184 GRAHAM bell's TETRAHEDRAL KITES. 



i'ace.s represent the combined effect of aeroplanes of the area of the 

 projections.'^ 



The box kite of triangular section is, however, not stiff as regards 

 longitudinal shear, and the next "milestone" marks the reduction of 

 the triangular or prismatic form to the tetrahedron, an essentially stiff* 

 framework for all directions. A tetrahedron of rods with two adja- 

 cent faces covered with fabric forms a tetrahedral kite cell which, on 

 the principle of projection before referred to, is equivalent to three 

 aeroplanes represented by the projections of the covered sides upon 

 planes at right angles. 



The further development of pure tetrahedral construction is obvious. 

 Four cells can be combined to form a tetrahedron of double linear 

 dimensions without additional framework; the weight and wing area 

 are both simpl}' proportional to the number of cells, and not to the 

 linear dimensions. For each set of four cells thus combined there is 

 an octahedral free space in the interior which corresponds to the free 

 space ])etwcen the two cells of the Hargrave kite. The tetrahedral 

 kites that have the largest central spaces preserve their equilibrium 

 best in the air. 



Combining 4 multiple cells to till the outline of a tetrahedron of 

 double size, again, we get a 16-cell kite, and repeating the process 

 again a 64-cell kite, occupying a tetrahedron eight times the dimen- 

 sions of a single cell. The building up of multicellular kites from 

 the units is represented in the figures here reproduced from illustra- 

 trations in Doctor Bell's article. Fig. 1, PI. I, represents the unit 

 cell; fig. 2 a combination of 4 cells; fig. 3 of 64 cells. 



The kites fl}" with the points of the wings upward; the line of junc- 

 tion of the covered faces of the tetrahedron forms a kind of keel. No 

 details as to the heights attainable are given. The most convenient 

 place for the attachment of the fl.ying end is said to be the extreme 

 point of the bow. If the cord is attached to points successively farther 

 back on the keel, the fl3"ing end makes a greater and greater angle 

 with the horizon, and the kite flies more nearly overhead; but it is not 

 advisable to carry the point of attachment as far back as the middle 

 of the keel. A good place for high flights is a point halfway between 

 the bow and the middle of the keel. 



"Tetrahedral kites combine in a marked degree the qualities of 

 strength, lightness, and stead}^ flight; but further experiments are 

 required before deciding that this form is the best for a kite or that 



«This principle to be generally applicable would require the normal component of 

 wind pressure to be uniform and independent of the angle between the plane and 

 the wind. This is not the case with an aeroplane (see Eayleigli, Nature, vol. xxv, 

 p. 108); and for the principle to be applied api^roximately in the case of the kites 

 some convention as regards the angle of exposure of the aeroplanes to the wind 

 would be required. 



