ttom. (Note.-If type c is for use on a three-support
stove, increase the number of tubes to 9, equally spaced, 40 degrees apart,
so that the kettle may be adjusted easily.)

The copper tubing should be annealed or softened by heating to a dull red
and plunging in cold water. Cut a wooden template of the exact outline of
the inside line of the shape that the tube is to assume, and secure this
firmly to a board. Fill the tube with melted resin, to prevent, as much as
possible, "buckling" or flattening on the curves. The tube must be kept up
to the template by a stop of hard wood, at the end at which bending
commences. Don't cut the tube into lengths before bending, as short pieces
are more difficult to handle. When a piece sufficient for a tube has been
bent, cut it oft, and remove the resin by heating.

The fitting of the tubes is an easy matter, as the holes are circular. Pair
off a tube with its holes and number it. A fluted reamer will be found
invaluable for enlarging them to the correct size. Tin all tubes at points
where they are to be attached to the kettle.

In Fig. 96 (c) and (d) care should be taken to make all the tubes project
the same distance, so that the kettle may be level when resting on them.

XX. A HOT-AIR ENGINE.

The pretty little toy about to be described is interesting as a practical
application to power-producing purposes of the force exerted by expanding
air. It is easy to make, and, for mere demonstration purposes, has an
advantage over a steam-engine of the same size in that it can be set
working in less than a minute, and will continue to act as long as a small
spirit flame is kept burning beneath it; it cannot explode; and its
construction is a simpler matter than the building of a steam-engine.

[Illustration: FIG. 97.--Vertical section of hot-air engine.]

Principles of the Hot-air Engine.--Fig. 97 gives a sectional view of the
engine. The place of what would be the boiler in a steam-engine of similar
shape is taken by an air chamber immediately above the lamp, and above that
is a chamber through which cold water circulates. In what we will call the
heating chamber a large piston, known as the displacer, is moved up and
down by a rod D and a connecting rod CR1. This piston does not touch the
sides of the chamber, so that the bulk of the air is pushed past it from
one end of the chamber to the other as the piston moves. When the displacer
is in the position shown--at the top of its