11. 



Wind and Flight. 



IT is generally known that all birds turn their heads to the wind 

 when they wish to rise. But the reason of this is not so simple 

 as it is thought to be. If the wind were of perfectly uniform velocity, 

 the bird could gain no assistance from it, and it would make no differ- 

 ence whether he looked down the wind or against it. The moment 

 he left the ground, becoming part of the moving current, he would 

 have, independently of what was gained by his own efforts, no relative 

 velocity. What we feel as a wind might be looked upon as due to 

 the earth revolving within its envelope of air, and this could not 

 possibly affect the bird when once he had left the ground. However 

 strong a breeze may be blowing, people in a balloon will not be con- 

 scious of it, unless they look down towards the earth and see the trees 

 and the houses passing rapidly beneath them. If, however, as he 

 rises, the bird is continually entering more rapid currents, then the 

 inertia due to his having just left a stratum where the velocity is less, 

 will cause the wind to act upon him as upon a kite, and assist in lifting 

 him ; whereas, if he has no inertia or momentum, he will not rise any 

 more than a kite that has broken its string. It is well to insist upon 

 this point, since even Professor Marey ("Vol des Oiseaux," p. 286) 

 seems to have fallen into the error of supposing that a perfectly 

 uniform breeze can be of assistance to a bird in mounting, if he turns 

 his head towards it. 



Mr. R. C. Gilson recently proposed to me to make some experi- 

 ments in order to discover the rate of increase of velocity with 

 increase of altitude, and to determine, if possible, up to what altitude 

 the increase continues. These experiments were carried out, as far 

 as circumstances allowed, with the assistance of Mr. J. T. Gilson 

 last January. The place chosen was New Romney. There the large 

 extent of flat sands and the absence of trees or cliffs render any 

 irregular diversion of current unlikely. Experiments were first made 

 at low levels with an anemometer. The following figures represent 

 the averages struck from a considerable series of observations. It 

 will be seen that with an occasional exception, due no doubt to the 

 gustiness common to almost all winds, the increase in velocity grows 

 less rapid as the elevation increases. 



