148 THE FLIGHT OF BIRDS 



slight one perceptible if one approached close to the 

 edge of the hill." 



What is going on in the air high aloft when soaring 

 takes place over a dead level I imagine to be this. 

 There is a wind sweeping over the plain, and at the 

 outset it is horizontal. Coming into contact with 

 an up-current from the heated surface below, it is 

 deflected upward. The soaring bird, then, gets 

 support not only from the ascending column of air, 

 but from the wind to which the ascending column 

 gives an upward trend. Were the heated air 

 mounting from below the sole support, birds might 

 soar in an almost dead calm, and that is a thing 

 which all observers agree does not take place. They 

 first ascend to some height— two or three hundred 

 feet — by beating with their wings, and then the 

 performance begins. 



The reason of the spiral movement is not, I believe, 

 far to seek. There are over the plain regions of 

 ascending and regions of descending air. It is 

 essential that the bird should not pass beyond the 

 boundaries of the upward stream that maintains 

 and lifts him. Had the wind over the whole extent 

 of the plain an upward incline, then the Kites and 

 the rest might soar, if the term will stand this strain- 

 ing of its use, in a straight line like the Eagles, whose 

 majestic advance, without deviation to right or left, 

 I have already described ; or, to take an example 

 more commonly seen, like the Gull that follows a 

 steamer, poised on an up -draught over the stern. 

 The up-and-down currents on which birds depend 

 for soaring are sometimes very formidable to an 

 aviator, who in a few seconds may pass through an 



