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 been “drawn.”  Emptying in this way is much quicker than when there is no pressure, and it assists to keep the boiler free from sediment.

[Illustration:  Fig. 92.—­Steam cock.]

Steam Cocks.-The screw-down type (Fig. 92) is very preferable to the “plug” type, which is apt to leak and stick.

Testing Boilers.—­The tightness of the joints of a boiler is best tested in the first instance by means of compressed air.  Solder on an all-metal cycle valve, “inflate” the boiler to a considerable pressure, and submerge it in a tub of water.  The slightest leak will be betrayed by a string of bubbles coming directly from the point of leakage.  Mark any leaks by plain scratches, solder them up, and test again.

[Illustration:  Fig. 94.—­Benzoline lamp for model central-flue boiler.]

The boiler should then be quite filled with cold water, and heated gradually until the pressure gauge has risen to over the working pressure.  There is no risk of an explosion, as the volume of the water is increased but slightly.

The third test is the most important and most risky of all-namely, that conducted under steam to a pressure well above the working pressure.