THEOEY OF FLIGHT. 



273 



notions on which may be established the mechanical theory of 

 jaight. 



From all these experiments we may deduce that it is 

 during the descent of the wing that the bird acquires all the 

 motive force which sustains and directs it in space. 



Theory of the flight of the bird. — On this subject, as on 

 almost all those that belong to this discussion, nearly every- 

 thing has been already said ; so that we must not expect to 

 find an entirely new theory arise from the experiments which 

 have been described. In the works of Borelli we find the 

 first correct idea of the mechanism of flight. The wing, 

 says this writer, acts on the air like a wedge. Developing 

 still farther the thought of the learned Neapolitan physiologist, 

 we should now say that the wing of the bird acts on the air 

 after the manner of an inclined plane, in order to produce a 

 re-action against this resistance which impels the body of the 

 bird upward and forward. This theory, confirmed by Strauss- 

 Durkheim, has been completed by Liais, who noticed the 

 double action of the wing ; first, that which in the phase of 

 depression of this organ, raises the bird and gives it an im- 

 pulse in a forward direction ; then, the action of the ascending 

 wing, which is guided in the same manner as a boy's kite, 

 and sustains the body of the bird until the following stroke 

 of the wing. 



We have been reproached for relying on a theory which 

 had its origin more than two centuries ago ; we much prefer 

 an old truth to the most modern error ; therefore we must be 

 allowed to render to the genius of Borelli the justice which 

 is due to him, and only claim for ourselves the merit of having 

 furnished the experimental demonstration of a truth already 

 suspected. 



But the theories which had been propounded up to the 

 present time neglected many important parts which experi- 

 ments reveal, and which we are about to endeavour to bring 

 clearly forward. 



Thus, the manner in which the change in the plane of the 

 wing is efi'ected in every part of the flight was necessary to 

 be known, in order to explain the re- actions which tend 

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