een
"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.

In order to carry out the test without risk, one needs to be able to watch
the steam-gauge from a considerable distance, and to have the fire under
control. My own method is to set the boiler out in the open, screw down the
safety-valve so that it cannot lift, and raise steam with the help of a
blow-lamp, to which a string is attached wherewith to pull it backwards
along a board. If the boiler is to be worked at 50 lbs., I watch the steam
gauge through a telescope until 100 lbs. is recorded, then draw the lamp
away. After passing the test, the boiler, when pressure has fallen, say, 20
lbs., may safely be inspected at close quarters for leaks.

This test is the only quite satisfactory one, as it includes the influence
of high temperature, which has effects on the metal not shown by "cold"
tests, such as the hydraulic.

Do not increase your working pressure without first re-testing the boiler
to double the new pressure to be used.

Fuels.--For very small stationary boilers the methylated spirit lamp is
best suited, as it is smell-less, and safe if the reservoir be kept well
apart from the burner and the supply is controllable by a tap or valve.
(See Fig. 104.)

[Illustration: FIG. 95.-Paraffin burner for vertical boiler.]

For medium-sized model boilers, and for small launch boiler