
1891.] Problem of the Soaring Birds. 791 
small fractions of force, so that if there be friction down the 
plane caused by slipping air, such resistance would act on the 
parallel scales, but, being of small magnitude, could not be read 
from the index. 
2. Neither the cross-arm in wind, nor the rotating arm in calm 
air, holds the plane against the air. They simply. neutralize the 
parallel factor of gravity, and prevent its acting upon the plane. 
3. The normal gravity component does work upon the air by 
disturbing it in various ways, such as changing its motion, and 
condensing it, and it is the only fraction of weight that meets 
with atmospheric resistance, if we neglect the unknown elements 
of friction. 
4. The experiments are in obedience to the law of normal 
resistance of fluids to surfaces compressing them. 
5. There is no resistance to the plane on the line of motion of 
either plane or air. Even the hypothetical friction is not on such 
line. 
6. Wind or calm air produce identical results ; the only require- 
ment being that air and plane shall meet, motion of either, related 
to the earth, being indifferent. 
The consequences flowing from the above are many and im- 
portant, and in the briefest manner I will notice those which 
seem to demand the earnest attention of every student of nature, 
however abhorrent they may be to’ text-book devotees. 
It is obvious that the wires of the experiment, when the plane 
is adjusted to any obliquity in calm air, resolves weight in the 
same way that an ordinary inclined plane would if the latter were 
rough, the stresses being of the nature of pressure instead of 
tension. 
Then we have in the mathematical formula of inclined planes 
without friction what also applies to oblique atmospheric resist- 
ance. For when the plane of the experiment, held by the wires, 
is submitted to air pressure, the shape, size, and obliquity of that 
plane determines the shape, size, and obliquity of the plane of air 
pressure beneath it, which plane being destitute of friction the 
component along it must be neutralized, as is done by the paral- 
Am. Nat—Sept.—2, 
. 


