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 stroke—­the air is heated by contact with the hot plate C, and expands, forcing up the piston of the power cylinder, seen on the left of the engine. (The power crank and the displacer crank are, it should be mentioned, set at right angles to one another.) During the second half of the power stroke the displacer is moved downwards, causing some of the air to pass round it into contact with the cold plate D. It immediately contracts, and reduces the pressure on the power piston by the time that the piston has finished its stroke.  When the power piston has reached the middle of its downward stroke, the displacer is at its lowest position, but is halfway up again when the power piston is quite down.  The air is once again displaced downwards, and the cycle begins anew.  The motive power is, therefore, provided by the alternate heating and cooling of the same air.