and screw, the first soldered to the boiler round a
hole of the same size as its internal diameter, make a convenient "filler;"
but a plain hole plugged with a tapered piece of wood, such as the end of a
penholder, will serve.

Half fill the boiler by immersion in hot water, the large hole being kept
lowermost, and one of the steam vents above water to allow the air to
escape.

A spirit lamp supplies the necessary heat. Or the boiler may be held in a
wire cradle over the fire, near enough to make the wheel hum. Be careful
not to over-drive the boiler. As a wooden plug will probably be driven out
before the pressure can become dangerous, this is a point in favour of
using one. Corrosion of the boiler will be lessened if the boiler is kept
quite full of water when not in use.

A Practical Steam Turbine.

The next step takes us to the construction of a small turbine capable of
doing some useful work. It is shown in cross section and elevation in Fig.
69.

[Illustration: FIG. 69.--Model steam turbine, showing vertical cross
section (left) and external steam pipe (right).]

The rotor in this instance is enclosed in a case made up of two stout brass
discs, D and E, and a 3/4-inch length of brass tubing. The plates should be
1/2-inch larger in diameter than the ring, if the bolts are to go outside.
The stouter the parts, within reason, the better. Thick discs are not so
liable to cockle as thin ones, and a stout ring will make it possible to
get steam-tight joints with brown-paper packing.

The wheel is a disc of brass, say, 1/25 inch thick and 4 inches in
diameter; the spindle is 3/16 inch, of silver steel rod; the bearings,
brass tubing, making a close fit on the rod.

If you cannot get the ring ends turned up true in a lathe--a matter of but
a few minutes' work--rub them down on a piece of emery cloth supported on a
true surface, such as a piece of thick glass.

Now mark out accurately the centres of the discs on both sides, and make
marks to show which face of each disc is to be outside.

On the outside of both scribe circles of the size of the bearing tubes, and
other circles at the proper radius for the bolt hole centres.

On the outside of D scribe two circles of 2-inch and 1-11/16-inch radius,
between which the steam pipe will lie.

On the inside of D scribe a circle of 1-27/32-inch radius for the steam
ports.

On the outside of E mark a 7/8-inch circle for the exhaust pipe.

On the inside of both m