66 THE ZOOLOGIST. 
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 
material 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, 
in respect to the earth’s surface its motion is in any direction 
whatever indifferently. These two unique characteristics of 
soaring constitute its value for artificial use. They are the 
results of the action of an inclined plane driven on air, and meet 
the requirements of atmospheric translation completely, so far as 
the direction of motion and supply of motive power is concerned. 
It effectually disposes of balloons to antagonise gravity, and of 
motors to drive the machine against the air. Two requirements 
are still needed. The device must be steered and kept in position. 
When it is remembered that the rear expansion drives the bird 
forward, any change in the extent or position of one wing which 
is not shared by the other would retard or accelerate the motion 
of the changed side relatively to the other, and serve to determine 
the direction of motion. A balance still needs to be preserved. 
The bird gives significant hints also. The heavier it is the 
steadier is its motion, and a device large enough to sustain the 
weight of a man may be qualified by its inertion to assist in 
preserving its own equipoise. 
The most important points of this subject have now been given. 
Experiments with artificially-induced currents of air, requiring a 
steam-engine to be transported to the sands of Florida, were ex- 
pensive, and productive of only negative results. Much time and 
means were wasted in this way, and all my efforts previous to the 
summer cyclone above recited seemed abortive, excepting in the 
settlement of the facts of soaring as shown by the birds. The 
moment the idea that gravity was the motive power, and not the air, 
was entertained, the whole matter became luminous. No steps sub- 
sequently taken have been in the wrong direction, and I shall return 
to the prosecution of the subject the moment prudence permits. 
