FLIGHT 271 



Upward currents of air are to be met with, when there is a 

 wind, on the windward side of a sloping mountain, or a house, 

 or the sail of a ship, or as a result of the inequalities of surface 

 produced by waves on the sea. For example. Gulls may be 

 seen to soar for a prolonged period of time in front of or above 

 cliffs on the shore when the wind comes from the sea. This, 

 however, cannot be taken as an example of soaring proper, seeing 

 that Gulls can only remain suspended in the air without loss of 

 relative horizontal motion under special conditions which do not 

 apply in the case of the typical soaring birds. Gulls are not seen 

 to soar except under conditions which point toward upward cur- 

 rents, being a very obvious explanation of the phenomenon. In 

 the case, on the other hand, of typical soaring birds, such as those 

 named above, soaring is observable under conditions and at heights 

 where there is no sufficient reason to assume that upward cur- 

 rents exist. For example. Eagles and Adjutants are seen to rise 

 continuously by soaring for miles above the surface of the ground 

 or sea. To explain such cases as being due to upward currents 

 would, we are of opinion, require a good deal more evidence of 

 continuous upward currents of air from the plains or seas than is 

 at present available. That the direction of the wind even at great 

 heights, and above a comparatively smooth sea or plain, is by no 

 means always parallel wdth the surface of the globe is more than 

 probable ; but we know of no reason for assuming that the upward 

 ciurrents are sufficiently predominant over the downward currents 

 to justify us in looking on the former as capable of explaining 

 observed facts as to soaring. In other words, the theory of upward 

 currents of the air as an explanation of sailing or soaring flight 

 requires more support in the way of facts than have been so far 

 produced. 



The theory of unequal velocity of the air at different heights as an 

 explanation of soaring is based on the fact that the motion of a 

 wind is retarded by the surface of the earth, so that considerable 

 variations in velocity may and do occur at different heights above the 

 ground. The matter is put very clearly by Hubert Airy,^ although 

 we are indebted, in this country at all events, to Lord Eayleigh for 

 having first formulated it. Airy says : " Suppose a bird at the 

 highest point of one of its gyrations, when it has mounted against 

 the wind and is wheeling to one or other side preparatory to the 

 descent with the wind. . . . Let us regard the air at the level 

 of the bird, at this turning-point, as still. Then, relative to this 

 point, the lower strata of air have a horizontal velocity in the 

 opposite direction to the wind (as perceived on earth) ; and the 

 bird in falling apparently down the wind will really be meeting 

 stronger and stronger adverse currents, and when it has reached 

 ^ Nature, xxvii. pp. .534 and 591. 



