192. THE STRUCTURE AND LIFE OF BIRDS cuHap, 
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! (fig. 52). 
The principle just explained can be seen at work 
in little paper contrivances. Take a piece of paper 
shaped thus : 
Fold it along the line AB so that the two sides slope 
upwards. Put ina 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 Dictvonary of Birds, p. 265. 
