PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 277 



points of the curve have been produced at equal intervals of time, each 

 of them, under the influence of the motion of the glass i)late, will have 

 a constant deviation toward the right, bearing a stated relation to the 

 preceding point. The correction thus consists in carrying: the second 

 point back toward the left twice this amount, the third point three times 

 Pi„ 27. t^^^ amount, and 



so on. The ascend- 

 ing portion of the 

 curve should also be 

 submitted to this 

 correction, and sim- 

 lilarly each part of 

 the tracing. But it 

 i s precisely the 

 height which the 

 wing attains in the 



different ascending 

 Ellipse traced by aWbeatstoue's rod upon a turning cylinder. „^,i (laonpvi(liii o- mn. 



tions of its course which we do not know; bnt this want can be snp- 

 plied by the apparatus in the following manner : 



Since the principle of this mechanism is founded upon the transmission 

 of two motions, perpendicular to each other, vertical and horizontal, it 

 suffices to suppress the transmission of the horizontal motion to obtain 

 the curv^e of elevation immediately ; that is to say, the expression of the 

 height of the wing at each instant of its course. For this I obstruct the 

 tube of lateral transmission, let the bird fly, and obtain the curve of the 

 heights of the wing at each moment. 



The correction being made, and Fig. 26 being selected to show the 

 course of the point of the wing during one of its evolutions, and pro- 

 jected upon a stationary plane, we obtain Fig. 28. 



Fie. 28. ^^^ arrows indicate the direction in which the wing 



moves. 



Is this the form characteristic of all birds ; or is it 

 only that of the harrier, in the conditions of flight 

 in which it has been placed ? 



The last supposition appears to be the most prob- 

 able ; we can see, even while comparing the form of 

 the tracmg at different instants of its flight while 

 under experiment, that the ellipse is greater and more 

 open in the first strokes of the wing than in the last. 

 It is, however, necessary to excei)t the second stroke 

 of the wing, which has given me a narrower ellipse 

 Conrsp in space of the thaii any Other, in all the experiments which I have 

 dnVert^froUtL'^nrotion'of made. I do uot know to what this special form is to 

 ^i^*" ^i'"*^- be attributed, but have thought it worth while to men- 



tion it on account of its constancy. 



Of the rotation of the hiimeriis and the clianges of the plane in the wing 

 during flight. — The wing of a bird, like that of an insect, must meet 

 with a sufficient resistance from the air in its motion upward and down- 

 ward to incline its flexible j^ortion, namely, that which forms the webs 

 and coverts. This cause does produce a change of the plane of the 

 wing, but there is another even more ijowerful, for it places the wing at 

 the outset of the depressing motion in a favorable position for the double 

 propulsion which is produced. I refer to the pivot motion which the 

 humerus executes around its axis at each contraction of the great pec- 

 toral. It is enough to examine the bony crest on Avhich the large tendon 



