PKOGRESSION IN OE THROUGH THE AIR. 



105 



body falls, and vice versa] the wing describing the arc of a 

 large circle (/ /'), the body {h), or the weights representing it 

 {v) to') describing the arc of a much smaller circle. The body, 



jr 



Fig. 52. 



therefore, as well as the wing, rises and falls in flight. When 

 the wing descends it elevates the body, the wing being active 

 and the body passive ; when the body descends it elevates 

 the wing, the body being active and the wing passive. The 

 elevator muscles, and the reaction of the air on the under 

 surface of the wing, contribute to its elevation. It is in this 

 manner that weight forms a factor in flight, the wing and the 

 weight of the body recij)rocating and mutually assisting and 

 relieving each other. This is an argument for employing 

 four wings in artificial flight, the wings being so arrranged 

 that the two which are up shall always by their fall mechani- 

 cally elevate the two which are down. Such an arrangement 

 is calculated greatly to conserve the driving power, and, as a 

 consequence, to reduce the weight. It is the upper or dorsal 

 surface of the wing which more especially operates upon the 

 air during the up stroke, and the under or ventral surface 

 which operates during the down stroke. The wing, which at 

 the beginning of the down stroke has its surfaces and margins 

 (anterior and posterior) arranged in nearly the same plane with 

 6 



