COMPARATIVE ANATOMY. 135 



for, though the stroke upon the air by theunderside o-f her 

 wing would carry her up, the stroke from the upper side, 

 when she raised her wing again, would bring her down. 

 In order, therefore, to account for the advantage which the 

 bird derives from her wing, it is necessary to suppose, that 

 the surface of the wing, measured upon the same plane, is 

 contracted, whilst the wing is drawn up ; and let out to it& 

 full expansion, when it descends upon the air for the pur- 

 pose of moving the body by the reaction of that element. 

 Now the form and structure of the wing, its external con- 

 vexity, the disposition, and particularly the overlapping, of 

 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 semirotatory motion, 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 row- 

 ing, whilst the rower advances his hand for a new stroke,, 

 is a similar operation to that of the feather, and takes its 

 name from the resemblance. I believe that this faculty is 

 not found in the great feathers of the tail. This is the 

 place also for observing, that the pinions are so set on up- 

 on 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 remarka- 

 ble. Birds 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 be- 

 come the rudder ; supplying that steerage which the tail 

 could not. 



From the icings of birds, the transition is easy to the 

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



^ There are three powerful muscles (the fleshy part of the 

 breast) called pectoral muscles, which, with other smaller on the bones 

 of the wing which are analogous to the arm, press with A'igour on the 

 air, the elasticity of which gives support. "And it is remarkable that 

 the general resemblance which the best form of windmill sails bears to 

 the feathers of the wings of birds is striking, and one of those beau- 

 tiful instances of truly mathematical principles on which the works of 

 creation are constructed." Paxton. 



