174 THE STORY OF THE BIRDS. 
light body, certain rather heavy vultures float for 
great periods in the air with a much shorter, broader, 
and more rounded wing. Just how the thing is done 
is by no means settled, but to some extent it must be 
a matter of skill on the part of the flyer. It is not 
probable, however, that any sort of wing could do it. 
While albatrosses and others appear so to float 
continuously in calms, it is fairly certain that this sort 
of flight is connected with strong horizontal currents 
by nice adjustment of wing, tail, neck, ete. It has 
been asserted that upward currents are always pre- 
vailing also, but this is not proved yet. 
Various theories have been (and are being) ad- 
vanced, but none of them yet account for all the 
facts, and many are based upon assumptions not yet 
established. In this outlook hes our human hopes of 
flying, and upon it some most abstruse physics are 
brought to bear, too technical for our discussion. 
But we may glance a few minutes at how a bird 
progresses by flapping. As the quills all lap under 
each other with their longer vanes backward, the wing 
is nearly air-tight as the down stroke is made, and it 
becomes loose, like the slats of a window blind, on 
the upstroke. We can see, therefore, how flapping 
lifts. Likewise, the downstroke is more rapid or at 
least more forcible than the upstroke, and the air is 
much more resisting to a rapid stroke than to a slow 
one. Again, the difference in the shape of the lower 
and upper side of the wing is helpful, as noted. 
Now, since the feathers have either their tips or 
their wider vanes backward, as the wing is extended, 
