240 THE STRUCTURE AND LIFE OF BIRDS chap. 



I long thought perfectly true what some writers 

 on flight still maintain, that a uniform horizontal wind 

 would lift the bird if he faced it, as it lifts a kite. But 

 a fact that ought to have been obvious has now been 

 pointed out to me — viz., that the bird, a moment after 

 he has left the ground, becomes part of the moving 

 current, so that it will make no difference as far as his 

 upward progress is concerned whether he fly with it 

 or against it. There might as well be not breeze 

 enough to shake an aspen leaf. You may imagine 

 him flying in a globe filled with air, the globe itself 

 moving with the current. The fact that the globe is 

 moving will not affect the bird's flight. 



Mr. R. C. Gilson first showed me how it is that a bird 

 wishing to rise derives advantage from facing the wind. 

 He is perpetually passing from a slower into a faster 

 current. Thus at every stage he has his own inertia, 

 which is equivalent to momentum, to help him. When 

 he first jumps from the ground, if we divide the wind 

 in theory into separate layers, he has at his service 

 the whole velocity of the lowest layer. As he passes 

 out of this he is helped by the difference in velocity 

 between layers Nos. I and 2. As he ascends higher, 

 the rate of increase in the wind's speed diminishing, 

 he will be helped less, but, as I have shown, he will 

 not be left entirely to his own unaided efforts, at 

 any rate until he has passed an altitude of 1,000 feet. 



Oinuard FligJit and Air Currents. 



When at the seaside it is blowing with a violence 

 that must startle an anemometer familiar only with 

 sheltered places inland, it is a beautiful sight to see a 



