118 BRITISH BIRDS. 



Rising. 



Everyone is familiar with the fact that birds face the 

 wind when they Tvish to rise, though the reason for this is 

 not so generally known. A wind blowing in the bird's 

 face would not help him to rise except for the fact that 

 the wind increases in velocity with altitude — a fact that can 

 be verified with an anemometer. I have fomid that when 

 there is a wind of 770 feet per minute at a height of 

 2 feet from the ground, the anemometer will record a 

 velocity of over 1000 feet at a height of 7| feet. We may 

 for the sake of clearness divide the air into distinct 

 successive streams, the second more rapid than the lowest, 

 and each, as we ascend, more rapid than the one below it. 

 Imagine a bird rising with wings held rigidly expanded 

 through these successive streams. The wind will, of 

 course, di-ive him backwards, but in each stream he will 

 have only the velocity of the one below, from which he 

 has just emerged. Consequently he will offer resistance 

 to the stream, will have, in fact, inertia, and the force of 

 the horizontal wind will be divided into two forces, one of 

 which will lift him. True, he cannot trust to the wind 

 alone, so he plies his wings vigorously. Even the Lark, 

 past master as he is in the art of upw^ard flight, always 

 lightens the work of his wings in this w^ay. Moreover, 

 the wind can depress as well as lift. Were the bird to 

 look doivn the wind as he rises, then the air striking upon 

 his back w^ould tend to force him downwards. 



Big birds such as Cormorants have the utmost difficulty 

 in rising without a wind to help them. Indeed, a 

 Cormorant will sometimes fly some distance in a direction 

 in which he, as far as one can judge, does not wish to go, 

 simply in order to get the help of the wind in rising. He 

 has had his meal, and that a heavy one, and his fellow 

 banqueters may be seen drying their wings upon a rock. 

 He also wishes to get to this rock, but he first flies some 

 distance in the opposite direction, because, as it so happens, 

 he has thus the wind in his face as he rises. After 

 attaining some little elevation, he swings romid and makes 

 for the rock. 



