A lamp hole is cut in the centre, and ventilation holes 1 inch apart, as shown in Fig. 103.  If the latter holes are made square or triangular (base uppermost), and the metal is cut with a cold chisel so as to leave the side nearest the edge unsevered, the parts may be turned up to form supports for the barrel.

[Illustration:  Fig. 103.—­Plate for lamp chamber cut out ready for bending.]

The slit lower side of the plate is splayed out into a series of “feet,” by three or more of which, the chamber is secured to the base.  Bend the plate round the barrel and put the two screws and bolts which hold the ends in place, and tighten them until the barrel is gripped firmly.  Screw the engine to its base, fit on the rubber water connections, and fasten down the tank by a screw through the centre of the bottom.  The screw should pass through a brass washer, between which and the tank should be interposed a rubber washer to make a water-tight joint.

The Lamp.—­The lamp shown in Fig. 104 was made out of a truncated brass elbow, a piece of 5/16-inch brass tube, and a round tin box holding about 1/3-pint of methylated spirit.  A tap interposed between the reservoir and burner assists regulation of the flame, and prevents leakage when the lamp is not in use.

Running the Engine.—­The power and displacer cranks must be set exactly at right angles to one another, and the first be secured by soldering or otherwise to the crank shaft.  The fly wheel will revolve in that direction in which the displacer crank is 90 degrees ahead of the other.

[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.  A water supply giving a pressure of 40 lbs. upwards per square inch must be available.

We begin operations by fashioning the case, which consists of three main parts, the centre and two sides, held together by brass screws.  For the centre, select a piece of oak 1 inch thick.  Mark off a square, 7 inches on the side; find the centre of this, and describe a circle 5 inches in diameter.  A bulge is given to the circle towards one corner of the square, at which the waste-pipe will be situated.