424 



MOTION. 



winged insects, so as to lie above the centre of 

 gravity. According to Dr. Derham, if either 

 a poiser or winglet be cut off, the insect flies as 

 if one side overbalanced the other, till it falls 

 to the ground ; and if both be removed, it flies 

 unsteadily. Shelver states that the removal of 

 either vvinglets or poisers deprived the insect of 

 the power of flight altogether. The pneumatic 

 pressure which retains them inverted to the 

 ceilings of rooms, gives them a position favour- 

 able for flying off instantaneously, the centre of 

 gravity being below the articulation of the 

 wings, enabling them to regain the pendant 

 position of the trunk in flight. 



In the crane fly the centre of gravity is ad- 

 justed, and the direction of flight directed by 



its long legs ; the two fore legs being extended 

 forwards, and the four hind legs backwards. 

 According to Kirby, the one represents the 

 prow, the other the stern of a ship. The velo- 

 city of the house, and large flesh flies (Muses 

 domestica et vomitoria) appears to be from 

 five to six feet in a second, or about four miles 

 in an hour ; but if favoured by the wind, they 

 are seen flying round the ears of horses when tra- 

 velling at the rate of from ten to twelvemilesm 

 an hour.* 



The following table presents at one view the 

 proportions of the areas of the wings in square 

 inches to their weight in grains of various spe- 

 cies of insects which have been already de- 

 scribed in the text. 



By the help of this table we are enabled to 

 compare the proportions of the area of the wing 

 to the weight of the insect in different orders, 

 and to estimate the relation between these pro- 

 portions and the absolute powers of flight, 

 when the latter have been ascertained by expe- 

 riment. 



If the velocity and power of suspension varied 

 in insects precisely in the same ratio as the 

 areas of the wings to the weight of their bodies, 

 we should be enabled to compare with tolerable 

 accuracy the relative powers of the flight of in- 

 sects from data similar to the preceding, but 

 there are several other mechanical and physio- 

 logical conditions involved ; such as the ratio 

 of the force of the muscles to the areas of the 

 wings, and the figure and structure of the latter. 

 The Lepidoptera which have the greatest 

 surface of wing in proportion to their weight, 

 should surpass all other insects in power of 

 flight, yet the diurnal section at least yield to 

 the Libellulte in velocity, if not in the duration 

 of their suspension in the air. From the pre- 

 ceding data we conclude, that to render a man, 

 whose weight is 150 pounds, capable of sus- 

 pending himself in the air by the assistance of 

 artificial wings, with the same facility as in- 

 sects, would require an extent of surface be- 

 yond the control of his muscular force, and 

 consequently that the act is impossible. 



F/ig/it of Birds. In the organization of 

 birds, we observe that many parts common to 

 other animals are modified, the power of the 

 muscular system is increased, and new forms 

 of matter are introduced to confer on them 

 the power of flight. The bulk of birds is 

 less than that of quadrupeds of equal strength, 

 and owing to many of their bones being per- 

 meated with air, and their skin clothed with 

 fr.itliPi's, their specific gravity, and conse- 

 quently the demand on tl.dr muscular power 



are diminished.f Instead of the cylindrical 

 form observed in animals which move ex- 

 clusively on solids, the anterior extremity of 

 birds is expanded into a triangular surface, of 

 which the apex is the distal, and the base is the 

 proximal section of the wing in reference to the 

 axis of rotation. The arm is articulated to the 

 trunk by a ball and socket joint, permitting all the 

 freedom of motion necessary for flight, whilst in 

 consequence of the axes of motion in the gyngli- 

 moid joints of the fore-arm being directed either 

 perpendicularly or obliquely to the ascent and 

 descent of the wing, it is prevented from yield- 

 ing to the resistance of the air during elevation 

 and depression, and is more conveniently folded 

 on itself during repose. The surface of the 

 wings may be increased or diminished by 

 abduction and adduction, in consequence of 

 which the resistance of the air to their motion 

 may be proportionally varied in the up and 

 down strokes. The amount of this resistance is 

 also varied by the surface of the wing being 

 convex above, and concave below. The feathers 

 are moreover provided with a curious me- 

 chanism by which the barbules lock into each 

 other, so as to unite all the parts of the vane, 

 and present a continuous surface to the air. 

 The ten primary and the secondary feathers, 

 which have the greatest leverage, are inserted 

 into the arm and fore-arm, and directed so 

 as to produce the greatest surface of wing 



* See Kirby and Spence, vol. ii. p. 357. 



t 1. The Pelicanus onocrotalus is five feet in 

 length, but its skeleton weighs only twenty-three 

 ounces, whilst the whole animal weighs twenty-five 

 pounds. See Roget's Bridg. Treat, vol. i. p. 490. 



2. The skeleton of the Carrion Crow when dry 

 weighs only twenty-three grains. Jaquamin, An. 

 Sci. Nat. scries 2, 11, p. 2718. 



3. Many entire Humming Birds weigh only one- 

 eighth of an ounce, or one drachm. 



