192 THE STRUCTURE AND LIFE OF BIRDS chap. 



man with his hands tied behind his back may appear 

 to walk with ease, but in the course of a long tramp, 

 it would hamper him much. In stopping-, in rising 

 and till great velocity is attained, a broad expanse of 

 wing is of use. 



For the understanding of gliding flight also, it is 

 most important to bear in mind the law of Avanzini. 

 If a bird wishes to descend rapidly, he must partly 

 flex his wings, so that they may present a less 

 extended front and, consequently, receive less support. 

 If he wishes to descend very gradually or maintain 

 his level or glide upward, he must open his wings to 

 their full stretch, so as to have the support of as long 

 a front line as possible. The amount of work done 

 by the front margin and, consequently, the trajectory 

 of his flight will of course vary with the pace x (fig. 52). 



The principle just explained can be seen at work 

 in little paper contrivances. Take a piece of paper 

 shaped thus 



Fig. 53- 



Fold it along the line AB so that the two sides slope 

 upwards. Put in a pin along the line with its head 

 near A. Hold it on a Siant with A at the lower end 

 and let it drop. It will glide some distance and very 

 likely show an upward tendency ending in a somer- 

 1 See Newton's Dictionary of Birds, p. 265. 



