Page:Popular Science Monthly Volume 64.djvu/136

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them hitched on one line. But Dr. Bell's object was great lifting power in one kite and not in a team of kites. He realized that he was thwarted at the very outset PSM V64 D136 Hargrave box kite.pngDiagram 1. Hargrave Box Kite. by an old law, which was recently formulated by Dr. Simon Newcomb and which has made many believe that the flying machine is impossible without the discovery of a new metal or a new force. This law is that the weight of kites or machines built on exactly the same model increases as the cube, when all the dimensions are increased alike, while the supporting or wing surface increases as the square.

A Hargrave box kite two meters on a side weighs eight times as much as one that is one meter on a side, but it has only four times as much sustaining or wing surface; the weight is tripled, while the wing surface is PSM V64 D136 Triangular cells.pngDiagram 2. Triangular Cells. doubled; hence as the size of a box kite is increased a point soon comes when the weight is so great that the wing or supporting surface will not lift the weight.

Dr. Bell then set to work to see if he could not outwit this law by devising a new form of kite which he could enlarge indefinitely without the weight increasing faster than the wing surface. He saw that if he could get a large kite by combining many small kites instead of by increasing the dimensions of his model the weight would not increase faster than the wing surface. He decided, therefore, to combine many small cells into one large kite instead of using two large cells each as big as a barn door. The Hargrave box cell however did not lend itself to combination. Two box cells fly well, but when a number PSM V64 D136 Regular tetrahedral winged cell.pngDiagram 3. Regular Tetrahedral Winged Cell. of them are tied together they do not act with the same harmony. A box cell is structurally weak in all directions and requires a great deal of bracing to keep it from being twisted in a strong breeze; this bracing adds to the weight and makes head resistance to the wind; the more cells combined together, the more bracing required proportionally. Furthermore, the cells must be grouped in two sets at a distance from each other, and as the sets tend to pull apart, the framework connecting the two sets has to be very strong and heavy. As a result the experiments showed that neither