Page:Popular Science Monthly Volume 64.djvu/139

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 wing-surface. The ratio of weight to surface, therefore, is the same for the larger kites as for the smaller. In the middle of the kites there is an empty space, octahedral in form, which seems to have the same function as the space between the two cells of the Hargrave box kite. The tetrahedral kites that have the largest central spaces preserve their equilibrium best in the air.

twelve sides, and cells of various other shapes were devised, tried and thrown away.

Finally the triangular cell was hit upon. It immediately proved an immense advance over the rectangular Hargrave, being stronger