[Illustration:  Fig. 101.-Details of built-up crank.]

Connecting Rods.—­Put a piece of card 1/16 inch thick in the bottom of the cylinder and push the piston home.  Turn the power crank down and mark off the centre of the hole for the crank pin in the connecting rod CR2.  Solder a piece of strip brass on each side of the rod at this point; measure again, and drill.

The top of the displacer rod D is now filed flat on two sides and drilled.  Slip a ring 1/16 inch thick over the rod and push the rod upwards through the guide tube till the displacer can go no farther.  Turn the displacer crank up and measure from the centre of the hole in the rod to the centre of the crank.  The top of the connecting rod should be filed to fit the under side of the crank, against which it should be held by a little horseshoe-shaped strap pinned on. (Fig. 102). (Be sure to remove the ring after it has served its purpose.)

The Water Circulation.—­The water chamber is connected by two rubber tubes with an external tank.  In Fig. 97 the cooling water tank is shown, for illustrative purposes, on the fly-wheel side of the engine, but can be placed more conveniently behind the engine, as it were.  Two short nozzles, E1 and E2, of 1/4-inch tube are soldered into the water chamber near the top and bottom for the rubber pipes to be slipped over, and two more on the water tank.  For the tank one may select a discarded 1 lb. carbide tin.  Cut off the top and solder on a ring of brass wire; make all the joints water-tight with solder, and give the tin a couple of coatings of paint inside and outside.

[Illustration:  Fig. 102.]

Closing the Hot-air Chamber.—­When all the parts except the lamp chamber have been prepared, assemble them to make sure that everything is in order.  The lower end of the hot-air chamber has then to be made air-tight.  Soldering is obviously useless here, as the heat of the lamp would soon cause the solder to run, and it is impossible to make a brazed joint without unsoldering the joints in the upper parts of the engine.  I was a bit puzzled over the problem, and solved it by means of the lower part of an old tooth-powder box stamped out of a single piece of tin.  This made a tight fit on the outside of the barrel, and as it was nearly an inch deep, I expected that if it were driven home on the barrel and soldered to it the joint would be too near the water chamber to be affected by the lamp.  This has proved to be the case, even when the water is nearly at boiling point.  If a very close-fitting box is not procurable, the space between box and barrel must be filled in with a strip of tin cut off to the correct length.

The Lamp Chamber.—­Cut out a strip of tin 4 inches wide and 1 inch longer than the circumference of the lower end of the hot-air chamber.  Scratch a line 1/2 inch from one of the sides, a line 3/4 inch from the other, and a line 1/2 inch from each of the ends.