a little disk of brass with the hole
drilled off center. The distances these holes are placed off center will
depend entirely upon the motion of the valve. It will be seen that the
valve is connected to the eccentric by means of the valve-rod _E_. The
valve-rod, in turn, is held to a circular strap which is placed around
the eccentric. A groove should be cut in the surface of the eccentric,
so that this strap will not slip off. If the strap is not put on too
tightly and the eccentric is free to revolve within it, the valve will
be forced up and down as the eccentric revolves.

The crank-shaft _B_ revolves in two bearings, _D D_. The flywheel is
held to the crank-shaft by means of a set-screw _S_.

Most small engines with a bore under one inch will operate nicely on
from 20 to 30 pounds of steam, and this pressure can easily be generated
in the boiler that was described in the chapter on model-boat power
plants.

CHAPTER XI

A MODEL FLOATING DRY-DOCK

AS many of the readers probably know, a dry-dock is used in assisting
disabled vessels. Some dry-docks are permanent, while others are built
so that they can be floated or towed to a disabled vessel that is not
able to get to a land dry-dock. The land dry-dock operates as follows.
It is first filled with water, and the disabled boat is towed in by
tugs. After the tugs leave, the gates are closed, and the water in the
dry-dock is pumped out, leaving the boat high and dry. Large props are
put in place to prevent the boat from tipping.

The dry-dock here described is a model that is towed to a disabled
vessel. It is then sunk until it passes under the boat. The sinking is
brought about by filling the dry-dock with water. After it has sunk to
the proper depth it is passed under the boat to be repaired, the water
is pumped out, and the dry-dock rises, lifting the disabled boat with
it. Repairs can then be made very easily.

The model here described does not possess all the fittings and additions
of the original. However, it is able to rise or sink as required,
carrying the machinery necessary to bring about these functions.

[Illustration: FIG. 120]

[Illustration: FIG. 121]

A general view of the completed model is shown in Fig. 120. The first
part to construct is the framework for the hull. Four pieces of wood
will be required for this, and they should be cut to the shape and size
shown in Fig. 121. To make this it is best to cut the two side parts
first,