414 DE EETTIGEEW ON THE PHYSIOLOGY OF WINGS. 



yielding is, however, in all cases very slight, and the little upward movement 

 there is, is in part the result of the posterior margin of the wing rotating 

 around the anterior margin as an axis. That the posterior margin of the wing 

 never yields in an upward direction until the under surface of the pinion makes 

 a backward angle of 45° with the horizon, as Marey remarks, is a matter of 

 absolute certainty. This statement admits of direct proof. If any one watches 

 the horizontal or upward flight of a large bird, he will observe that the pos- 

 terior or flexible margin of the wing never rises during the down stroke to a 

 perceptible extent, so that the under surface of the wing never looks backwards. 

 On the contrary, he will find that the tinder surface of the iving (during the 

 down stroke) invariably looks forwards — the posterior margin of the wing being 

 inclined doivnwards and backwards, the anterior one upwards and forwards, as 

 shown at c d ef j k I m of fig. 15, page 345 ; h j of fig. 38, page 361 ; 1, 2, 3 ; 

 4, 5, 6 of figs. 18 and 19, Plate XIV. ; and qp o of figs. 14 and 15, Plate XIII. 



The under surface of the wing, as will be seen from this account, not only 

 looks forwards, but it forms a true kite with the horizon, the angles made by 

 the kite varying at every part of the down stroke, as shown more particularly 

 at c, d, e,f; j, k, I, m of fig. 15, page 345. 



Professor Marey goes on to state that not only does the posterior margin of 

 the wing yield in an upward direction during the down stroke until the under 

 surface of the pinion makes a backward angle of 45° with the horizon (page 

 415, fig. 55, x c; a b), but that during the up stroke it yields to the same extent 

 in an opposite direction {xd; ab). The posterior flexible margin of the wing, 

 according to Marey, thus passes through a space of 90° every time the wing 

 reverses its course, this space being dedicated to the mere adjusting of the 

 planes of the wing for the purposes of flight. The planes, moreover, he 

 asserts, are adjusted not by vital and vito-mechanical acts but by the action of 

 the air alone ; this operating on the under surface of the wing and forcing its 

 posterior margin upwards during the down stroke ; the air during the up stroke 

 acting upon the posterior margin of the upper surface of the wing, which it 

 forces doivnwards. Marey thus delegates to the air, the difficult and delicate 

 task of arranging the details of flight, The time, power, and space occupied 

 in reversing the wing alone, according to this view, are such as to render flight 

 impossible. That the wing does not act as stated by Marey, may be readily 

 proved by experiment, It may also be proved diagrammatically, as a reference 

 to fig. 55, page 415, will show. 



Let a, b of fig 55 represent the horizon ; m, n the line of vibration ; x, c the 

 wing inclined at an upward backward angle of 45° in the act of making the 

 down stroke, and x, d the wing inclined at a downward backward angle of 45 

 and in the act of making the up stroke. When the wing x c descends it will tend 

 to dive downwards in the direction / (giving very little of any horizontal sup- 



