quickly, it is important that the safety-valve should be large enough to
master the steam. If the valve is too small, the pressure may rise to a
dangerous height, even with the steam coming out as fast as the valve can
pass it.

[Illustration: FIG. 90.-Steam gauge and siphon.]

Steam Gauges.--The steam gauge should register pressures considerably
higher than that to be used, so that there may be no danger of the boiler
being forced unwittingly beyond the limit registered. A siphon piece should
be interposed between boiler and gauge (Fig. 90), to protect the latter
from the direct action of the steam. Water condenses in the siphon, and
does not become very hot.

[Illustration: FIG. 91.-Water gauge.]

Water Gauges should have three taps (Fig. 91), two between glass and
boiler, to cut off the water if the glass should burst, and one for blowing
off through. Very small gauges are a mistake, as the water jumps about in a
small tube. When fitting a gauge, put packings between the bushes and the
glass-holders, substitute a piece of metal rod for the glass tube, and pack
the rod tightly. If the bushes are now sweated into the boiler end while
thus directed, the gauge must be in line for the glass. This method is
advisable in all cases, and is necessary if the boiler end is not perfectly
flat.

Pumps.--Where a pump is used, the supply should enter the boiler below
low-water level through a non-return valve fitted with a tap, so that water
can be prevented from blowing back through the pump. As regards the
construction of pumps, the reader is referred to p. 164 and to Chapter
XXII.

Filling Caps.--The filling cap should be large enough to take the nozzle
of a good-sized funnel with some room to spare. Beat the nozzle out of
shape, to give room for the escape of the air displaced by the water.

The best form of filling cap has a self-seating ground plug, which, if
properly made, is steam-tight without any packing. If needed, asbestos
packing can easily be inserted between plug and cap.

Mud-holes.--All but the smallest boilers should have a mud-hole and plug
in the bottom at a point not directly exposed to the furnace. In Fig. 82 it
is situated at the bottom of the barrel. In Figs. 86 and 87 there should be
a mud-hole in one end of each of the three drums, A, B, and C. The plug may
be bored at the centre for a blow-off cock, through which the boiler
should be emptied after use, while steam is up, and after the fire has b