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 boilers, benzoline or petrol blow-lamps and paraffin stoves have become very popular, as they do away with stoking, and the amount of heat is easily regulated by governing the fuel supply.  Fig. 94 is a sketch of a blow-lamp suitable for the horizontal boiler shown on pp. 204, and 206, while Fig. 95 shows a convenient form of paraffin stove with silent “Primus” burner, which may be used for a horizontal with considerable furnace space or for vertical boilers.  In the case of all these liquid fuel consumers, the amount of heat developed can be increased by augmenting the number of burners.  Where a gas supply is available its use is to be recommended for small stationary boilers.

Solid Fuels.—­The chief disadvantages attaching to these are smoke and fumes; but as a solid fuel gives better results than liquid in a large furnace, it is preferred under certain conditions, one of them being that steam is not raised in a living room.  Charcoal, coke, anthracite coal, and ordinary coal partly burned are the fuels to use, the fire being started with a liberal supply of embers from an open fire.  Every solid-fuel boiler should have a steam-blower in the chimney for drawing up the fire; and if a really fierce blaze is aimed at, the exhaust from the engine should be utilized for the same purpose.

XIX.  QUICK BOILING KETTLES.

[Transcriber’s note:  Do not use lead solder on articles associated with human or animal consumption.]

The principles of increasing the area of heating surface in model boilers may be applied very practically to the common kettle.  The quick-boiling kettle is useful for camping out, for heating the morning tea water of the very early riser, and for the study “brew,” which sometimes has to be made in a hurry; and, on occasion, it will be so welcome in the kitchen as to constitute a very useful present to the mistress of the house.