220 ANIMAL MECHANISM. 



hawking. Without going further back, we find in Iluber* the 

 description of these curvilinear movements of falcons, to which 

 they gave the name of passades, and which consisted of an 

 oblique descent of the bird, followed by a re-ascent, which 

 they called ressource (from the Latin, resurgere). " The bird," 

 says Iluber, "carried forward by its own velocity, would dash 

 itself against the ground, were it not that it exercises a cer- 

 tain power which it possesses of stopping when at its utmost 

 speed, and turning directly upwards to a sufficient height to 

 enable it to make a second descent. This movement is able 

 not only to arrest its descent, but also to carry it without any 

 further effort, as high, as the level from which it started." 



Surely, there is some exaggeration in saying that the bird 

 can rise, without any active effort, to the height from which 

 it stooped; the resistance of the air must destroy a portion of 

 the force which it had acquired during its descent, and which 

 must be transformed into a rising impulse. We see, how- 

 ever, that the phenomenon of the ressource has been noticed 

 by many observers, and that it has been considered by them 

 as, to a certain extent, a passive motion in which the bird 

 has to employ no muscular force. 



The act of hovering presents, in certain cases, a great ana- 

 logy with the phenomena just described. When a bird a 

 pigeon, fur example has traversed a certain distance by flap- 

 ping its wings, we see it suspend all these movements for 

 some instants, and glide on either horizontally, ascending or 

 descending. The latter kind of hovering motion is that which 

 is of longest duration ; in fact, it is only an extremely slow 

 fall, but in which the weight assists the movement, while 

 it checks it in the horizontal or ascending course. la the last 

 two forms, the wing, directed more or less obliquely, derives 

 its point of resistance from the air, like the child's plaything 

 called a kite, but with this difference, that the vi-lnrity is 

 given to the kite by the tractile force exerted on the string 

 when the air is calm, while the bird when it hovers utilizes 

 the speed which it has already acquired, either by its oblique 

 fall or by the previous flapping of its wings. 



We have already said, that observers had admitted that 



* 8vo. (iclirva, 1784. 



