PROGRESSION IN OR THROUGH THE AIR. 



151 



pushing the body uptuards and forwards. It is necessary that 

 the wings descend further than they ascend ; that the wings 

 be convex on their upper surfaces, and concave on their under 

 ones: and that the concave or biting surfaces be brought 

 more violently in contact with the air during the down stroke 

 than the convex ones during the up stroke. The greater 

 range of the wing below than above the body, and of the 

 posterior margin below than above a given line, may be 

 readily made out by watching the flight of the larger birds. 

 It is well seen in the upward flight of the lark. In the 

 hovering of the kestrel over its quarry, and the hovering of 

 the gull over garbage which it is about to pick up, the wings 

 play above and on a level with the body rather than below 

 it ; but these are exceptional movements for special purposes, 

 and as they are only continued for a few seconds at a time, 

 do not aff'ect the accuracy of the general statement. 



Points wherein the Screws formed by the Wings differ from those 

 employed in navigation. — 1. In the blade of the ordinary screw 

 the integral parts are rigid and unyielding, whereas, in the 

 blade of the screw formed by the wing, they are mobile and 

 plastic (figs. 93, 95, 97, pp. 174, 175, 176).. This is a curious 

 and interesting point, the more especially as it does not seem 

 to be either appreciated or understood. The mobility and 

 plasticity of the wing is necessary, because of the tenuity of 

 the air, and because the pinion is an elevating and sustaining 

 organ, as well as a propelling one. 



2. The vanes of the ordinary two-bladed screw are short, 

 and have a comparatively limited range, the range corre- 

 sponding to their area of revolution. The v/ings, on the 

 other hand, are long, and have a comparatively wide range; 

 and during their elevation and depression rush through 

 an extensive space, the slightest movement at the root or 

 short axis of the wing being followed by a gigantic up 

 or down stroke at the other (fig. 56, p. 120; figs. 64, 65, 

 and 66, p. 139 ; figs. 82 and 83, p. 158). As a consequence, 

 the wings as a rule act upon successive and undisturbed strata 

 of air. The advantage gained by this arrangement in a thin 

 medium like the air, where the quantity of air to be com- 

 pressed is necessarily great, is simply incalculable. 



