple lever, lifting it up ; the other extremity opens the steam and exhaust valves, and closes the equilibrium, and in going down it reverses the process. Thus the slides are worked. The water is lifted on the down stroke of the piston by the extremity of the beam by means It is not elevated of a pipe passing down the mine shaft. right up to the top at once, but is driven from the bottom by an ordinary single acting valve, consisting of a plunger of suitable size, the water being forced into a cistern or tank at the first level, and not allowed to return by means of valves ; the next stroke forces it up to another and higher level, and so on. If possible, they make the water drainage of each level run into its own tank. To give an idea of their size, the following are the dimensions of one of the engines erected by Boulton and Watt:

Diameter of cylinder,                28 inches. Stroke,                              8 feet. Number of strokes per minute,       14 Diameter of lifting pump,           17 1/2 inches. Stroke of lifting pump,             8 feet. Water raised,                      126 feet.

The distinctive feature of the Cornish single acting pumping engines are: the large employment of the principle of expansion, by which a very great economy of fuel has been realized ; the use of the cataract to ensure a slow stroke, by regulating the supply of condensing water; the mode in which the valves are worked ; the employment of steam for the down stroke, and the up stroke being performed by the weight of the rods, etc., at the other end of the beam ; and a plunger is employed in the pumps, and not a lift or distinctive features bucket.

The eduction valve allows the steam to escape to the condenser, when the down stroke is to be made ; it is opened a little before the steam or expansion valve, so that it may