DK PETTTGREW ON THE PHYSIOLOGY OF WINGS. 431 



The instant the wing begins to descend, the cord is more or less relaxed, and a 

 struggle ensues between the air, which endeavours to keep the wing open, and 

 the elastic substances (k k) which endeavour to close it. If the wing is very 

 forcibly depressed, it is kept open till quite near the end of the down stroke, 

 when the elastic bands close it (C), destroy its momentum, and prepare it for 

 the up stroke. This form of wing acts as a short lever (C) during the up 

 stroke, and a long one (A) during the down stroke. It therefore eludes the 

 superimposed air to a great extent when it is being elevated. If it is thought 

 desirable to differentiate the wing still further in imitation of the bird's wing, 

 it is only necessary to add a series of segments similar to those represented at 

 fig. 65, page 425, these segments representing the individual rowing feathers. 

 What especially struck me on analysing the movements of the artificial bat and 

 bird's wing, was the fact, that during their vibrations figure of 8 curves were 

 developed along their anterior and posterior margins similar to those found in 

 the living wings ; that the under surfaces of the pinions made various angles of 

 inclination with the horizon analogous to those made by the natural wings ; 

 these changes being induced in a great measure independently of the air, in 

 virtue apparently of inherent structural peculiarities. This I regard as a very 

 remarkable circumstance, and one well worthy the attention of the physiologist 

 and mechanician. 



How to apply Artificial Wings to the Air. — Borelli, Durckheim, Marey, and 

 all the writers with which I am acquainted, assert that the wing should be 

 made to vibrate vertically. I believe that if the wing be in one piece it should 

 be made to vibrate obliquely and more or less horizontally. If, however, the wing 

 be made to vibrate vertically, it is necessary to supply it with a ball and socket 

 joint, and with springs at its root (m n of fig. 62, page 423), to enable it to 

 leap forward in a curve when it descends, and in another and opposite curve 

 when it ascends {vide a, c, e, g, i of fig. 14, page 344). This arrangement practi- 

 cally converts the vertical vibration into an oblique one. If this plan be not 

 . adopted the wing is apt to foul at its tip. In applying the wing to the air it 

 ought to have a figure of 8 movement communicated to it either directly or 

 indirectly. It is a peculiarity of the artificial wing properly constructed, (as it 

 is of the natural wing), to tivist and untwist and make figure of 8 curves during 

 its action (see a b, c d of fig. 62, page 423), this enabling it to seize and let go 

 the air with wonderful rapidity, and in such a manner as to avoid dead points. 

 If the wing be in several pieces it may be made to vibrate more vertically than 

 a wing in one piece, from the fact that the outer half of the pinion moves for- 

 wards and backwards when the wing ascends and descends so as alternately to 

 become a short and long lever. (Compare C with A of fig. 69, page 430), this 

 arrangement permitting the wing to avoid the resistance experienced from the air 

 during the up stroke, while it vigorously seizes the air during the clown stroke. 



VOL. XXVI. PART II. 5 T 



