240 THE STRUCTURE AND LIFE OF BIRDS cnap. 
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 
afact 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, tohelp 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. 1 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. 
Onward Flight 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 
