BIRD-FLTING. 
53 
the balloons of the Teas; lighter than the waters of the 
ocean, on the furface of which they float, to navigate it 
were as impoflible to them as are the ftretches of the air 
to veflels floated by hydrogen were the deep charged with 
water-forces of analogous character to thofe that fill our 
atmofphere. 
The problem of air-locomotion is, therefore, one of 
Force ; and bird-flying can only become poflible to man 
when men are changed to birds. 
If we regard the wings of birds in their primary 
capacity of fan-blowers for creating air-currents to fly in, 
then the longer the wing the ftronger the current of air, 
and the greater the volume of it when produced by the 
wings. For example, let us fuppofe the wings when 
defcribing a complete circle to fet an air-current in motion 
of a diameter equal to the ftretch of the wings from tip to 
tip. Wings then meafuring two feet from tip to tip 
fhould create an air-column four times as great as wings 
meafuring but one foot from tip to tip; and wings 
meafuring three feet from tip to tip fhould create an air- 
current or column nine times as great as wings meafuring 
but one foot from tip to tip; and wings meafuring four 
feet from tip to tip fhould create an air-current or column 
fixteen times as great as wings meafuring but one foot 
from tip to tip; and fo on by fquares. The long axis of 
the bird being praddically the axis of the air-column 
created by the wings. 
