er
the balls fall toward it. By proper attachments the valve openings
admitting steam are widened or narrowed accordingly. Thus the higher
speed of the engine, the less steam admitted, the slower the speed the
more steam admitted. Hence any uniform speed desired can be maintained:
should the engine be called upon to perform greater service at one
moment than another, as in the case of steel rolling mills, speed being
checked when the piece of steel enters the rolls, immediately the valves
widen, more steam rushes into the engine, and _vice versa_. Until the
governor came regular motion was impossible--steam was an unruly steed.

Arago describes the steam-gauge thus:

It is a short glass tube with its lower end immersed in a
cistern of mercury, which is placed within an iron box screwed
to the boiler steam-pipe, or to some other part communicating
freely with the steam, which, pressing on the surface of the
mercury in the cistern, raises the mercury in the tube (which is
open to the air at the upper end), and its altitude serves to
show the elastic power of the steam over that of the atmosphere.

The indicator he thus describes:

The barometer being adapted only to ascertain the degree of
exhaustion in the condenser where its variations were small, the
vibrations of the mercury rendered it very difficult, if not
impracticable, to ascertain the state of the exhaustion of the
cylinder at the different periods of the stroke of the engine;
it became therefore necessary to contrive an instrument for that
purpose that should be less subject to vibration, and should
show nearly the degree of exhaustion in the cylinder at all
periods. The following instrument, called the Indicator, is
found to answer the end sufficiently. A cylinder about an inch
diameter, and six inches long, exceedingly truly bored, has a
solid piston accurately fitted to it, so as to slide easy by the
help of some oil; the stem of the piston is guided in the
direction of the axis of the cylinder, so that it may not be
subject to jam, or cause friction in any part of its motion. The
bottom of this cylinder has a cock and small pipe joined to it
which, having a conical end, may be inserted in a hole drilled
in the cylinder of the engine near one of the ends, so that, by
opening the small cock, a communication may be effected between
the inside of the