THE MASTERY OF THE AIR 249 



vulture, gull, raven. It is seen only when 

 there is some breeze, but there may be consid- 

 erable wind overhead when there is little or 

 none near the ground. For long intervals there 

 are no ordinary strokes of the wings, though it 

 is a bold thing to assert that the wings are not 

 moving at all. It is often associated with a 

 tilting of the body, which can be effected by 

 movements of head and neck, shoulder-joint, 

 and tail. It is not due to massive up-currents 

 of air playing upon the under surface of the 

 bird, for it is sometimes exhibited when light 

 objects like feathers are seen sinking slowly in 

 the air. It is highly probable, however, that 

 the sailing bird takes advantage of horizontal 

 currents of unequal velocity in the air. It is 

 also highly probable that the bird having got 

 up some speed by strong strokes sustains this 

 velocity against the wind and rises in its sail- 

 Ing; that it turns and comes down with the 

 wind, getting up, without strokes, sufficient 

 speed to rise again. In other words, it is con- 

 tinually changing "energy of position" into' 

 "energy of motion," and conversely. 



It is important, we think, to remember one's 

 own experience in such an exercise as skating, 

 that, given a certain speed, slight movements 



