[Illustration:  Fig. 100.—­Exterior view of hot air engine.]

Putting in the Water-chamber Discs.—­Clean the inside of the barrel thoroughly with sandpaper; also discs A and B round the edges and the central holes.  Disc A is forced in from the crank end a little further down than it is to be finally, and then driven up from below until at all points its lower side is exactly three inches from the bottom edge of the barrel.  Disc B is then forced up 1-1/2 inches from the bottom end.  The guide tube—­ which should have been cleaned—­having been driven into place, solder is run all round the joints.  If the barrel is heated over a spirit lamp, this operation is performed very quickly. ("Tinol” soldering paste is recommended.) Before soldering in B, drill a small hole in the barrel between A and B to allow the air to escape.

Attaching the Cylinder.—­Scratch a bold line through the centre of one of the crank holes to the bottom of the barrel, to act as guide.  Drill a 5/32-inch hole in the barrel on this line just below plate B, and a similar hole in the bottom of the cylinder. (The cylinder end should be put in position temporarily while this is done to prevent distortion.) Flatten down the cylinder slightly on the line of the hole, so that it may lie snugly against the barrel, and clean the outside of the barrel.  Lay the cylinder against the barrel with the holes opposite one another, and push a short piece of wood through to exclude solder from the holes and keep the holes in line.  Half a dozen turns of fine wire strained tightly round cylinder and barrel will hold the cylinder in place while soldering is done with a bit or lamp.  The end of the cylinder should then be made fast.

The Displacer.—­This is a circular block of wood—­well dried before turning—­5/8 inch thick and 3/32 inch less in diameter than the inside of the barrel.  The rod hole in it should be bored as truly central as possible.  A hole is drilled edgeways through the block and through the rod to take a pin to hold the two together.  To prevent it splitting with the heat, make a couple of grooves in the sides to accommodate a few turns of fine copper wire, the twisted ends of which should be beaten down flush with the outside of the block.  The bottom of the block is protected by a disc of asbestos card held up to the wood by a disc of tin nailed on.

The Crank Shaft and Crank.—­The central crank of the crank shaft—­that for the displacer—­has a “throw” of 1/4 inch, as the full travel of the displacer is 1/2 inch.  If the bending of a rod to the proper shape is beyond the reader’s capacity, he may build up a crank in the manner shown in Fig. 101.  Holes for the shaft are bored near the tops of the supports, and the shaft is put in place.  After this has been done, smoke the shaft in a candle flame and solder two small bits of tubing, or bored pieces of brass, to the outside of the supports to increase the length of the bearing.  The power-crank boss is a 1-1/2-inch brass disc.  This crank has a “throw” of 1/2 inch.