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, 



