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time with his wings, before the impulse communicated by the tirst stroke 

 is over, he rises rapidly, but, on the contrary, descends, it' this motion is 

 delayed. If he allow himself to fall only to the height whence he began 

 to rise, he muy. by a continuance of equal vibrations, keep at the same 

 height. A bird, sometimes, ceases altogether to move his wings, closes 

 them against his sides and falls, with a precipitate motion, like any other 

 weighty body. The name of pouncing is given to the rapid descent of 

 predacious birds on their prey. Observe a falcon drop suddenly on a 

 poultry yard: if on the point of reaching the ground he perceives dan- 

 ger, he immediately spreads his wings, and thus saves himself from 

 falling ; for, whatever velocity he may have acquired in this rapid motion, 

 the resistance of the air always increases, as the squares of the velocity; 

 he then rises anew and takes to flight. This peculiar act is called 

 resource. 



The oblique motions differ from the .vertical motion which has just 

 been described, in this, that the bird rises by a series of curves which 

 are more or less extended, as the motion is more horizontal or vertical. 

 In consequence of the peculiar strength of their wings, birds of prey have 

 a very powerful horizontal motion, so that in soaring, the curves which 

 they describe are so slight, that the motion seems quite horizontal. 



Swimming, to many birds, is a more natural mode of progression, than 

 flying; these birds are very light, their body is covered with a light 

 down; and with feathers over which the waters glides very readily: their 

 body is flattened and rests on the fluid, by a broad surface. Their pelvis 

 is shaped like the keel of a ship ; lastly, their toes, united by webs, strike 

 the water with a very broad surface. This is the case with the numerous 

 tribes of web-footed or water-fowl*. 



They who have conceived it to be possible for man to support himself 

 in the air, by rendering his body specifically lighter, have not consider- 

 ed, that it is impossible to give to the muscles which move the arms, a 

 sufficient degree of strength, to enable them to move the machines, which 

 are adapted to them; and all who have ventured to try such machines, 

 have suffered for their rashness. 



CXCII. Of craiding. All the motions of progression, of which man 

 and animals are capable, may be referred to the theory of the lever of the 

 third kind. The body, in leaping, as in walking, may be compared to 

 an elastic curve, since the point of support, or fulcrum, is in the ground; 

 the force, the spring of power, in the extensor muscles, and the resistance 

 in the weight of the body. What is running, but a succession of short 

 leaps, and is not its mechanism intermediate between walking and leap- 

 ing? Are not flying and swimming real leaps, in which the body of the 

 animal alternately bends and unbends, having its support on media of 

 much less resistance than the ground, on which walking, running, and 

 leaping, are generally performed ? The mode of progression peculiar to 

 serpents and soft reptiles, furnishes an additional application of the lever 

 of the theory of the third kind, The snake, which moves by forming 

 with its body horizontal and vertical undulations, forms, in the course of 



* The faculty of diving 1 , &.c. into a denser medium, possessed by some aquatic birds, 

 is exerted in the same manner as that of flying 1 in the air. Swimming on the surface 

 of the water is performed entirely by means of the webbed feet of this class of 

 birds, Authors- Note. 



