In the foregoing article the heat motor described is made of metal. This can be improved upon and made much more attractive by constructing the apparatus of glass. The entire operation is then visible, and if the source of heat be hidden, the result is very mystifying to the uninitiated. A set of glass balls are procured, each pair connected by a small glass tube as shown, and one of them filled with a volatile liquid, alcohol or ether, the air being exhausted before the bulb is closed.

A set of these bulbs, three or four, is arranged on a central support which revolves on pins. The support can be made as follows: Procure a cylindrical piece of wood, as shown at . A, and drill holes at angles, dividing the circumference equally. Then with a fine saw cut through the center of these holes at right angles to the axis of the wood. This will provide a set of clamps, which will hold the rods connecting the bulbs in the position as shown on the assembled sketch. The bulbs are then balanced in their support and securely fastened, and the whole mounted on two vertical supports, which are in turn placed on top of a box.

This box is best made of tin or light sheet iron, and contains inside a deflector through the top of which some small holes are bored, to allow the heat from a small spirit lamp to strike the lowest bulb only. Air holes are provided at one side, on the bottom as shown.

The operation is as follows: When the heat strikes the lower bulb, the liquid evaporates quickly, forming sufficient pressure to blow the remaining liquid into the opposite bulb directly on top. This displaces the center of gravity, as the bulbs overhang the center, and causes the set to revolve and bring each bulb in succession over the hot gas from the lamp, producing a rotary motion. Of course, very little power, if any, can be obtained from such a machine, but as a window attraction it is very suc-cessful. The source of heat being hidden, the sudden upward flow of the liquid is mystifying, and very pretty effects can be obtained by using four different colored solutions of alcohol.

Fig. 189 - An ornamental heat motor.

The sizes of the bulbs should be about 1 1/2 inches diameter, one of each pair being filled slightly over one-half its height with liquid. The distance between the centers of bulbs of the above diameter should be not less than 9 inches. The tubes should extend to within 1/2 inch of the outer side of each bulb, and the connecting tube should be not less than 1/8 inch inside.

The bulbs complete can be had made to order as described for about 50 cents at any glass blower's.