he boiler
is sometimes performed by a pump driven from the main engine, sometimes
by an independent steam-pump, and sometimes by means of an injector. The
feed-water is admitted by a "check-valve," the lift of which is
regulated by a screw and hand-wheel, and which when the feed-pump is not
working is kept on its seating by the boiler pressure.

[Illustration: FIG. 16.--Yarrow Water-tube Boiler.]

Every boiler is in addition supplied with a steam-gauge to indicate the
steam-pressure, with a stop-valve for regulating the admission of steam
to the steam-pipes, and with one or two safety-valves. These last in
stationary boilers usually consist of valves kept in their seats against
the steam-pressure in the boiler by levers carrying weights, but in
marine and locomotive boilers the valves are kept closed by means of
steel springs. One at least of the safety-valves is fitted with easing
gear by which it can be lifted at any time for blowing off the steam.
Blow-out cocks are fitted for emptying the boiler.

Openings must always be made in boilers for access for cleaning and
examination. When these are large enough to allow a man to enter the
boiler they are termed man-holes. They are usually made oval, as this
shape permits the doors by which they are closed to be placed on the
inside so that the pressure upon them tends to keep them shut. The doors
are held in place by one or two bolts, secured to cross-bars or "dogs"
outside the boiler. It is important in making these doors that they
should fit the holes so accurately that the jointing material cannot be
forced out of its proper position. In the few cases where doors are
fitted outside a boiler, so that the steam-pressure tends to open them,
they are always secured by several bolts so that the breakage of one
bolt will not allow the door to be forced off.

_Water-softening._--Seeing that the impurities contained in the
feed-water are not evaporated in the steam they become concentrated in
the boiler water. Most of them become precipitated in the boiler either
in the form of mud or else as scale which forms on the heating surfaces.
Some of the mud and such of the impurities as remain soluble may be
removed by means of the blow-off cocks, but the scale can only be
removed by periodical cleaning. Incrustations on the heating surface not
only lessen the efficiency of the boiler by obstructing the transmission
of heat through the plates and tubes, but if excessive they become