432 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



If the body of a flying animal be in a horizontal position, a wing attached to 

 it in such a manner that its under surface shall look forwards, and make an up- 

 ward angle of 45° with the horizon, is in a position to be applied either verti- 

 cally (figs. 5 and 6, Plate XL), or horizontally (figs. 3, 4, 5, and 6, page 338). 

 Such, moreover, is the conformation of the shoulder joint in insects, bats, and 

 birds, that the wing can be applied vertically, horizontally, or at any degree of 

 obliquity without inconvenience."" It is in this way that an insect which may 

 begin its flight by causing its wings to make figure of eight horizontal loops 

 (vide fig. 8, page 340), may gradually change the direction of the loops, and make 

 them more and more oblique until they are nearly vertical (see fig. 13, page 342). 

 In the beginning of such flight the insect is screwed nearly vertically upwards ; 

 in the middle of it, it is screwed upwards and forwards; whereas, towards the 

 end of it, the insect advances in a waved line almost horizontally (see q, r,s, t, of 

 fig. 11, page 341). The muscles of the wing are so arranged that they can propel 

 it in a horizontal, vertical, or oblique direction. It is a matter of the utmost 

 importance that the direction of the stroke and the angles made by the surfaces 

 of the wing during its vibration with the horizon should be distinctly under- 

 stood, as it is on these that all flying creatures depend when they seek to elude 

 the upward resistance of the air, and secure a maximum of elevating and pro- 

 pelling power with a minimum of slip. 



Nature of the Forces required for Propelling artificial wings. — Borelli, 

 Durckheim, and Marey affirm that it suffices if the wing merely elevates 

 and depresses itself by a rythmical movement in a perpendicular direction, 

 while Chabrier is of opinion that a movement of depression only is required. 

 All those observers agree in believing that the details of flight are due to the 

 reaction of the air on the surface of the wing. Repeated experiment has, how- 

 ever, convinced me that the artificial wing must be thoroughly under control, 

 both during the down and up strokes — the details of flight being in great measure 

 due to the movements communicated to the wing by an intelligent agent. In order 

 to reproduce flight by the aid of artificial wings I find it necessary to employ a 

 power which varies in intensity at every stage of the down and up strokes. The 

 power which I find suits best is one which is made to act very suddenly and 

 forcibly at the beginning of the down stroke, which gradually abates in intensity 

 until the end of the down stroke where it ceases to act in a downward direction. 

 The power is then made to act in an upward direction, and gradually to decrease 

 until the end of the up stroke. The force is thus applied more or less con- 

 tinuously, its energy being increased and diminished according to the position 



* The human -wrist is so formed that if a wing be held in the hand at an upward angle of 45 , 

 the hand can apply it to the air in a vertical or horizontal direction without difficulty. This arises 

 from the power which the hand has of moving in an upward and downward direction, and from side 

 to side with equal facility. The hand can also rotate on its long axis, so that it virtually represents all 

 the movements of the wing at its root. 



