244 THE STRUCTURE AND LIFE OF BIRDS cnap. 
Birds, like all matter living or dead, are subject 
to physical laws. We must, therefore, not relegate 
soarjng to the realm of miracle, but try to find some 
solution of the problem. 
There are three explanations possible, all of which 
may be true, each of them being applicable to par- 
ticular circumstances. 
(1) There may be upward currents. We have 
already condemned as fantastic Lilienthal’s notion 
that the general tendency of the wind is upward. 
But, as I have shown, in particular places it un- 
doubtedly is so. A Raven or a Vulture soaring over 
a hilltop is probably making use of alternating 
upward and downward currents, or else, if the streams 
of air in that particular spot flow steadily, sweeping 
from a downward into an upward one. This means 
of soaring is, it cannot be doubted, put at his disposal 
through the diversion of the wind when it comes in 
contact with a high ridge. If.the results I have 
described could be produced by a bank only a few 
feet high, what great irregularities, what tremendous 
up and down blasts, may be caused by such a barrier 
as a hill of only moderate height! Gulls are very 
quick in finding out such currents, and enjoy playing 
in them. When the wind is blowing hard against a 
cliff and is diverted upward, they will fly again and 
again to its base and let themselves be carried to the 
top. Or they will float and circle high above a cliff, 
profiting no doubt by the upward slant of the breeze. 
They will often follow a steamer with wings perfectly 
motionless, and the explanation of this, probably, is 
that the passage of the vessel through the air causes 
