246 FACULTIES OF BIRDS. 



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 contraction, so to speak, 

 continues during the flight. 



" After having darted forwards, 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 de- 

 parture. 



" The ascension 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 

 iaculty of retaining. This air, which is perhaps a 

 light gas, being dilated and rarefied by great heat, not 

 only is its specific gravity probably diminished, but it 

 must also contribute to diminish that of the bird, 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 the rapidity of the descent, permit it to 

 alight gently on the earth. 



" It is by the assistance of the tail that certain birds 

 are enabled to descend with precipitation from a great 



