1170 The Problem of the Soaring Bird. { December, 
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 motion- 
less 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 prop- 
erly 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 dis- 
turbing air. The motive power required for driving the mechan- 
ism is its gravitating force. Its effectiveness consists in the 
amount of disturbance which returns to the normal condition of 
the surrounding atmosphere. ; 
There are two peculiarities connected with its action, which 
may be considered accidental or rather incidental, which deserve 
attention, for they are of the first importance in all questions 
relating to artificial air navigation. One of them is, that the ma- 
‘ terial upon which the machine does work becomes the frame 
which supports it, and which is not connected with the earth in 
any way. The other is, that while the motion of the device is in 
all cases in two directions on the air upon which it operates, viz., 
in the direction of the gravitating force and at right angles to it, 
~ inrespect to the earth’s surface its motion is in any direction what- 
_ ever indifferently. These two unique characteristics of soaring 
Constitute its value for artifical use. They are the results of the 
2 ctio n of an inclined plane driven on air, and meet the require- 
ments of atmospheric translation completely, so far as the direc- 
n of mee and supply of motive power is concerned. It 
es ~ balloons to antagonize gravity, and of 
