WIND AND FLIGHT 135 
splendid up-currents must mountain ridges put at 
the service of a soaring Eagle ! 
Advance in a Direct Line Without Movement of Wing. 
There is a feat perhaps more striking than any of 
those already described, a feat which, nevertheless, 
Gulls often achieve. A steamer is advancing against 
a fairly strong wind which, if not absolutely a head- 
wind, strikes the vessel at an acute angle. There 
results a steady up-current over the stern of the 
vessel, or slightly to one side or the other of the stern. 
Poised on this up-current the Gulls hang in mid-air, 
their wings held rigidly expanded. Only very slight 
wing-movements, evidently for purposes of balance, 
can be detected. Standing on the deck and watching 
these Gulls one is irresistibly reminded of the poising 
of the Kestrel high in air with wings held motionless, 
when he finds a wind that is all that he could wish. 
It is sometimes easy to forget that, unlike the 
Kestrel, they do not remain in one spot, but that 
all the while they are moving onward and, in fact, 
keeping pace with the steamer. The Gulls, like the 
Kestrel, are poising on an up-current of air, but 
they give their bodies a rather different incline, with 
the result that they keep travelling forward. The 
diagram will explain this. The general incline of 
their body and wing surfaces is slightly downwards. 
Hence the upward-streaming wind not only main- 
tains them in air or lifts them higher, but, acting at 
a right angle, also drives them forward. Imagine 
a bird with his body sloped much more steeply 
downward. Obviously, the wind would then give 
him a shove forward. What the Gull does is like, 
