vii 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. And 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. 



