Vil FLIGHT 233 
The great principle to be borne in mind is that a 
bird can be helped to rise by a horizontal wind, only 
if he have a velocity of his own relatively to it. A 
man in a balloon that is sailing swiftly with the wind, is 
not conscious of his motion unless he looks down at 
the earth and marks his progress by the hills, trees, 
and houses that he leaves behind. Instead of con- 
sidering that there is a breeze blowing, we may think 
of the earth as turning round without carrying its 
envelope of air with it. Ard this would have no 
effect on a balloon and its occupants. 
A bird is often compared to a kite, and the simile is 
a correct or misleading one according to circumstances. 
If he has velocity of his own and inclines his body 
upward, the wind due to his own passage through the 
air will lift him like a kite. His own momentum takes 
the place of the string, and the force of the current 
that meets him is resolved into two, one acting upward, 
and the other horizontally. But supposing that he 
has no momentum, no velocity of his own, he is a kite 
without a string and is carried with the wind, not 
lifted by it. 
It is possible that a bird may obtain relative velocity 
without any effort of his own by the help of irregularities 
in the wind. There may be currents of different 
velocity forming strata, or it may come in gusts. It 
has long been known that the wind near the earth’s 
surface is retarded by friction. With a view to obtaining 
accurate measurements as to the amount of retardation 
and of the height to which the speed of the wind con- 
tinues to increase, some experiments were undertaken 
by Mr. R. C. Gilson, Mr. J. P. Gilson, and myself, 
