, in the course of which the valve is pushed back until,
just at the end of the stroke, the steam port begins to open again.

Notes.--
(l.) The connecting rod may be made shorter than shown in Figs.
51 and 52; but in that case the piston also must be shortened to allow for
the greater obliquity of the rod at half-stroke.

(2.) If two opposed cylinders are made to operate the one crank, a
double-acting engine is obtained. Both valves may be operated by a single
eccentric, the connecting rod of one being pivoted to a small lug
projecting from the eccentric strap. If three cylinders are set 120 degrees
apart round the crank shaft, a continuous turning effect is given. This
type will be found useful for running small dynamos.

(3.) If it is desired to use the exhaust steam to promote a draught in the
boiler furnace, it should be led away by a small pipe from the rear end of
the valve tube.

XV. A HORIZONTAL SLIDE-VALVE ENGINE.

The reader who has succeeded in putting together the simple engine
described in the preceding chapter may wish to try his hand on something
more ambitious in the same line. The engine illustrated in Figs. 53 to 66
will give sufficient scope for energy and handiness with drill and
soldering iron. The writer made an engine of the same kind, differing only
from that shown in the design of the crosshead guides, without the
assistance of a lathe, except for turning the piston and fly wheel--the
last bought in the rough. Files, drills, taps, a hack saw, and a soldering
iron did all the rest of the work.

Solder plays so important a part in the assembling of the many pieces of
the engine that, if the machine fell into the fire, a rapid disintegration
would follow. But in actual use the engine has proved very satisfactory;
and if not such as the highly-skilled model-maker with a well-equipped
workshop at his command would prefer to expend his time on, it will afford
a useful lesson in the use of the simpler tools. Under 50 lbs. of steam it
develops sufficient power to run a small electric-lighting installation, or
to do other useful work on a moderate scale.

[Illustration: Fig. 53.--Elevation of a large horizontal engine.]

The principal dimensions of the engine are as follows:

Bedplate (sheet zinc), 13-1/2 inches long; 4-1/2 inches wide; 1/8 inch
thick.

Support of bedplate (1/20 inch zinc), 3 inches high from wooden base to
underside of bedplate.

Cylinder (mandrel-drawn brass tubing)