ell governor employs a link motion. You must here picture the
balls raising and lowering the _free end_ of the valve rod, which
carries a block moving in a link connected with the eccentric rod. The
link is pivoted at the upper end, and the eccentric rod is attached to
the lower. When the engine is at rest the end of the valve rod and its
block are dropped till in a line with the eccentric rod; but when the
machinery begins 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
the same water over and over again in the boilers. On the sea, where
fresh water is not obtainable in large quantities, this is a matter of
the greatest importance. (2) It adds to the power of a compound engine
by exerting a back pull on the piston of the low-pressure cylinder while
the steam is being exhausted.

[Illustration: FIG.