[Illustration: FIG. l04.-Spirit lamp for hot-air engine, with regulating
tap.]
The packing of the piston should be sufficiently tight to prevent leakage
of air, but not to cause undue friction. When the packing has settled into
place, an occasional drop of oil in the cylinder and guide tube will assist
to make the piston and slide air-tight.
The engine begins to work a quarter of a minute or so after the lamp is
lit, and increases its speed up to a certain point, say 300 revolutions per
minute. When the water becomes very hot it may be changed. The power might
be applied, through demultiplying gear, to a small pump drawing water from
the bottom of the tank and forcing it through the water chamber and a
bent-over stand pipe into the tank again. This will help to keep the water
cool, and will add to the interest of the exhibit by showing "work being
done."
XXI. A WATER MOTOR.
FIG. 105 is a perspective view of a simple water motor which costs little
to make, and can be constructed by anybody able to use carpenter's tools
and a soldering iron. It will serve to drive a very small dynamo, or do
other work for which power on a small scale is required.
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