236 THE STRUCTURE AND LIFE OF BIRDS chap. 



inflated with hydrogen gas in different degrees, and 

 allowing them to race : those which were more dis- 

 tended, rising to a height of 500 feet or more, quite 

 outpaced the smaller ones floating below them. But, 

 as far as I could judge, small differences of elevation 

 at the higher levels did not much affect the velocity. 



Since making these experiments, I have found that 

 far more elaborate ones, with kite-wire suspended 

 anemometers, had already been recorded. They 

 showed that the velocity of the wind increased up to 

 a height of nearly 1,100 feet above the ground, but 

 that the rate of increase there was very slight, the 

 difference being only forty-four feet per minute 

 between the velocities at altitudes ot ;g$ feet and 

 1,095, and eighty-five between 549 and 795. l 



It seems probable, then, that the wind increases in 

 velocity up to a certain altitude, but that above that 

 there is no appreciable increase. This being so, we 

 must look for some other irregularity of which a bird 

 soaring over a level plain can make use when he has 

 passed this limit. It is within even* one's experience 

 that wind often comes in gusts. But we not unfre- 

 quently speak of a steady breeze, meaning one which 

 is almost or entirely free from such fitfulness. Our 

 experiments with an anemometer at New Romney 

 went to show that a stead}- wind, in the strict sense 

 of the term, does not exist. We had to expose the 

 instrument for not less than a minute at a time, make 

 a series of experiments, and strike averages, in order 

 to obtain dependable figures. Professor Langley has 

 by means of a delicate anemometer made more thorough 

 1 Xature, April 22, 1886. 



