278 FACULTIES OF BIRDS. 
strokes upon the air in quick succession; and in 
descending obliquely, these actions are proportion- 
ally slower. 
“In birds of prey the form of the wings is very 
oblique, so that they cannot rise in the air perpen- 
dicularly unless they fly against the wind; they 
have, however, a greater power of horizontal mo- 
tion than other birds, because the extreme parts of 
the wings are long, and the ends of the feathers lap 
over each other, which opposes a uniform resist- 
ance to the air; while in other birds the air passes 
through between the feathers, which lessens the 
power of keeping the wing oblique. To enable 
themselves to turn to the right or left, they move 
one wing more rapidly than the other. This is at- 
tended with difficulty when the flight is rapid; they 
therefore make a large sweep before they can turn 
round.”* 
In the ingenious attempts which have been made 
to devise wings to enable men to fly in the air, it 
has rarely been taken into account that the muscles 
of the most powerful arm are proportionably slen- 
der and weak when compared with the wing-mus- 
cles of birds; and, therefore, even if wings suffi- 
ciently efficient could be contrived, the arms would 
be too feeble to wield them, considering also that 
there are no aircells distributed through the human 
body as in birds, to diminish its specific gravity by 
inflation 
* Home, Comp. Anat., i., 8. 
