346 NATURAL SCIENCE. May. 1894. 



another occasion several were progressing at right angles to the wind, 

 without any apparent movement of their wings, by flying downwards 

 and then making a rapid turn and facing the wind, which, naturally, 

 lifted them as they rose from the slower to the more rapid currents. 



The annexed diagram will show this clearly. 



At the same place I also made some experiments with a 

 vane working vertically up and down. These were suggested by 

 Lilienthal's book, " Der Vogelflug als Grundlage der Fliegekunst," 

 where he comes to the fantastic conclusion that the normal direction 

 of a wind blowing over a level plain is from 3'^ to 4'"^ upward. If a 

 vane under these conditions were to point upward, it would be 

 more reasonable to regard the fact as an indication of the eccentricity 

 of all vertical vanes, or of that particular one. My own vane had 

 for its larger arm a piece of thin deal i ft. long by 6 in. broad, 

 exactly balanced by a lump of lead attached to the shorter arm. It 

 is true that so small a deviation as 3^' to 4*"^ would be hard to detect, 

 but this instrument seemed to indicate that a wind blowing over a 

 level expanse is perfectly horizontal. Experiments on the direction of 

 the wind on, or on either side of, a small barrier, had more interest 

 for me. While standing on a bank only 2 ft. high, its tripod lifting 

 it 4 ft. above the bank, the vane pointed decidedly upward. Five 

 yards to leeward of a bank 8 ft. high, it indicated that the wind 

 blew downward, making a large angle with the horizon ; there was 

 but rarely an upward gust. Ten yards from the bank the direction 

 was still mainly downward, but with not infrequent upward move- 

 ments. At twenty and thirty yards distance the wind came in wild 

 gusts, as often upward as downward. To windward of a bank only 

 rising 5 ft. above the level at a distance of 12 yards, the vane was 

 not quite steady, but, on the whole, horizontal. At six yards there 

 were occasional upward swings ; at four yards there was a decided 

 upward tendency, and this though the bank itself presented only a 

 very gentle upward incline. 



These results seem to me most important in connection with the 

 soaring of birds. Near a mountain top there must be a perpetual 

 alternation of up and down gusts which a bird can utilise. Happily, 

 there are still hilltops in Great Britain over which the raven and the 

 buzzard can be seen rising in majestic spirals without a motion of 

 the wings. Over a level plain is there any varying velocity available ? 

 It is hardly likely that after, say, 400 feet from the ground there is 

 any appreciable increase of velocity with increase of altitude. There 

 is, however, as I have remarked above, an irregularity of another 

 kind even in the steadiest wind. It comes in gusts, and a gyrating 

 adjutant or vulture, long practised in feeling the pulse of the wind, 

 may perhaps make use of this fact, descending when the breeze blows 

 strong, turning when it slackens, and holding himself so that the 

 breeze, when it freshens again, may lift him like a kite. 



F. W. Headley. 



