d be silver-soldered, and other
joints as well if the working pressure is to exceed 50 lbs. to the square
inch. Silver-soldering requires the use of a powerful blow--lamp or
gas-jet; ordinary soft soldering bits and temperatures are ineffective.
Brazing is better still, but should be done by an expert, who may be relied
on not to burn the metal. It is somewhat risky to braze brass, which melts
at a temperature not far above that required to fuse the spelter (brass
solder). Getting the prepared parts of a boiler silver-soldered or brazed
together is inexpensive, and is worth the money asked.

[Illustration: FIG. 78.]

Some Points in Design.

The efficiency of a boiler is governed chiefly (1) by the amount of heating
surface exposed to the flames; (2) by the distribution of the heating
surface; (3) by the amount of fuel which can be burnt in the furnace in a
given time; (4) by avoiding wastage of heat.

The simplest form of boiler, depicted in Fig. 78, is extremely inefficient
because of its small heating surface. A great deal of the heat escapes
round the sides and the ends of the boiler. Moreover, a good deal of the
heat which passes into the water is radiated out again, as the boiler is
exposed directly to the air.

Fig. 79 shows a great improvement in design. The boiler is entirely
enclosed, except at one end, so that the hot gases get right round the
barrel, and the effective heating surface has been more than doubled by
fitting a number of water-tubes, aaa, bbbb, which lie right in the flames,
and absorb much heat which would otherwise escape. The tubes slope upwards
from the chimney end, where the heat is less, to the fire-door end, where
the heat is fiercer, and a good circulation is thus assured. The Babcock
and Wilcox boiler is the highest development of this system, which has
proved very successful, and may be recommended for model boilers of all
sizes. The heating surface may be increased indefinitely by multiplying the
number of tubes. If a solid fuel-coal, coke, charcoal, etc.-fire is used,
the walls of the casing should be lined with asbestos or fire-clay to
prevent the metal being burnt away.

[Illustration: FIG. 79--Side and end elevations of a small water-tube
boiler.]

The horizontal boiler has an advantage over the vertical in that, for an
equal diameter of barrel, it affords a larger water surface, and is,
therefore, less subject to "priming," which means the passing off of minute
globules o