plode between December 16 and 24 in a store has seriously affected the
Christmas trade. Shoppers are incredibly nervous. One store advertises,
'No boilers are being used; lifts running electrically.' All stores have
had their boilers inspected."

THE WATER-GAUGE.

No fitting of a boiler is more important than the _water-gauge_, which
shows the level at which the water stands. The engineer must continually
consult his gauge, for if the water gets too low, pipes and other
surfaces exposed to the furnace flames may burn through, with disastrous
results; while, on the other hand, too much water will cause bad
steaming. A section of an ordinary gauge is seen in Fig. 12. It consists
of two parts, each furnished with a gland, G, to make a steam-tight
joint round the glass tube, which is inserted through the hole covered
by the plug P^1. The cocks T^1 T^2 are normally open, allowing the
ingress of steam and water respectively to the tube. Cock T^3 is kept
closed unless for any reason it is necessary to blow steam or water
through the gauge. The holes C C can be cleaned out if the plugs P^2
P^3 are removed.

Most gauges on high-pressure boilers have a thick glass screen in front,
so that in the event of the tube breaking, the steam and water may not
blow directly on to the attendants. A further precaution is to include
two ball-valves near the ends of the gauge-glass. Under ordinary
conditions the balls lie in depressions clear of the ways; but when a
rush of steam or water occurs they are sucked into their seatings and
block all egress.

[Illustration: FIG. 12.--Section of a water-gauge.]

On many boilers two water-gauges are fitted, since any gauge may work
badly at times. The glasses are tested to a pressure of 3,000 lbs. or
more to the square inch before use.

THE STEAM-GAUGE.

It is of the utmost importance that a person in charge of a boiler
should know what pressure the steam has reached. Every boiler is
therefore fitted with one _steam-gauge_; many with two, lest one might
be unreliable. There are two principal types of steam-gauge:--(1) The
Bourdon; (2) the Schaeffer-Budenberg. The principle of the Bourdon is
illustrated by Fig. 13, in which A is a piece of rubber tubing closed at
one end, and at the other drawn over the nozzle of a cycle tyre
inflator. If bent in a curve, as shown, the section of the tube is an
oval. When air is pumped in, the rubber walls endeavour to assume a
circular section, because th