1168 The Froblem of the Soaring Bird. [ December, 
to soar in wind moving at the rate of thirty feet per second hori- 
zontally. Why does not the body fall? It is true that there is a 
stiff wind moving against it horizontally, but the gravitating force 
is vertical and can be in no way influenced by a horizontal force. 
The ball shot from a level cannon falls precisely as fast as one 
dropped from the mouth of the gun. It is evident that the body 
is indifferent to the horizontal air. “This does not act upon it at 
all. No particle of air influences it but what is in contact with 
its surface, and the instant it is in contact it ceases to be horizon- 
tal, being deflected in numberless different directions. In a strict 
sense, in a sense which alone represents the true character of this 
phenomenon, the air can only be considered as quiescent in every 
case of soaring. In every case the air is a dead calm until it 
comes in actual contact with the body, and the movement of the 
body on the air is a consequence of force derived from the body 
and not from the air. It is a parallel case with the boy and grind- 
stone. From the reciprocal nature of action and reaction, the 
air is doing as much work on the bird as the latter is on the air. 
The grindstone is doing as much work on the boy as he is on the 
grindstone, still it would never do to say that the latter turned the 
boy. 
If gravity, then, be the motive power of a soaring bird, how 
does it act to produce the results? Vertically downwards to- 
wards the center of the earth precisely as it does in all other 
cases, and the reason that the body manifesting it does not get 
lower, is because something is pushing up, against the under sur- 
face, just hard enough to balance the weight. It may be hard to 
follow all the peculiarities of the disturbances going on under 
the bird, but it is certain that they serve to hold it up. They are 
mainly condensations of air upon which the body is falling, and 
are equal to ten pounds in each foot of air passing to the rear. 
_ This ten pounds of force is moving at the rate of thirty feet per: 
=~- Second, as we assumed at the start, and it follows that an amount 
capable of holding up 300 pounds each second is passing the 
rear edge of the bird’s wings and is wasted in falling to the ten- 
sion of the surrounding air, 
: Bat this is not all the force of disturbance which passes to the 
ear. The reactions against air resistance also go there. These, 
the others, consist in condensations, accelerations and deflec- 
ig to the: Mr of the composition and EE 
