AERONAUTICS. 225 



and Physiology, and M. Liais " On the Flight of Birds and 

 Insects " in the Annals of Natural History, all assert that the 

 stroke is delivered downwards and more or less backwards. 



To obtain an ujpward recoil, one would naturally suppose all 

 that is required is a downward stroke, and to obtain an ujpward 

 and forward recoil, one would natvirally conclude a downward 

 and backward stroke alone is requisite. Such, however, is not 

 the case. 



In the first place, a natural wing, or a properly constructed 

 artificial one, cannot be depressed either vertically downwards, 

 or downwards and bachvards. It will of necessity descend 

 downwards and forwards in a curve. This arises from its 

 being flexible and elastic throughout, and in especial from its 

 being carefully graduated as regards thickness, the tip being 

 thinner and more elastic than the root, and the posterior 

 margin than the anterior margin. 



In the second place, there is only one direction in which 

 the wing could strike so at once to support and carry the bird 

 forward. The bird, when flying, is a body in motion. It has 

 therefore acquired momentum. If a grouse is shot on the 

 wing it dees not fall vertically downwards, as Borelli and his 

 successors assume, but downwards and forwards. The flat 

 surfaces of the wings are consequently made to strike down- 

 wards and forwards, as they in this manner act as kites to 

 the falling body, which they bear, or tend to bear, upwards 

 and forwa.rds. 



So much for the direction of the stroke during the descent 

 of the wing. 



Let us now consider to what extent the posterior margin 

 of the wing yields in am upward direction when the wing 

 descends. Borelli does not state the exact amount. The 

 Duke of Argyll, who believes with Borelli that the posterior 

 margin of the wing is elevated during the down stroke, avers 

 that, " whereas the air compressed in the hollow of the wing 

 cannot pass through the wing owing to the closing upwards of 

 the feathers against each other, or escape forwards because of 

 the rigidity of the bones and of the quills in this direction, it 

 passes backwards, and in so doing lifts by its force the elastic 

 ends of the feathers. In passing backwards it communicates 

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