AERIAL LOCOMOTION 419 



like a man, and an engine for motive power. This conclusion 

 is undoubtedly correct in the case of structures that are " exactly 

 alike, excepting in their dimensions," but it is not true as a 

 general proposition. 



A small bird could not sustain a heavy load in the air ; and 

 while it is true that a similar bird of double the dimensions would 

 be able to carry a less proportionate weight because it is itself 

 heavier in proportion to its wing surface than the smaller bird 

 — eight times as heavy in fact, with only four times the wing 

 surface — still it is conceivable that a flock of small birds could 

 sustain a heavy load divided equally among them, and it is 

 obvious that in this case the ratio of weight to wing surface 

 would be the same for the whole flock as for the individual 

 bird. If then we build our large structure by combining together 

 a number of small structures each light enough to fly, instead of 

 simply copying the small structure upon a larger scale, we 

 arrive at a compound or cellular structure in which the ratio of 

 weight to supporting surface is the same as that of the individual 

 units of which it is composed, thus overcoming entirely the really 

 valid objections of Professor Newcomb to the construction of 

 large flying-machines. 



In my paper upon the tetrahedral principle in kite structure, I 

 have shown that a framework having the form of a tetrahedron 

 possesses in a remarkable degree the properties of strength and 

 lightness. This is especially the case when we adopt as our 

 unit structure the form of the regular tetrahedron, in which the 

 skeleton frame is composed of six rods of equal length as this 

 form seems to give the maximum of strength with the 

 minimum of material. When these tetrahedral frames or cells 

 are connected together by their corners they compose a struc- 

 ture of remarkable rigidity, even when made of light and fragile 

 material — the whole structure possessing the same properties of 

 strength and lightness inherent in the individual cells them- 

 selves. 



The unit tetrahedral cell yields the skeleton form of a solid, 

 and it is bounded by four equal triangular faces. By covering 

 two adjoining faces with silk or other material suitable for use 

 in kites, we arrive at the unit "winged cell " of the com- 



