FLIGHT OF BIRDS. 513 



oar in rowing is a similar operation, performed 

 with the same intention, and deriving its name 

 from this resemblance. Independently of this cir- 

 cumstance, however, the force exerted by the wing 

 in its descent is considerably greater than that with 

 which it rises ; and consequently the resistance and 

 reaction of the air is greatest in the opposite direc- 

 tion to the descent of the wing, which strikes 

 obliquely backwards, as well as downwards: and 

 the effect of the muscular action is, in consequence 

 of that resistance and reaction, to raise the bird, 

 and, at the same time, propel it forwards.* 



As the inclination of the wing is chiefly back- 



* Erroneous views have frequently been entertained respecting tlie 

 theory of the progressive and other movements of animals, from 

 mistaking the reaction of the ground, or medium against which 

 pressure is made, for a moving or propelling force, analogous to 

 that arising from elasticity : whereas it is merely a force, which, 

 being equal and opposed to that pressure, only destroys or neutralizes 

 its effect ; and thus enables the moving power from which the pres- 

 sure is derived, to exert its full efficiency in exactly the opposite 

 direction. This error, which will be found to vitiate many of the 

 reasonings of Borelli, was clearly pointed out by Barthez, in his 

 work entitled " Nouvelle Mechanique des Mouvemens de I'Homme 

 et des Animaux," published in 1798. 



M. Navier has undertaken a very long and elaborate mathematical 

 investigation of the dynamic conditions which obtain during the 

 flight of birds, and the swimming of fishes, in the Memoires de 

 I'Academie Royale des Sciences de I'lnstitut. de France, for 1832 ; 

 Tome xi. p. Ixi — cxiii ; under the title of " Rapport sur un Memoire 

 de M. Chabrier, concernant les moyens de voyager dans I'air, et de 

 s'y diriger ; contenant une nouvelle theorie des mouvemens pro- 

 gressifs." He asserts that a bird in flying folds its wings but little 

 while raising them to give the effective stroke; and ascribes the 

 effect of the movement of the wings to the greater velocity with 

 which they are made to descend, compared with that of their eleva- 

 tion. His calculations lead him to the startling conclusion, that a 

 swallow, while simply sustaining itself in the air without either rising 

 or advancing, must cause its wings to make 23 strokes in each 

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