COMPARATIVE ANATOMf. 157 



expansion when it descends upon the air for the purpose oi 

 moving the body by the reaction of that element. Now, 

 the form and structure of the wing, its external convexity, 

 the disposition and particularly the overlapping of its larger 

 feathers, the action of the muscles and joints of the pinions, 

 are all adapted to this alternate adjustment of its shape and 

 dimensions. Such a twist, for instance, or semirotary mo- 

 tion, is given to the great feathers of the wing, that they 

 strike the air with their flat side, but rise from the stroke 

 slantwise. The turning of the oar in rowing, while the 

 rower advances his hand for a new stroke, is a similar oper- 

 ation to that of the feather, and takes its name from the 

 resemblance, I believe that tliis faculty is not found in the 

 great feathers of the tail. This is the place also for observ 

 ing, that the pinions are so set upon the body as to bring 

 down the wings not vertically, but in a direction obliquely 

 tending towards the tail ; which motion, by virtue of the 

 common resolution of forces, does two things at the same 

 time — supports the body in the air, and carries it forward. 

 The steerage of a bird in its flight is effected partly by the 

 wings, but in a principal degree by the tail. And herein 

 we meet with a circumstance not a little remarkable, Bird^ 

 with long legs have short tails, and in their flight place 

 their legs close to their bodies, at the same time stretching 

 them out backwards as far as they can. In this position the 

 legs extend beyond the rump, and become the rudder ; sup- 

 plying that steerage which the tail could not. 



From the icings of birds, the transition is easy to the^?zs 

 of fish. They are both, to their respective tribes, the instru- 

 ments of their motion ; but, in the work which they have to 

 do, there is a considerable difierence, founded in this circura- 

 Btance. 



Fish, unlike birds, have very nearly the same specific 

 gravity with the element in which they move. In the cas(j 

 of fish, therefore, there is little or no weight to bear up ; 

 what is wanted is only an impulse sufficient to carry the 



