212 L. HARGRAVE. 



the relative wind acting on its upper surface, and be in effect a. 

 kite wrong side up. 



The position of the tip, that is, whether it incline up or droop 

 when viewed from front or rear, is a clear indication of whether 

 the bird is soaring in a horizontal or downward blowing wind, 

 or merely being supported by an upward trend of wind. 



The albatross and frigate bird show the drooping tip to per- 

 fection. Hawks and eagles frequently show the upward bent tip, 

 and when they do so we may safely conclude that any flat object 

 would be up-borne by the wind in their neighbourhood. Birds 

 that circle in calm or nearly calm air, have the wing tips turned 

 up ; and if the performance takes place over a hot and dusty plain, 

 the conditions are favourable for the formation of a sand column 

 or "whirley." The bird that soars in a gale has a deeper concavity 

 of wing than the one' that soars in a moderate breeze, from which 

 I deduce that the velocity of rotation of the vortex must have 

 some point of maximum efficiency. In other words, the small 

 vortex cannot attain an infinite velocity, and the large vortex 

 loses its efficiency when its speed of revolution is reduced below a 

 certain point. Each form of soaring wing is evolved by the 

 average velocity of wind in the latitudes frequented by the bird. 



A kite (0.) was made four feet wide and seven inches in a fore 

 and aft direction. Two feet six inches of the middle was shaped 

 to a soaring curve and the rest left flat. The inclination of the 

 flat part to the chord of the soaring part was 5° and unadjustable. 

 A rod with weighted ends and small tail was added. This kite 

 soared several times but was crank athwartships. 



It was thought desirable to reject all horizontal surfaces as it 

 appeared that their only use to a bird was to enable it to fly when 

 there was no wind; and as these soaring kites had no motor but 

 the soaring curve, the flat surfaces only increased the drift. At 

 this point the soaring machine develops into a form that has no 

 counterpart in nature. The rod now having no horizontal surface 

 at the ends ; could not, by the inertia of the lead weights alone,. 



