vii FLIGHT 



investigations, and his results show that the actual 

 variation is far greater than would ever have been 

 thought likely. 1 When he measured the velocity at 

 intervals of seven to seventeen seconds at an altitude of 

 fifty-three feet, it varied from ten to twenty-five miles 

 per hour, and the inertia of the anemometer may, as he 

 says, have reduced the apparent variability. The greater 

 the velocity of the wind, the greater was the fluctua- 

 tion. On one occasion he found that a wind blowing 

 forty miles an hour would almost in an instant drop to 

 a calm. His anemometer once stopped dead for one 

 second in a high wind. Another trial, when the 

 velocity was measured every second, showed that a 

 wind of twenty-three miles an hour may in ten seconds 

 rise to thirty-three miles, within ten seconds fall 

 again to twenty-three, then in another thirty seconds 

 rise to thirty-six. As to the cause or the nature of 

 the apparent gusts it is difficult to speak positively. 

 It is impossible that the onward movement of a large 

 volume of air in motion can be suddenly checked. 

 Possibly the irregularity may be due to eddies, the 

 anemometer during an apparent lull being really within 

 the centre or the back current of an eddy, whereas 

 during what seems a sudden gust it is in the eddv's 

 onward sweep. But, whatever the explanation, the 

 inequality certainly exists, ready for a bird who has 

 the skill to make use of. 



But is the wind blowing over the level plain hori- 

 zontal ? No one would imagine that it is otherwise, 



1 American Journal of s v. January 1S94: '"Internal 



Work of the Wind." by Professor S. P. Langley. [Since 

 published separately.] 



