THE PROBLEM OF THE SOARING BIRD. 65 
it is struck by the plane is, to some extent, carried with it, as 
well as condensed, while those below are at rest. Thus a rotary 
motion may be supposed to take place on the entire lower surface, 
when the air-resistant factor would be neutralised by the excess 
of gravity in each molecule, instead of at the rear edge. 
It has been objected that this is a disguised form of “‘ perpetual 
motion.” Remember that there is a great difference between 
heaping absurdity on a thing and finding absurdity init. It is 
urged that gravity can only do work by the fall of the body 
manifesting it, and that in this case it does not fall; that there 
is no sacrifice of its energy of position, and hence it cannot do 
work. This is true of actions in which the earth, or anything 
fast to or supported by it, is a part, and the gravitating body 
another part; and it is true in the sense that the word “fall” 
means getting nearer the earth. But this is not true with soaring, 
nor with allied phenomena, and I will illustrate the matter by an 
example. Suppose that our hypothetical bird rest in the air in 
a horizontal position, and that the wind moves vertically upwards 
against it at the rate of twenty feet per second. To get the 
attention on the significant features of the case, we will suppose 
the bird to weigh the same as the air which it displaces, thus 
obliterating the gravity factor. It will then have no motion with 
respect to the air, but will move with it. In this condition it is 
not a falling body, and is doing no work. It is simply a body 
resting motionless in air. We will now suppose ten pounds of 
weight to be added to it. It instantly becomes a gravitating body 
doing work on the air. Its motion is accelerated until a velocity 
is reached at which the work done on the air is equal to the force 
doing it, when its motion becomes uniform. Suppose this to be 
twenty feet per second. What have we? A case precisely 
analogous to that of a soaring bird, which is a falling body 
doing work on the air without losing its energy of position. Its 
“fall” is properly related, in a strictly scientific sense, to that 
upon which it is moving, and upon which it is doing work. In 
relation to the earth, or the moon, or the seven stars, it may 
be at rest, as they are not even remotely concerned in the 
matter. 
A soaring bird may therefore be considered a machine for 
distributing air. The motive power required for driving the 
iiechanism is its gravitating force. Its effectiveness consists in 
