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STEAM-ENGINE

1816

STEAM-ENGINE

admission o£ steam when the piston had made only a part of its stroke, and allowing the rest of the stroke to be performed by the expansion of the steam already in the cylinder. He also made many inven-

PISTON-CYLINDER AND   SLIDE-VALVE

tions of important details, as the throttle-valve for regulating the admission of steam, the centrifugal governor (see GOVERNOR), the indicator for studying the actions in the cylinder (see INDICATOR), and, in conjunction with Murdoch, the slide-valve for controlling the admission and release of steam. He also patented the use

The action of a simple steam-engine with the slide-valve, one of the simplest forms, can be easily studied from the diagram. This is a horizontal section of a cylinder, showing the piston in the cylinder and the valve below. As shown, the piston has reached the extreme left of the cylinder, and the port B on the right is connected with the exhaust E. This exhaust connects with the atmosphere in nonconden.-ing engines, or with the cold condenser in condensing engines. Live steam is at the same time entering the left end of th cylinder at B from the steam-chest A, thus moving the piston back to the right. The time of the admission and cut-off of the steam and the connection with the exhaust is fixed by the mechanism which moves the valve. The number of valve-mechanisms in use is very great, but they all serve the general purpose seen above, Among the best known valve-mechanisms is that due to George XL Corliss. In the Corliss valve-mechanism there are two separate ports at each end of the cylinder, one for the steam-chest and one for the exhaust. This or a modified form is often used on large engines. To convert the straight-line backward-and-forward -motion of the piston into rotary motion, a connecting rod and crank on a, fly-wheel are used. The efficiency of a steam-engine depen 1s

SECTION OF  STEAM-ENGINE

of two or more successive cylinders on one engine, or the modern compound-engine plan, which is used in all large engines where economy of fuel is important. Watt's inventive career extended over more than a half century, and he is rightly called the father of the modern steam-engine. For the full history of the steam-engine see Thurston's Growth of the Steam-Engine,

largely upon allowing the steam to expand so far as possible, so that its temperature and pressure are as low as possible when it lea/es the cylinder. Steam is consequently not used at full pressure through the whole stroke, but connection with the steam-chest is cut off at a proper fraction of the stroke and the piston is then moved by the expansion of the steam. Steam-