428 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



imitate the bat and bird's wing successfully it is necessary to introduce joints : 

 the artificial wing, in fact, requires to be composed of several pieces, so that 

 it will flex or fold towards the end of the down stroke, and open out or expand 

 towards the end of the up stroke. This is requisite for several reasons. In 

 the first place, the wing of the bat and bird is made to vibrate in a much more 

 vertical direction (figs. 5 and 6, Plate XL, figs. 18 and 19, Plate XIV.,) than 

 that of the insect (fig. 4, Plate XL) They have therefore to contend directly 

 with the resistance furnished by the superimposed air. As a consequence, the 

 wing in such of the bats and birds as do not sail or skim must be folded more 

 or less completely during the up stroke to diminish the wing area, so as to 

 elude the resistance offered by the air when the wing is being elevated. It is 

 for this reason too, that in the bird the rowing feathers open up or separate 

 during the up stroke. As the wings of the bat and bird afford comparatively 

 little support during the up stroke, it follows that the wing area must be 

 increased to its utmost during the clown stroke. But for the folding or clos- 

 ing of the wing towards the termination of the down stroke, the downward 

 passage of the pinion, as I have repeatedly satisfied myself by experiment, 

 could not be suddenly arrested and a new upward passage commenced. In 

 other words, the wing could not be reversed. At the beginning of the down 

 stroke the wing is a long lever, and acts as such, [vide c d of fig. 6, Plate XL) 

 It is depressed with extreme energy and acquires during its descent a degree 

 of momentum which could not possibly be checked if the wing was not sud- 

 denly flexed and instantly converted from a long into a short lever, (vide a b of 

 fig. 6, Plate XL) The wing is therefore by this very simple contrivance, not 

 only robbed of its momentum, but what is quite as important, it is prepared 

 for making the up or return stroke. If the wing of a gull just dead be taken, 

 and the air winnowed by it in a more or less vertical direction, it will be found 

 to fly open and to extend itself during the down stroke, and to fold up or close 

 during the up stroke. The quicker the wing is made to vibrate, the more 

 admirable is the result. Indeed, the gull's wing, when made to oscillate as 

 recommended, reverses perfectly and has no dead points. It moreover furnishes 

 a steady persistent buoying power which is quite remarkable when the limited 

 dimensions of the pinion are taken into account. 



To construct a bat or bird's wing, I take a tapering flexible reed, and cut 

 it into three pieces, each piece varying in length. These I bend to the shape 

 required as shown at a, d, and g of figs. 67 and 68, page 429. 



The shortest and thickest piece (a) I furnish with a ball and socket joint at 



one end (x), and a hinge joint (b) at the other. The second shortest and 



strongest piece (d) I supply with a hinge joint at either end (b and e) ; and the 



• third piece, which is the longest and weakest, I provide with a hinge joint at its 



thicker or proximal end (<?). When the three pieces are joined together as 



