22 The American Naturalist. [January, 
not added. pis a reaction against gn and hot a force equalto 
gn, acting with it against the soaring force. Atmosphere 
resistance to the bird must be overcome, but weight overcomes 
it, and is itself used up in the exertion. 
I once saw a parody on Jack, the Giant Killer. Jack was ~ 
set upon simultaneously by two giants, either of which could i 
have demolished him at once. He adroitly set them to fight- __ 
ing each other and then cut off their heads. Nature met with 
much the same problem. She desired a soaring bird. Two 
antagonists confronted her, air and weight. She so fashioned 
a bird as to take advantage of the law of fluid pressure, which 
set weight upon air resistance, in which contest they were both : 
destroyed. She outdid Hercules in details of destruction. l 
Pressure first cut weight into two unequal parts, then fell upon : 
the greater and transformed it into a stream of escaping air 
condensation, while the smaller still offered resistance. Then | 
from these condensations, equal to the total normal part, she 3 
obtained enough force through wing details to destroy the ¥ 
parallel part, and still have a large surplus. And all this was 
accomplished by a bird’s wing. It seems a pity that such a 
magnificent piece of work should belong only to fishhawks, 
carrion crows, and the like. 
Further, a single matter must be noticed. By referring to 
3, it will be seen that there is a region behind and above the 
lower surface of the bird, and beneath the front edge, of tri- 
angular shape, that is not affected by the air. If the bird had 
but one motion on the upward slant, any thickness of parts, 
either of wing or body, would resist. But normal motion 
confines air collision to the under side, leaving a confused 
mass of eddies and reacting currents at the upper side. This 
region of eddies may be filled with solid materials and still 
not destroy the effect of a mathematical plane devoid of 
thickness. The front edge must be sharp but the rear edge 
may be overreached, as in 4, without further resistance. Su 
shape will move farther in the same time unit normally than a 
flat shape, while parallel motion is not changed in the slightest 
degree. The least projection above a b increases resistance. 
