3), while their plungers are connected to a disk crank which rotates above the point 0, which is the centre of the main crank; 0 S being the crank length or half stroke of the engine, any variation in its length will vary the power of the engine and at the same time the quantity of water used.
The movement of S is obtained by means of a relay engine, in which there are two rams of different diameters; a constant pressure is always acting on the smaller of these when the motor is at work, while the governor (or handpower if desired) admits or exhausts pressurewater from the face of the other, and the movements to and fro thus given to the two rams alter the position of the stud S, and thus change the stroke of the plungers of the main engine.
General Principle.A mass of fluid is used in mechanism to transmit motion and force between two or more movable portions (called pistons or plungers) of the solid envelope or vessel in which the fluid is contained; and, when such transmission is the sole action, or the only appreciable action of the fluid mass, its volume is either absolutely constant, by reason of its temperature and pressure being maintained cisrnstant, or not sensibly varied.
Alloys prepared in this way, and known as phosphor bronze, may contain only about 1% of phosphorus in the ingot, reduced to a mere trace after casting, but their value is nevertheless enhanced for purposes in which a hard strong metal is required, as for pump plungers, valves, the bushes of bearings, &c. Bronze again is improved by the presence of manganese in small quantity, and various grades of manganese bronze, in some of which there is little or no tin but a considerable percentage of zinc, are extensively used in mechanical engineering.
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