190 THE STRUCTURE AND LIFE OF BIRDS chap. 



But it is not only when we compare base and 

 extremity that the division of labour is unequal. If 

 we divide the wing in imagination by a line drawn 

 down its middle from base to tip, then the front half 

 will do far more work than the hinder half, when the 

 bird is gliding at great speed or moving his wings 

 rapidly through the air. A boat sailing at an angle 

 to the wind with a sail slung obliquely across it 

 supplies an illustration of this. If she moves rapidly, 

 only the forepart of the sail will do much work. The 

 wind is blowing, say, at right angles to the boat. It 

 will strike the forepart of the sail, over all the rest of 

 which there will only be a backward current of air, 

 which has been turned from its course by the forepart. 

 The faster the boat sails, and, also, the nearer to the 

 wind she sails, the truer this will be ; the narrower 

 will be the margin of sail that really works. This 

 is called the law of Avanzini. It holds with regard 

 to the bird's wing, which during the down stroke moves 

 rapidly forward as well as downward, and, of course, 

 shares the onward movement of the whole bird. It is 

 truer of the swiftly moving extremity than of the 

 slower inner part, and this accounts for the remarkable 

 way in which long wings, notably those of the Swift 

 and Gannet, narrow towards their ends. Professor 

 Pettigrew made some experiments which illustrate 

 this. He cut away the hinder part of a Bluebottle's 

 wings and apparently it could fly equally well. In 

 the same way with Sparrows, the removal of the same 

 part of the wing seemed to do little damage. Still to 

 describe the flight after these mutilations as " perfect " 

 is to go too far. No failing may strike the eye. A 



