Page:Popular Science Monthly Volume 64.djvu/140

136 in construction, lighter in weight and offering less head resistance to the wind.

Diagram 2 shows a drawing of a kite built of two triangular cells. The triangular cell needs bracing in one direction only, on its flat surfaces; in a transverse direction it is self-braced, so that internal bracing, which causes head resistance, is unnecessary.

By tying a number of kites built of triangular cells corner to corner, as shown in Fig. 1, Dr. Bell was able to construct a giant kite, Fig. 2, in which the ratio of weight to wing surface is not much more than that of the smaller kites of which it is composed. Combinations of

triangular kites, however, must be arranged in two sets with a powerful connecting framework as shown in Fig. 2. The larger the two sets, the farther apart must they be, and, therefore, the connecting frame becomes exceedingly stout and heavy. This connecting framework is of course dead weight; it is a very serious handicap and soon limits the size of kites that can be built of triangular cells.

By his invention of the triangular cell Dr. Bell was able to build larger kites than he had been able to build before. The old limit of size was stretched considerably, but a limit remained none the less.

The principal improvements of the triangular cell, greater lightness and strength, are due to the cell being self-braced in a transverse direction, from side to side. Longitudinally, fore and aft, it is, however, very weak, like the box cell. Dr. Bell reasoned that a cell could be made self-bracing in every direction by making it triangular in all directions or tetrahedral in form.

Accordingly a number of regular tetrahedral cells, Diagram 3, were