74 THE BOOK OF MIGRATORY BIRDS 



"While all this is performed with extreme quickness, 

 several muscles of the wings, besides, among others, the 

 extensors of the tail, strive to extend the wing; but, as 

 the resistance of the air on the extremities of the plumage 

 is very great, and this fluid opposing all rapid movement 

 on their part, these muscles then direct their power against 

 the sides of the trunk. Taking, then, their position on 

 the bone of the wing, on the external side of the wing, 

 and acting by their upper extremity, they extend the main 

 wing-bone over the fore wing-bone, and, as this action 

 and that of the great pectorals occur at the same instant 

 and in concert on each side of the trunk, the latter is 

 forced up in a middle direction. 



"Thus the combination of these various efforts impart 

 to the trunk a force of projecting forward and ascending, 

 by which it is propelled with the wings. This projection 

 is evidently similar to the leap of other animals. The 

 great pectorals then relax, and the wings immediately 

 reascend, partly by the reaction of the air on their lower 

 surface and the descent of trunk, and partly by the action 

 of the middle pectorals and their congeners, whose con- 

 traction, so to speak, continues during the flight. 



"After having darted forward, the bird remains for an 

 instant unsupported by the air; this fluid then, by its 

 reaction, repels and tends to raise it still higher than the 

 leap alone could have done, and afterwards prevents it 

 again falling as low as the point of departure. The ascen- 

 sion of the trunk is doubtless favoured by the internal air, 

 which insinuates itself into every part of the animal, and 

 which the latter has the faculty of retaining. The air, 

 which is perhaps a light gas, being dilated and rarified 

 by great heat, not only is its specific gravity probably 

 diminished, but it must also contribute to diminish that of 

 the air by inflating it and supplying all vacancies during 

 the flight. 



"If the bird which descends precipitately fears to hurt 

 itself on approaching the earth, it opens its wings, and 

 its tail, and takes several little leaps, which, diminishing 



