Action of Engine.—­During the forward motion of the piston the valve is pushed back by the eccentric until the steam port is fully opened, and is then drawn forward, covering the port.  At the end of the power stroke the port has begun to open to the air, to allow the steam to escape throughout the exhaust stroke, 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), 1-1/2 inches internal diameter; 2-13/16 inches long over all.