J?K0GKESS10N IN OR THROUGH THE AIR. 



109 



In this figure (5 4) the conditions represented at fig. 5 3 (p, 

 107) are exactly reproduced, the only difference being that in 

 the present figure the wing is applied to the air in a more or less 

 horizontal direction, whereas in fig. 53 it is applied in a more 

 or less vertical direction. The letters in both figures are the 

 same. The insects whose wings tack upon the air in a more 

 or less horizontal direction, have an extensive range, each 

 wing describing nearly half a circle, these half circles corre- 

 sponding to the area of support. The body of the insect is 

 consequently the centre of a circle of motion. It corresponds 

 to % of the present figure (fig. 5 4). When the wing is seized 

 by the hand at a, and the root made to travel in the direction 

 n e, the body of the wing travels in the direction ; /. While 

 so travelling, it flies upwards in a double curve, kite-fashion, 

 and elevates the weight L When it reaches the point /, it 

 reverses suddenly to prepare for a return stroke, which is 

 produced by causing the root of the wing to travel in the 

 direction m a, the body and tip travelling in the direction i b. 

 During the reverse stroke, the wing flies upwards in a double 

 curve, kite-fashion, and elevates the weight L The more 

 rapidly these movements are repeated, the more powerful the 

 wing becomes, and the greater the weight it elevates. This 

 follows because of the reciprocating action of the wing, — the 

 wing, as already explained, always drawing a current of air 

 after it during the one stroke, which is met and utilized by 

 it during the next stroke. The reciprocating action of the 

 wing here referred to is analogous in all respects to that ob- 

 served in the flippers of the seal, sea-bear, walrus, and turtle ; 

 the swimming wing of the penguin ; and the tail of the whale, 

 dugong, manatee, porpoise, and fish. If the muscles of the 

 insect were made to act at the points a e, the body of the 

 insect would be elevated as at k I, by the reciprocating action 

 of the wings. The amount of tractile power developed in the 

 arrangement represented at fig. 53 (p. 107), can be readily 

 ascertained by fixing a spring or a weight acting over a pulley 

 to the anterior margin (a h ov e f) of the wing ; weights acting 

 over pulleys being attached to the root of the wing {a or e). 



The amount of elevating power developed in the arrange- 

 ment represented at fig. 54, can also be estimr.ted b}' 



