AERIAL LOCOMOTION 423 



the wind. The change is usually small in steady winds ; but 

 in unsteady winds great and sudden changes often occur. 



The extreme possible range of fluctuation is, of course, from 

 the extreme front of the aeroplane to the rear, or vice versa, and 

 the possible amount of change, therefore, depends upon the 

 dimensions of the aeroplane — especially in the fore and aft 

 direction. With a large aeroplane the center of pressure may 

 suddenly change to such an extent as to endanger the equilibrium 

 of the whole machine. Whereas, with smaller aeroplanes, 

 especially those having slight extension in the fore and aft 

 direction, the change, though proportionally as great, is small 

 in absolute amount. Where we have a multitude of small sur- 

 faces well separated from one another, as in the tetrahedral con- 

 struction, it is probable that the resultant center of pressure for 

 the whole kite can shift to no greater extent than the centers of 

 pressure of the individual surfaces themselves. It is, therefore 

 extremely unlikely that the equilibrium of a large kite could be 

 endangered by the shifting of the centers of pressure in small 

 surfaces within the kite. This may be the cause of the auto- 

 matic stability of large structures built of small tetrahedral cells. 

 If so, one principle of stability would be: Small surfaces — 

 well separated — and many of them. The converse proposition 

 would then hold true if we desired to produce instability and a 

 tendency to upset in a squall — namely : Large surfaces — 

 continuous — and few of them. 



Another source of danger with large continuous surfaces is 

 the fact that a sudden squall may strike the kite on one side, 

 lifting it up at that side and tending to upset it. But the com- 

 pound tetrahedral structure is so porous, that a squall passes 

 right through and lifts the other side as well as the side first 

 struck ; so that the kite has not time to be upset before the blow 

 on one side is counterbalanced by a blow on the other. I have 

 flown a Hargrave box kite simultaneously with a large kite of 

 many tetrahedral cells in squally weather for the purpose of 

 comparing them under similar conditions. The tetrahedral 

 structure often seemed to shiver when struck by a sudden squall, 

 whereas the box kite seemed to be liable to a swaying or tipping 

 motion that would be exceedingly dangerous in a structure of 

 large size forming part of a flying machine. 



