hat when the piston is at one end of the
cylinder the opposite end is exhausting. By carefully studying Fig. 118
the action of the _D_ valve will be understood. The connecting-rod _E_
is connected to the crankshaft and in this way the engine is caused to
revolve.

[Illustration: FIG. 118]

A cylinder similar to that shown in Fig. 118 is called a double-acting
cylinder. This is because the steam acts on both sides of the piston.
Single-acting cylinders are cylinders in which the steam expands on only
one side of the piston. In the single-acting engines the _D_ valve is
modified.

The "stroke" of a steam-engine depends upon the length of the cylinder;
really, the stroke is the distance travelled by the piston. In model
engines it ranges from 3/8 of an inch to 1-1/2 inches. The bore of a
cylinder is its internal diameter. The bore is usually a trifle smaller
than the stroke. Thus it is common to have a stroke of 7/8 inch and a
cylinder-bore of 3/4 inch.

At this juncture the author would caution the more inexperienced young
mechanics not to build double-acting engines. The valve mechanism is
somewhat intricate and very difficult to regulate. The construction is
also much more complicated, and this also holds true of the designing.
On the other hand, single-acting engines, while not so powerful for a
given size, will do very nicely in driving model boats, and will deliver
sufficient power for all ordinary purposes.

[Illustration: FIG. 119]

Your attention is directed to Fig. 119. This shows a design for a model
single-cylinder, single-acting steam-engine. The reader should carefully
study each drawing before continuing to digest the following matter. The
cylinder _L_ can be made from a piece of tubing. This can be either
brass or copper. Aluminum should not be used, owing to the fact that it
is difficult to solder and difficult to work with. The piston is made so
that it will fit nicely into the cylinder and move up and down without
binding. It will be seen that a groove, _M_, is cut around the piston
near the top. String soaked in oil is placed in this groove. This is
called packing, and the presence of this packing prevents steam leakage
between the piston and the cylinder walls and thereby materially
increases the efficiency of the engine.

In this case the connecting-rod _R_ is made in a circular piece. It is
attached to the piston by a pin, _F_. The connecting-rod must be free to
revolve upon this pin. The en