Chap. XXI. STEAM CONSIDERED AS A MOVING POWER.

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cold water by the pump N; the governor, put in motion by bevelled wheels on the shaftof the fly-wheel P, regulates the throttle-valve in the steam-pipe S: in the pumpingapparatus, the rod M is the pump-rod for raising water; when the rod descends, the waterwill be forced through G into the upper air-vessel E, from whence it passes to the reservoir,

DOt'BLE-ACTING ENGINE.

at a distance and height proportionate to the power of the engine: the barrel of the pumpis refilled from the pipe F, which communicates with the lower air-vessel II.

The Double-acting Engine of Boulton and Watt (fig. 1962.) differs from that already described,by having a passage from the boiler, both at the top and bottom of the cylinder, and similarcommunications from both to the condenser : the counter-weight at the end of the beamis not here required to raise the piston. The force of the steam impels the piston bothupwards and downwards, and a double power in the same space and time is obtained bythe use of a double quantity of steam: the steam enters the cylinder at S, passing by Finto the upper, and by D into the lower part: through D the steam escapes into thecondenser B, and during the ascending stroke, the uncondensed gases and water escape bythe valve G ; when descending, they pass through the valve of the pump-bucket, and atthe ascending stroke are driven out at the valve Q into the hot well. The steam passesthrough F, down the pipe E, to the condenser, from the upper portion of the cylinder;during the time the piston P is in the action of ascent, D slides open and closes thepassages at D and F, through the means of the rod O. The diameter of the cylindershould be half its height; the velocity of the piston in feet per minute is found bymultiplying the square root of the length of the stroke by 103 for machinery, or by 98 forraising water. The area of the cylinder multiplied by the velocity of the piston in feetper minute, divided by 4800, gives the area of the steam passages. The air-pump shouldhave half the diameter of the cylinder, and half the length of stroke, or one-eighthof the capacity of the cylinder, and the condenser should be equal to it. The area