e that the indicator should
act immediately in the production of the final form of diagram.

DYNAMOMETER, GAUGES, AND CATARACT.

242. _Q._--What other gauges or instruments are there for telling the
state, or regulating the power of an engine?

_A._--There is the counter for telling the number of strokes the engine
makes, and the dynamometer for ascertaining the tractive power of steam
vessels or locomotives; then there are the gauge cocks, and glass tubes, or
floats, for telling the height of water in the boiler; and in pumping
engines there is the cataract for regulating the speed of the engine.

243. _Q._--Will you describe the mechanism of the counter?

_A._--The counter consists of a train of wheel work, so contrived that by
every stroke of the engine an index hand is moved forward a certain space,
whereby the number of strokes made by the engine in any given time is
accurately recorded. In most cases the motion is communicated by means of a
detent,--attached to some reciprocating part of the engine,--to a ratchet
wheel which gives motion to the other wheels in its slow revolution; but it
is preferable to derive the motion from some revolving part of the engine
by means of an endless screw, as where the ratchet is used the detent will
sometimes fail to carry it round the proper distance. In the counter
contrived by Mr. Adie, an endless screw works into the rim of two small
wheels situated on the same axis, but one wheel having a tooth more than
the other, whereby a differential motion is obtained; and the difference in
the velocity of the two wheels, or their motion upon one another, expresses
the number of strokes performed. The endless screw is attached to some
revolving part of the engine, whereby a rotatory motion is imparted to it;
and the wheels into which the screws work hang down from it like a
pendulum, and are kept stationary by the action of gravity.

244. _Q._--What is the nature of the dynamometer?

_A._--The dynamometer employed for ascertaining the traction upon railways
consists of two flat springs joined together at the ends by links, and the
amount of separation of the springs at the centre indicates, by means of a
suitable hand and dial, the force of traction. A cylinder of oil, with a
small hole through its piston, is sometimes added to this instrument to
prevent sudden fluctuations. In screw vessels the forward thrust of the
screw is measured by a dynamometer constructed on the pri