H. Sxey.—Introduction of the Tension Wheel in Aerial Transit. 175 
tip end is also raised, so that the feather becomes less eurved longitudinálly, 
and assumes the figure of a thin and slightly bent knife, with the concave 
side downwards, the serew of the feather unwinding as it were. 
If the feathers and wings of birds were straight when quiescent, and if 
every transverse section were inclined to the same angle—if, for instance, 
they were planes when not in action—then, when they came to be acted 
. on by the air, they would, unless perfectly rigid, lose that proper figure 
essential to buoyancy, for the tips would twist more than the other parts, 
and the longitudinal section would become convex downwards. Perfect 
rigidity of a plane would necessitate too much weight. 
We therefore see them formed with a twist or screw in the reverse 
direction to that in which the air itself twists them when in action. Then, 
when they come into action, they assume of themselves the proper and 
best figure, so that every part can act at nearly the same angle upon the 
air. 
The action of the air itself confers the requisite rigidity, and the greater 
the speed or pressure, so much greater is the rigidity ; the wing then assumes 
a knife-like figure approaching to a plane but slightly concave downwards. 
This appears to be the figure of the wings of birds while sailing and 
wheeling, for when they are viewed when the eye is in the same plane as 
the wing they appear as shown in fig. 3, in which the wings are represented 
as mere lines. It will be noticed in the case of this wheel that this is also 
the figure which the tensile radii assume when in action. "e 
The method here adopted allows of the very important rente of 2 
gene extremely thin anterior edges for the planes, whereby the air is cut, so 
speak, for the nearer they approach toa mathematical line the better, as 
the we to the horizontal motion of t he radii is surprisingly lessened. 
When these planes are revolved with the same velocity as the wind, it is 
 elear that even in a gale the wind cannot give any pressure — them 
except on the under and elevating sides. 
Sailing birds first acquire a great initial esed generally by flapping 
their wings. Naturalists and observers differ as to the direction of the 
 vibratory motion of the wing in this kind « of flight, probably on account of 
different birds being selected as examples, and omitting in some cases to 
mention whether the forward motion of the bird itself is included in the 
asserted direction of the stroke. For instance, a bird fastened by a string, 
or one which may be rising vertically, or, better still, a hovering bird, 
whieh we will assume to strike vertically downwards with its wings, would 
not really when in transit move them vertically downwards, although the 
stroke or attempt might be so; for the „downward stroke compounds with 
the horizontal motion of the bird producing a forward and downwards or 
m 
