THE MASTERY OF THE AIR 249 



gull, raven. It is seen only when there is some 

 breeze, but there may be considerable 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 velo- 

 city against the wind and rises in its sailing ; 

 that it turns and comes down with the wind, 

 getting up, without strokes, sufficient speed to 

 rise again. In other words, it is continually 

 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 



