AERONAUTICS. 



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downwards and forwards in the direction 5, t; the forward angle 

 which it makes with the horizon increasing as the wing 

 descends (compare with fig. 85 (t^Jc), p. 160, and fig. 88 

 (c def),'p, 166). The air is thus seized by a great variety 

 of inclined surfaces, and as the under surface of the wing, 

 which is a true kite, looks upwards and forwards, it tends to 

 carry the body of the bird u;pwards and forwards in the direc- 

 tion xw. When the wing xt makes the up stroke, it rotates 

 in the direction ts to prepare for the second down stroke. 

 It does not, however, ascend in the direction 1^5. On the 

 contrary, it darts up like a true kite, which it is, in the direc- 

 tion X V, in virtue of the reaction of the air, and because the 

 body of the bird, to which it is attached, has a forward 

 motion communicated to it by the wing during the down 

 stroke (compare with ghi of fig. 88, p. 166). The resultant 

 of the forces acting in the directions x v and x b, is one acting in 

 the direction x w, and if allowance be made for the operation 

 of gravity, the flight of the bird will correspond to a line 

 somewhere between w and b, probably the line x r. This 

 result is produced by the wing acting as an eccentric — by 

 the upper concave surface of the pinion being always directed 

 upwards, the under concave surface downwards — by the 

 under surface, which is a true kite, darting forward in wave 

 curves both during the down and up strokes, and never 

 making a backward angle with the horizon (fig. 88, p. 166); 

 and lastly, by the wing employing the air under it as a ful- 

 crum during the down stroke, the air, on its own part, react- 

 ing on the under surface of the pinion, and when the proper 

 time arrives, contributing to the elevation of the wing. 



If, as Borelli and his successors believe, the posterior 

 margin of the wing yielded to a marked extent in an upward 

 direction during the down stroke, and more especially if it 

 yielded to such an extent as to cause the under surface of the 

 wing to make a backward angle ivith the horizon of 45°, one of 

 two things would inevitably follow — either the air on which 

 the wing depends for support and propulsion would be per- 

 mitted to escape before it was utilized ; or the wing would 

 dart rapidly dowmvard, and carry the body of the bird with 

 it. If the posterior margin of the wing yielded in an upward 

 direction to the extent described by Marey during the down 



