ON SOARING FLIGHT. 205 



in a wind of a certain velocity, to 1 foot per second as the velocity 

 becomes greater, while for a sufficiently high velocity there will be no 

 descent at all, and as the velocity still further increases the bird may 

 ascend where before it had been descending. In direct tlight in a 

 calm the black vulture will be found to maintain his elevation only by 

 flapping its wings one-third of the time; in a light wind it will flap less 

 frequently; in a moderate wind still less frequently; while in a high 

 wind it will cease flapping entirely. The supporting power of the wind 

 thus varies not only as its velocity relative to the bird, but also as its 

 absolute velocity. In moderate winds, therefore, the bird, if it does not 

 flap, can only increase its altitude by soaring- in circles, but in direct 

 flight itsdescentis less rapid than in light winds, and it can consequently 

 sail further and need not so frequently resort to spiral flight. But as 

 an ascending column of air can not maintain a fixed locality, but must 

 be carried forward with the wind, the bird, if it utilizes such currents, 

 must also drift with the wind. This it will almost invariably be found 

 doing when the winds are light or moderate. Thus if it be advancing 

 into a wiud it will turn upon its course and drift with it whenever it 

 attempts to rise by soaring in circles. The bird may, of course, in 

 high winds circle without drifting, as there is then no necessity for 

 utiliziug ascending currents, and we find in this fact a complete refu- 

 tation of the supposition that ascending currents depend for their 

 maintenance upon wind-breaks, since wind-breaks can not move across 

 the country as ascending currents do. 



As some writers have seemingly regarded the effect of wind-breaks 

 as an important factor in the problem of flight, I may state on the 

 authority of Professor Eidgway that on the level, treeless prairies of 

 the West, where no wind-breaks occur, the birds soar as perfectly as in 

 regions where wind-breaks are of common occurrence. My own obser- 

 vation has been that the birds habitually take advantage of existing 

 wind-breaks, such as hill slopes, ridges, buildings, and railroad embank- 

 ments, as they may often be seen following the crest of a ridge upon 

 the windward side for long distances, but that when every allowance 

 has been made for their possible influence the i^roblem of flight in its 

 essential features remains unchanged when this factor has been taken 

 out. 



What I have said seems in general to be corroborated by the obser- 

 vations of M. Mouillard, who says of the great tawny vulture: ''If the 

 wind be feeble he can climb into the air by circling round and. drifting 

 back somewhat. If the wind be brisk enough to sustain him thor- 

 oughly he can rise perpendicularly, facing the wind, without wheeling- 

 round, and even advance into the wind while rising. This unlimited 

 advance against the wind current without beat of wing, without appar- 

 ent effort, and almost without act of guidance, is performed every day 

 by myriads of winged creatures; and if the reader doubts the fact he 

 can thoroughly satisfy himself by a trip to the trade- wind latitudes." 



