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.

Construction.--The barrel and supports were made out of a single piece of
thin brass tubing, 2-7/16 inch internal diameter and 5-5/8 inch long. The
heating end was filed up true, the other cut and filed to the shape
indicated in Fig. 98 by dotted lines. The marking out was accomplished with
the help of a strip of paper exactly as wide as the length of the tube, and
as long as the tube's circumference. This strip had a line ruled parallel
to one of its longer edges, and 2-1/2 inches from it, and was then folded
twice, parallel to a shorter edge. A design like the shaded part of Fig. 98
was drawn on an end fold, and all the four folds cut through along this
line with a pair of scissors. When opened out, the paper appeared as in
Fig. 98.

[Illustration: FIG. 98.]

We now--to pass into the present tense--wrap this pattern round the
tube and scratch along its edges. The metal is removed from the two hollows
by cutting out roughly with a hack saw and finishing up to the lines with a
file.

The next things to take in hand are the displacer rod D and the guide tube
in which it works. These must make so good a fit that when slightly
lubricated they shall prevent the passage of air between them and yet set
up very little friction. If you cannot find a piece of steel rod and brass
tubing which fit close enough naturally, the only alternative is to rub
down a rod, slightly too big to start with, until it will just move freely
in the tube. This is a somewhat tedious business, but emery cloth will do
it. The rod should be 3-3/8 inches, the tube 2-1/8 inches, long. I used rod