134 THE FLIGHT OF BIRDS 
their disposal up-currents at short intervals, and 
these are just as serviceble 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 J 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 
