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 
nonenear 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 
