54 
BIRD-FLYING. 
So that when large and heavy birds are feen, as 
Captain Hutton defcribes the albatrofs, whofe wings 
meafure fourteen feet from tip to tip, <c failing and wheel¬ 
ing about in all directions for more than an hour without 
the flighted: movement of the wings,” we are to conclude 
that the ftrength or force-denflty exifting in the winds 
that float the bird is equal to what the wings of the bird 
indicate to be its ability to create did neceflity require it. 
In illuftration of the power of wings in rapid vibration 
to produce flotation, let us for a moment conflder the law 
of atmofpheric reflftance to moving bodies. I quote again 
from Lardner:— 
“ Refiftance of the air to the motion of falling bodies .—- 
It has been fhown that a body obedient to the adion of 
gravity would defcend in a vertical line with a uniformly 
accelerated motion. Its velocity would increafe in pro¬ 
portion to the time of its fall, fo that in ten feconds it 
would acquire ten times the velocity which it acquired in 
one fecond; but thefe concluflons have been obtained on 
the fuppofltion that no mechanical agent ads upon the 
body, fave gravity itfelf. If, however, the body fall through 
the atmofphere, which in pradice it muft always do, it 
encounters a reflftance which augments with the fquare of 
the velocity. Now, as the accelerating force of gravity 
does not increafe, while the reflftance continually increafes, 
this reflftance, if the motion be continued, muft at length 
become equal to the gravitation of the falling body; and, 
