230 



AEKONAUTICS. 



stroke, it would be tantamount to removing the fulcrum (the 

 air) on which the lever formed by the wing operates. 



If a bird flies in a horizontal direction the angles made by 

 the under surface of the wing with the horizon are very slight^ 

 but they always look forwards (fig. 60, p. 126). If a bird 

 flies upwards the angles in question are increased (fig. 59, p. 

 126), In no instance, however, unless when the bird is 

 everted and flying downwards, is the posterior margin of the 

 wing on a higher level than the anterior one (fig. 106, p. 

 203). This holds true of natural flight, and consequently 

 also of artificial flight. 



These remarks are more especially applicable to the flight 

 of the bat and bird where the wing is made to vibrate more 

 or less perpendicularly (fig. 17, p. 36; figs. 82 and 83, p. 

 158. Compare with fig. 85, p. 160, and fig. 88, p. 166). If 

 a bird or a bat wishes to fly upwards, its flying surfaces 

 must always be inclined upwards. It is the same with the 

 fish. A fish can only swim upwards if its body is directed 

 upwards. In the insect, as has been explained, the wing 

 is made to vibrate in a more or less horizontal direction. 

 In this case the wing has not to contend directly against 

 gravity (a wing which flaps vertically must). As a conse- 

 quence it is made to tack upon the air obliquely zigzag fashion 

 as horse and carriage would ascend a steep hill {vide figs. 67 

 to 70, p. 141. Compare with figs. 71 and 72, p. 144). In 

 this arrangement gravity is overcome by the wing reversing its 

 planes and acting as a kite which flies alternately forwards and 

 backwards. The kites formed by the wings of the bat and bird 

 always fly forward (fig. 88, p. 166). In the insect, as in the bat 

 and bird, the posterior margin of the wing never rises above the 

 horizon so as to make an upward and backward angle with it, as 

 stated by Borelli, Marey, and others (cxa of fig. 114, p. 228). 



While Borelli and his successors are correct as to the wedge- 

 action of the wing, they have given an erroneous interpretation 

 of the manner in which the wedge is produced. Thus Borelli 

 states that when the wings descend their posterior margins 

 ascend, the two wings forming a cone whose base is repre- 

 sented by cle of fig. 113, p. 220); its apex being repre- 

 sented by o.f of the same figure. The base of Borelli's cone, 

 it will be observed, is inclined forwards in the direction of 



