A wheel one-sixteenth of an inch thick will add little weight to your model and will run much more smoothly. They may be bought very cheaply from clockmakers. The ingenious miniature ball bearings constructed for model aeroplanes are excellent for mounting gears, but they are not essential by any means, and the ordinary arrangement of washers used for motor axles will be delicate enough for gear adjustments. Complete sets of gear wheels mounted on metal frames, ready to be attached to the motor bases, may be purchased from the supply houses, but they are so simple that almost any bright boy can construct them for himself. In experimenting with gears it will be well to begin with but two sets of strands with gears of the same number of teeth. Several models equipped in this way have been flown with success in America. The best flights are those made by a model built by Mr. Frank Schoeber of New York, a winner in several competitions. Mr. Schoeber’s model is driven by two motors of three strands each. He uses wheels of the same size and number of teeth. The support on which the gear wheels are mounted must be perfectly rigid, since any play will result in a serious loss of power through increased friction. Compressed-air motors have not yet been developed to a form practical for use in small model aeroplanes. [Illustration: An aeroplane of simple construction that flies remarkably well, built by R. S. Barnaby] A number of inventors are at work on this problem and high hopes are entertained that a practical form of compressed-air engine will soon be invented. An unlimited supply of compressed air may of course be carried without adding to the weight of the model. It costs nothing again to get a supply. Anyone with a bicycle pump can get up enough energy for his model as easily as he can inflate a bicycle tire. The best compressed-air engines at present weigh from one-third to one-half as much as the model they drive, and will run only from ten to thirty seconds. Spring motors hold out a promise for the model aeroplane builder. A number of fascinating experiments have been made to utilize the power of a coil spring. One young aviator mounted the spring of an ordinary shade roller upon a model aeroplane. The spring was connected up for a direct drive, a single propeller being used. It was found that such a spring could turn fifty times, storing up a relatively large amount of energy. Once released, however, it unwound with startling rapidity. The unwinding process was so rapid that the propeller spun through the air without taking hold or exerting any appreciable driving power.