THE PROBLEM OF THE SOARING BIRD. 63 
man and car is due to the man’s force, and not to the train’s 
force. This force works a pair of legs, which set up motion. 
Gravity works in a different way. It requires a device which 
compresses air, as found in the soaring birds. 
Notice also that the mechanical action known as “soaring” 
takes place only between the minimum and maximum velocities 
with which body and air meet. An initial impulse is required in 
all cases to carry the body within those limits. In a calm the 
body would have to be pushed on the air until the minimum was 
reached. In a breeze it would be forcibly held to reach the same 
result. The first impulse resembles pushing an engine off of the 
dead centre. It simply starts the machine. It has nothing what- 
ever to do with its continuous running. Once within the limits 
of ‘‘soaring,” the gravitating force of the body gives a liberal 
supply of power for all the purposes of air-navigation. 
Let us suppose the wing-surfaces to be twelve inches in width, 
and the bird to weigh ten pounds, with wing-expanse sufficient to 
soar in wind moving at the rate of thirty feet per second horizon- 
tally. 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 
horizontal, 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 in 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 
grindstone. 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 towards 
66 
