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 WDSTG — ADVANCE SIDEWAYS IN A DERECT 

 LINE SOARING SOARING IN A HORIZONTAL WESD 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 manoeuvres 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 



