246 THE STRUCTURE AND LIFE OF BIRDS cuap, 
be seen doing when they advance at right angles to 
the wind without moving their wings. Descending 
with all the velocity of the upper current, he would 
rebound from the slower one below, and the inertia 
due to this undercurrent would make him rise like a 
kite when he re-entered the more rapid stream at a 
higher level. But, as I have shown, it is very improb- 
able that there is a regular progressive increase in 
the velocity of the wind at high altitudes. We know, 
too, that it comes in gusts, and this would bring it 
about that the bird, if he depended on the arrange- 
ment of the air in strata of increasing velocity, would, 
when the breeze happened to strengthen inopportunely, 
find himself in a slower instead of a more rapid 
current. 
Whatever the difficulties that meet us when we 
try. to explain by this principle the phenomenon of 
soaring, it is certain that near the surface of land or 
sea the increase of velocity with altitude is rapid 
enough and constant enough to assist a bird in rising 
or in making progress at right angles to the wind. 
(3) I believe myself that the irregularity of the 
wind may supply the explanation of soaring. The 
wind is a “chartered libertine,” and, even when 
steadiest, blows, as Professor Langley has shown, with 
great fitfulness. A bird, when soaring, if this ex- 
planation be sound, will face a strengthening breeze : 
when it begins to slacken will turn and go with it, 
until conscious it is freshening again, when once more 
he will sweep round and face it, his aim being always 
to feel the wind blowing in his face, sure evidence 
that he has momentum that will lift him. To do 
