is shape encloses a larger area than an oval
of equal circumference, and therefore makes room for a larger volume of
air. In doing so the tube straightens itself, and assumes the position
indicated by the dotted lines. Hang an empty "inner tube" of a pneumatic
tyre over a nail and inflate it, and you will get a good illustration of
the principle.

[Illustration: FIG. 13.--Showing the principle of the steam-gauge.]

[Illustration: FIG. 14.--Bourdon steam-gauge. Part of dial removed to
show mechanism.]

In Fig. 14 we have a Bourdon gauge, with part of the dial face broken
away to show the internal mechanism. T is a flattened metal tube
soldered at one end into a hollow casting, into which screws a tap
connected with the boiler. The other end (closed) is attached to a link,
L, which works an arm of a quadrant rack, R, engaging with a small
pinion, P, actuating the pointer. As the steam pressure rises, the tube
T moves its free end outwards towards the position shown by the dotted
lines, and traverses the arm of the rack, so shifting the pointer round
the scale. As the pressure falls, the tube gradually returns to its zero
position.

The Schaeffer-Budenberg gauge depends for its action on the elasticity of
a thin corrugated metal plate, on one side of which steam presses. As
the plate bulges upwards it pushes up a small rod resting on it, which
operates a quadrant and rack similar to that of the Bourdon gauge. The
principle is employed in another form for the aneroid barometer (p.
329).

THE WATER SUPPLY TO A BOILER.

The water inside a boiler is kept at a proper level by (1) pumps or (2)
injectors. The former are most commonly used on stationary and marine
boilers. As their mechanism is much the same as that of ordinary force
pumps, which will be described in a later chapter, we may pass at once
to the _injector_, now almost universally used on locomotive, and
sometimes on stationary boilers. At first sight the injector is a
mechanical paradox, since it employs the steam from a boiler to blow
water into the boiler. In Fig. 15 we have an illustration of the
principle of an injector. Steam is led from the boiler through pipe A,
which terminates in a nozzle surrounded by a cone, E, connected by the
pipe B with the water tank. When steam is turned on it rushes with
immense velocity from the nozzle, and creates a partial vacuum in cone
E, which soon fills with water. On meeting the water the steam
condenses, but not b