264 THE STRUCTURE AND LIFE OF BIRDS chap. 



often long beyond all justification) we find a certain 

 proportion maintained with no great deviation, that 

 when their bulk has been trebled their wing surface has 

 not at the same time been trebled, in defiance of the 

 laws of symmetry and geometry. But, looked upon 

 as flying-machines, why do small birds require larger 

 wings, in proportion to their bulk, than big birds ? 



Since, allowing for his greater weight, the big bird 

 has less wing to support him than the small bird, we 

 must see what advantages he has that enable him to 

 do with less. To begin with, if we compare the sustain- 

 ing powers of two wings of different lengths, we shall 

 find that the superior power of the long one is altogether 

 out of proportion to its superiority in length. Suppose 

 the shorter one to be one foot, the longer two feet, 

 long. Then if the two are worked by muscles of 

 equally rapid contraction,the extremity of the latter will 

 move with a velocity that is far more than double that 

 attained by the extremity of the former. And since 

 the resistance of the air increases nearly as the square 

 of the velocity, it is clear that to judge even by the 

 comparative velocities of the extremities is to under- 

 estimate by a great deal the superiority of the longer 

 wing. Secondly, it has been found by experiment 

 that the air passes lightly off the margins of a plane 

 surface which moves through it, and so does not offer 

 much resistance to them. 1 The loss is proportionately 

 greater with small planes than with large ones, since 

 for each square inch of area they have more margin. 

 The resistance of the air in the case of a square plane 

 having an area of two square feet, moving in a direction 

 1 Marey, Vol des Olseaux, p. 214. 



