1885.] The Problem of the Soaring Bird. 1167 
thrown into confusion, and the gravitating force of the bird’s 
mass instantly carried it to the water. It was evident that the 
internal adjustments to environing conditions, going on through 
a line of ancestry reaching to the reptiles of the secondary age, 
omitted summer cyclones. They were too rare to count. It was 
also pretty clear that the gravity of the bird’s mass was the source 
of the entire motive power concerned in the act of soaring. 
Were we dealing with wind-mills, sailing vessels, tornadoes or 
any other phenomenon in which the air was one factor anda 
body connected with the earth the other, the force would prop- 
erly be spoken of as coming from the air. The amount of force 
would vary with the velocity of the wind. The work done would 
be referred to the mechanical agency which set the air in motion. 
But a body suspended in free air is part of the atmosphere, and 
at rest with it, unless it employs some activity not derived from 
it. The same mechanical agent which moves the air equally 
moves the body. The active birds derive the force to move 
themselves in the air from their muscular efforts, the soaring 
birds from gravity. Gravity gives a// the motive power; that 
which antagonizes itself and that which antagonizes air resistance. 
he case is analogous to that of a man ona moving train of 
cars. He is at rest with the train throughout unless he employs 
muscular power to set up motion with it. All activity between 
man and car is due to the man’s force and not to the train’s force. 
His 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. Ina breeze it would be forcibly held to reach the same 
result. The first impulse resembles pushing an engine off of 
the dead center. It simply starts the machine. It has nothing 
whatever to do with its continuous running. Once within the 
limits of “soaring,” the gravitating force of the body gives a lib- 
eral 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 
