400 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1923 



Lilienthal (1911, p. 78) advanced the somewhat surprising theory 

 that the general trend of the wind everywhere is upward at an angle 

 3° to 4° to the horizon. The logical difficulties of such a theory 

 are rather obvious, as at this rate we should shortly be living in 

 a vacuum; and Headley (1895, p. 238) has comfortingly demon- 

 strated that the direction of a wind over a level plain is horizontal, 

 although a very slight obstruction may cause a pronounced upward 

 draft. 



Opponents of the ascending current theory have proposed numer- 

 ous other, and often less adequate, hypotheses to account for soar- 

 ing flight. 



Some have postulated a wavelike or pulselike motion of the air; 

 according to this theory, the bird gains momentum by gliding with 

 the wind in the interim between gusts, and gains altitude by turn- 

 ing to face each freshening breeze (Headley, 1895, p. 246). Others 

 have maintained that small eddies or whirlpools in the air are taken 

 advantage of, the bird meeting them and gaining energy by ex- 

 tinguishing their motion (Hankin, 1913, p. 62). A few have even 

 urged that soaring flight is an illusion, the wings really being in 

 motion, slight, but sufficient to keep the bird aloft. This rather 

 strained hypothesis has probably been suggested by the occasional 

 balancing movements which soaring birds are seen to make. 



In the American Naturalist for 1886 we find a very remarkable 

 theory advanced by I. Lancaster, which, stated briefly, is this: A 

 properly constructed glider will move in a horizontal direction much 

 more rapidly than it descends vertically. The more the wings are 

 inclined, the greater becomes the horizontal motion relative to the 

 vertical. If the wings are sufficiently inclined, as he assumes to 

 be the case in the soaring bird, theoretically ( ?) the vertical motion 

 should entirely cease, the pull of gravity causing only horizontal 

 motion. This seems to be a roundabout way of stating that a soar- 

 ing bird is really held up by the force of gravity ! 



A curious consequence of this theory was that Professor Hendricks 

 (1886) thought it necessary to reply in a subsequent issue of the same 

 journal with several pages of complicated mathematical disproof, 

 demonstrating by various formulae that the effect of gravity would 

 actually be, not to support a soaring bird, but rather to bring it to 

 earth ! 



A more recent investigator (Hankin, 1913) has discarded all 

 theories having a basis in any known physical laws, and insists, on 

 the grounds, be it said, of much excellent observation, that soaring 

 flight must be referred to some entirely unknown quality of the 

 atmosphere, which he terms " soarability." Of this he postulates 

 two kinds, " sun soarability " and " wind soarability." Neither of 



