134 THE FLIGHT OF BIRDS 



their disposal up-currents at short intervals, and 

 these are just as serviceable to them as the travelling 

 up-current behind a steamer is to the Gull. 



Before I go further it will be best to mention some 

 experiments I once made with a vane which, instead 

 of swinging round to show from what point of the 

 compass the wind was blowing, worked vertically 

 up and down and detected up-currents and down- 

 currents of air. For its large arm it had a thin piece 

 of deal, one foot long by six inches broad, and this 

 was exactly balanced by a lump of lead at the end 

 of the shorter arm. There happened to be at New 

 Romney, where I made these experiments, a number 

 of banks forming barriers of a very convenient 

 height. While standing on a bank only two feet 

 high, its tripod lifting it four feet above the bank, 

 the vane pointed decidedly upwards. Five yards to 

 leeward of a bank six feet high it indicated that the 

 wind blew downwards, making a large angle with the 

 horizon ; there was but rarely an upward gust. Ten 

 yards to leeward of the bank the direction was still 

 mainly downward, but with not unfrequent upward 

 movements. At twenty and at thirty yards' dis- 

 tance the wind came in wild gusts, as often upward 

 as downward. On the windward side of the bank 

 the results were no less remarkable. Twelve yards 

 to windward the vane was not quite steady, but on 

 the whole horizontal. At a distance of six yards 

 there were occasional upward swings ; at four yards' 

 distance there was a decided upward tendency, and 

 this though the bank itself presented only a very 

 gentle incline. These facts set one thinking. If a 

 bank only six feet high is capable of so much, what 



