WIND AND FLIGHT 131 
exertion on the bird’s part unnecessary, any more 
than it does in the case of the Snipe that faces the 
wind in rising. 
There is no reason why a bird should not advance 
in the same way in the teeth of a wind when the 
conditions are favourable, and occasionally Gulls 
may be seen employing this method behind a steamer 
when the wind is directly or almost directly ahead. 
As the vessel moves onward, there is a down-current 
behind the stern, since the air rushes down to fill 
the space just vacated by the advancing ship. The 
down-rush of air strikes the water and, rebounding 
ee eee eo ee 
Ww 
Fia. 25. 
Flight at right angles to the wind, with motionless wings. W: 
the direction of the wind. 
from it, forms an up-current a little way farther 
back. Here, then, is an up-current extending only 
over a very small area of water, but, since it moves 
on with the steamer, it opens up great possibilities. 
To start with, imagine the Gull flying a little in rear 
of the up-current. He glides swiftly downward and 
onward, and when near the water finds himself in 
the convenient up-draught, which lifts him to his 
former level, so that he is able again to glide down- 
ward and make much headway. At the end of his 
K2 
