to work the block is gradually drawn up by the governor, diminishing the movement of the valve, and so shortening the period of steam admission to the cylinder.
Governors are of special importance where the load of an engine is constantly varying, as in the case of a sawmill. A good governor will limit variation of speed within two per cent.—that is, if the engine is set to run at 100 revolutions a minute, it will not allow it to exceed 101 or fall below 99. In very high-speed engines the governing will prevent variation of less than one per cent., even when the load is at one instant full on, and the next taken completely off.
MARINE GOVERNORS.
These must be more quick-acting than those used on engines provided with fly-wheels, which prevent very sudden variations of speed. The screw is light in proportion to the engine power, and when it is suddenly raised from the water by the pitching of the vessel, the engine would race till the screw took the water again, unless some regulating mechanism were provided. Many types of marine governors have been tried. The most successful seems to be one in which water is being constantly forced by a pump driven off the engine shaft into a cylinder controlling a throttle-valve in the main steam-pipe. The water escapes through a leak, which is adjustable. As long as the speed of the engine is normal, the water escapes from the cylinder as fast as it is pumped in, and no movement of the piston results; but when the screw begins to race, the pump overcomes the leak, and the piston is driven out, causing a throttling of the steam supply.
CONDENSERS.
The condenser serves two purposes:—(1) It makes it possible to use