Tetrahedral Principle in Kite Structure 225 



gular, we arrive at the form of cell 

 shown at B, in which the framework 

 forms the outline of a tetrahedron. In 

 this case the aeroplanes are triangular, 

 and the whole arrangement is strongly 

 suggestive of a pair of birds' wings 



FIG. II — ONE-CELLED TETRAHEDRAL FRAME 



raised at an angle and connected to- 

 gether tip to tip by a cross-bar (see B, 

 Fig. 9 ; also drawings of winged tetra- 

 hedral cells in Fig. 10). 



A tetrahedron is a form of solid 

 bounded by four triangular surfaces. 



In the regular tetrahedron the bound- 

 aries consist of four equilateral triangles 

 and six equal edges. In the skeleton 

 form the edges alone are represented, 

 and the skeleton of a regular tetrahedron 

 is produced by joining together six equal 



FIG. 12 — FOUR-CELLED TETRAHEDRAL FRAME 



rods end to end so as to form four equi- 

 lateral triangles. 



Most of us no doubt are familiar with 

 the common puzzle — how to make four 

 triangles with six matches. Give six 

 matches to a friend and ask him to ar- 

 range them so as to form four complete 

 equilateral triangles. The difficulty 

 lies in the unconcious assumption of the 

 experimenter that the four triangles 

 should all be in the same plane. The 

 moment he realizes that they need not 

 be in the same plane the solution of the 

 problem becomes easy. Place three 

 matches on the table so as to form a 

 triangle, and stand the other three up 



over this like the three legs of a tripod 

 stand. The matches then form the 

 skeleton of a regular tetrahedron. 

 ( See figure 1 1 . ) 



A framework formed upon this model 

 of six equal rods fastened together at 

 the ends constitutes a tetrahedral cell 

 possessing the qualities of strength and 

 lightness in an extraordinary degree. 



It is not simply braced in two direc- 

 tions in space like a triangle, but in 

 three directions like a solid. If I may 

 coin a word, it possesses " three-dimen- 

 sional" strength; not "two-dimen- 

 sional ' ' strength like a triangle, or 

 "one-dimensional" strength like a rod. 

 It is the skeleton of a solid, not of a 

 surface or a line. 



fig. 13- 



-SIXTEEN-CELLED TETRAHEDRAL 

 FRAME 



It is astonishing how solid such"' a 

 framework appears even when composed 

 of very light and fragile material ; and 

 compound structures formed by fasten- 

 ing these tetrahedral frames together at 

 the corners so as to form the skeleton 

 of a regular tetrahedron on a larger scale 

 possess equal solidity. 



Figure 1 2 shows a structure composed 

 of four frames like figure 1 1 , and figure 

 13 a structure of four frames like figure 

 12. 



When a tetrahedral frame is provided 

 with aero-surfaces of silk or other mate- 

 rial suitably arranged, it becomes a tetra- 



