8(> ON AERIAL JifAVIGATION. 



dicular to A B, to rppresent the whole pr<issure ; E G being 

 parallfc-'l to the horizon, will represent the propelling forpe ; 

 and G F, perpendicular to it, the supporting- -power. A 

 jbird is supported as efFectuall}' during the return as duraig 

 the beat of its wing; this is chiefly effected by receiving the . 

 resistrtnce of the current under that portion of the wing 

 next the body where its receding motion is so slow as to be 

 of scarcely any effect. The extreme portion of the wing, 

 owing to its velocity, receives a pressure downward and ob- , 

 Hquely forward, which forms a part of the propelling force j 

 gnd at the riame time, by forcing the hinder [)art of the 

 middle portion of the wing downward, so increases its angle 

 with the current, as to enable it still to receive nearly ita 

 usual prcsj;uie from beneath. 

 Flight of tlie As the common rook has its surface and weig))t in tlie 

 common rook. j-^j-Jq ^f ^ square foot to a pound, it may be considered as 

 a standar<l for calculations of this sort; and I shall therefore 

 State, from the average of many careful observations, the 

 movements of that bird, its velocity, represented by C D, 

 Fig. 4, is 34*5 feet per second. It moves its wing up and 

 down ouce in flying over a space of ]2"9feet. Bence, as 

 the centre of resistance of the extreme portion of the wing 

 moves over a space of 0*75 of a foot each beat or return, 

 its velocity is about 4 feet per second, represented by the 

 line E D. As the wing certainly overtakes the current, it 

 must be iaiclined. from it in an angle something less than 7° 

 for at this angle it would scarcely be able to keep parallel 

 with it. unless the waft downward were performed with more 

 x^elocity th;in the return ; which may be and probably is 

 tiie case, though these movements appear to be of equal 

 duvation. The propelling power, represented by EG, un- 

 der these circumstances, cannot be equal to an eighth part 

 of the supporting power G F, exerted upon this portion of 

 the wing; yet this, together with the aid from the return 

 of the wing, has to overcome all the retarding power of the 

 surface, and the direct resistance occasioned by the bulk of 

 the body. 

 Very acutean- It has been before suggested, and I believe upon good 

 hn\c in theii " g''oiiTids, that very acute angles vary little in the degree of 

 Tesistancc resyrtance they make under a ai^pilar velocity of current. 



Hence 



