
798 The American Naturalist. [September, 
plain. Magnitude of force depends on weight of plane, and on 
that alone. The gravity stress existing between the plane and 
the earth’s center is the only conceivable force in action, and air 
will meet any demand made upon it by the normal factor of this 
force, against which it is a reaction. This factor is determined 
by the angle of obliquity of plane in each case, and the work 
done upon the air goes on independently of the parallel motion 
of the plane on the upward slant, which alone gives a soaring 
movement. It is obvious that whether wind should move from 
the zenith downwards, or from the earth vertically upwards, or 
horizontally, or from any angle of obliquity whatever, the soaring 
activity is not affected. That goes on as it does in calm air. 
Motion of plane as related to the earth would of course vary with 
wind, if inclination remained the same, but the soaring action, 
considered by itself, is absolutely independent of wind,—meaning 
by that word motion of air as related to the earth. 
I had prepared specimen feathers from the wings of various 
birds during the past twenty-five years, intending to prepare a 
paper for publication on the subject of surface manipulation, 
which I hold to be the most curious and interesting feature about 
a bird, yet almost paralleled by the wonder that it has hitherto 
escaped notice. But we have a rat, if it be a rat, that is endowed 
with a peculiar moral nature. With it, exchange is no robbery. 
A nest of them inhabited the building in which I kept my speci- 
mens, and they bartered what I laid claim to for an indescribable 
lot of miscellaneous rubbish of no earthly use to me. Their 
views and mine were very different. They had cut the feathers 
into small bits and mixed them with dirt, and stowed them away 
in all sorts of holes and corners. I shut acat in the room for 
some days, which caused the rodents to vanish, but my feathers 
were ruined. 
If you will take a primary wing feather of any large bird— 
say a wild goose, or especially a frigate bird——and cut squarely 
across the ribs, and examine the divided part endwise with a 
magnifying glass of low power, you will see nature’s plan of 
surface manipulation to get the forward thrust against the small 
component of weight and air friction. Cut on 4c, and examine 
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