4 2 4 



BELL 



Another element of stability in the tetrahedral structure lies 

 in the fact that the winged surfaces are elevated at a greater 

 angle above the horizon than 45 °. 



Supposing the wings of a cell to be opened out until they are 

 nearly flat — or at least until they each make a comparatively 

 small angle with the horizon — say 20 — then if, from any 

 cause, the cell should tip so as to elevate one wing (say to 25 °) 

 and depress the other (say to 15 °) then the lifting-power of the 

 wind will be increased upon the elevated wing and diminished 

 on the depressed wing, so that there would be no tendency to a 

 recovery of position, but the very reverse. The pressure of the 

 wind would tend to increase the tipping action, and favor the 

 production of oscillation and a tendency to upset. The lifting- 

 power of the wind upon a surface inclined at io° is less than at 

 20 ; and greater at 25 than 20 . The more the wings are 

 opened out, and the flatter they become, the more essentially 

 unstable is the arrangement in the air. 



Now suppose the wings to be raised until they are nearly 

 closed, or at all events until they make a small angle with the 

 vertical (say 70 from the horizontal), then if from any cause 

 the cell should tip so as to elevate one wing (say to 75 °) and 

 depress the other (say to 65 °), the lifting-power of the wind will 

 be increased upon the depressed wing and diminished on the 

 elevated wing ; for the lifting-power of the wind is greater at 

 65 ° than at 70 and less at 75 °. Thus the moment a tipping 

 action begins the pressure of the wind resists it, and an active 

 force is invoked tending to restore the structure to its normal 

 position. The more the wings are raised, and the more they 

 approach the perpendicular position the more stable essentially 

 is the arrangement in the air. 



The dividing line between these two opposite conditions seems 

 to be drawn about the angle of 45 °. As the tetrahedral wing- 

 surfaces make a greater angle than this with the horizontal they 

 constitute an essentially stable arrangement in the air ; whereas 

 a horizontal surface represents the extreme of the undesirable 

 unstable condition. 



These considerations have led me to prefer a structure com- 

 posed of winged tetrahedral cells alone, without horizontal sur- 



