246 



AERONAUTICS. 



without inconvenience.^ It is in this way that an insect 

 which may begin its flight by causing its wings to make 

 figure-of-8 horizontal loops (fig. 71, p. 144), may gradu- 

 ally change the direction of the loops, and make them more 

 and more oblique until they are nearly vertical (fig. 7 3, p. 

 144). In the beginning of such flight the insect is screwed 

 nearly verticolly upwards; in the middle of it, it is screwed 

 ujnvards and forwards; whereas, towards the end of it, the 

 insect advances in a ivaved line almost horizontally (see 

 q\r\s\t' of fig. 72, p. 144). 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 nature of the angles 

 made by the surface of the wing during its vibration with 

 the horizon be distinctly understood ; as it is on these that 

 all flying creatures depend when they seek to elude the up- 

 ward resistance of the air, and secure a maximum of elevating 

 and propelling power with a minimum of slip. 



As to the nature of the Forces required for propelling Arti- 

 ficial Wings. — Borelli, Durckheim, and Marey affirm that it 

 suffices if the wing merely elevates and depresses itself by 

 a rhythmical 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 sur- 

 face of the wing. Eepeated experiment has, however, con- 

 vinced me that the artificial wing must be thoroughly under 

 control, both during the down and up strokes — the details of 

 flight being in a great measure due to the movements com- 

 municated 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 



^ 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 withont difficulty. This arises from the power which the hand has 

 of moving in an upward and downward direction, and from side to side with 

 eqnal facility. Tlie hand can also rotate on its long axis, so that it virtually 

 represents all the movements of the wing at its root. 



