3/16 inch in diameter; but a smaller rod would do equally well.

[Illustration: FIG. 99.]

The two plates, A and B, are next prepared by filing or turning down thin
brass[1] discs to a tight fit. (Note.--For turning down, the disc should
be soldered centrally to a piece of accurately square brass rod, which can
be gripped in a chuck. I used a specially-made holder like that shown in
Fig. 99 for this purpose.)

[Footnote 1: Thin iron plate has the disadvantage of soon corroding.]

When a good fit has been obtained, solder the two discs together so that
they coincide exactly, and bore a central hole to fit the guide tube
tightly. Before separating the plates make matching marks, so that the same
parts may lie in the same direction when they are put in position. This
will ensure the guide tube being parallel to the barrel.

The power cylinder is a piece of brass tubing 2 inches long and of 7/8-inch
internal diameter. The piston is of 7/8-inch tubing, fitting the cylinder
easily, and thick enough to allow a shallow packing recess to be turned in
the outside. Brass washers turned or filed to size form the ends of
cylinder and piston. The connecting rod CR2 is a piece of strip brass,
3-3/16 inches long, between centres of holes. This had better be cut off a
bit long in the first instance, and be fitted to the little stirrup which
attaches its lower end to the piston. The drilling of the crank pinhole
should be deferred till the cylinder and crank are in position.

[Illustration: FIG. 100.--Exterior view of hot air engine.]

Putting in the Water-chamber Discs.--Clean the inside of the barrel
thoroughly with sandpaper; also discs A and B round the edges and the
central holes. Disc A is forced in from the crank end a little further down
than it is to be finally, and then driven up from below until at all points
its lower side is exactly three inches from the bottom edge of the barrel.
Disc B is then forced up 1-1/2 inches from the bottom end. The guide tube--
which should have been cleaned--having been driven into place, solder is
run all round the joints. If the barrel is heated over a spirit lamp, this
operation is performed very quickly. ("Tinol" soldering paste is
recommended.) Before soldering in B, drill a small hole in the barrel
between A and B to allow the air to escape.

Attaching the Cylinder.--Scratch a bold line through the centre of one of
the crank holes to the bottom of the barrel, to act as guide. Drill a