0?l ABJllAI. N,AYiGATrON. §7 



Hence it is pro.babl.e, tliat this propelling; part of the wini'f 

 receives little more than its comnion proportion of resist- 

 ance, (luring the wait downward. If it be taken fit one 

 tjiird of the whole SLirface, and one eighth of this be al- 

 lowed as the propelling power, it will fmly amount to one 

 tweiity-fonrth of the weight of the bird; .and even this is 

 exerted only half the duration of the flight. The power 

 gained in th.e return of the wing must be added, to render 

 this statement correct, and it is diflicult to estimate this ; 

 yet the following Rtatement prove?, that a greater degree of 

 propelling force is obtained, upon the whole, than the fore- 

 going observations will justify. Suppose the largest circle 

 that can be described in the breast of a crow, to be 12 

 inches in area. Sueh a surface, moving at the velocity of 

 34'5 I'eet per second, would meet a resistance of 0'2l6 of 

 a pound, which, reduced by the proportion of the resist- 

 ance of a spiiere to its great circle (given by Mr. Robins ag 

 1 to 2-,;7) leaves a resistance of 0-09i> of a pound, had the 

 breast been hemispherical. It is probable however, that 

 the curve made use of by Nature to avoid resistance, 

 being so exc[viisitely adapted to its purpose, will reduce this 

 quantity to one half less than the resistance of the sphere, 

 which would ultimately leave 0*0475 of a pound as some- 

 what approaching the true resistance. Unless therefore the 

 return of the wing gives a greater degree of propelling force 

 than the beat, which is improbable, no sycb resistance of 

 the body eould be sustained. Hence, though the eye can- 

 not perceive any distinction between the velocities of the 

 beat and return of the wing, it probably exists, and ex- 

 periment alone can determine the proper ratios between 

 them. 



From these observations we may, however, be justified The act of fly- 

 in the remark — that the act of flying, when properly ad- J^\"^ "xmion 

 justed by the Supreme Author of every power, requires less than common- 

 exortions than, from the appearance, is supposed. ly supposed. 



Broitiplon, Nov.dtli, I80y. 



{To be continued in our next.) 



II. 



