CHAPTER X. 
WIND AND FLIGHT. 
RISING—FLIGHT WITH THE WIND—UNDULATING FLIGHT WITH- 
OUT MOVEMENT OF THE WINGS—-ADVANCE IN A DIRECT LINE 
WITHOUT MOVEMENT OF WING—ADVANCE SIDEWAYS IN A DIRECT 
LINE—SOARING—SOARING IN A HORIZONTAL WIND IMPOSSIBLE. 
No incline, however slight, escapes the notice of 
the bicyclist. He is as sensitive as a spirit-level. 
In the same way there are many birds that detect 
every up-current, even the most local, and make 
use of it, and, no doubt, detect the down-currents 
no less. When they have no serious business on 
hand they will practise manceuvres in the air, making 
the wind, as far as possible, do the work that would 
otherwise fall on their muscles. A wind with an 
upward trend will often lift them as if they were 
feathers and nothing else, provided they throw 
themselves into the correct attitude and hold their 
wings rigidly extended. Nothing but an upward- 
blowing wind can do the whole work of lifting, but 
a wind that is not of uniform velocity may be of 
assistance. In order to profit by the inequality, 
the bird must pass from a comparatively slow current 
of air into a comparatively rapid one. There is 
hardly a bird possessed of the power of flight that 
has not the skill to turn to account the fact that the 
