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1886. ] The Wings of Birds. 703 
the plane of the wings, or the bird’s horizon, so that no weight 
is lifted in the lateral motion, as the body is continually descend- 
ing on its own vertical. This view completely transforms the 
aspect of atmospheric navigation, and brings order out of chaos. 
I have hitherto spoken of the expansion of the compressed air 
in the rear of the wings producing the thrust needed for the lat- 
eral motion. This was done to avoid complicating the case. 
The significant thing is the resolution of gravity and consequent 
shifting of the vertical, and if this fact is seen the method of 
utilizing the pressures presents no difficulty. 
I now wish to direct attention to the structure of a bird’s wing, 
The wing of a pigeon or any of the barnyard fowls may be ex- 
amined, as the peculiarities I desire to point out are found in all 
of the large birds. First, let us examine a feather. If we place 
one on the table with the lower surface upwards and pass the 
7 _hand from the quill to the tip, it will seem smooth to the touch, 
+ while reverse motion meets with much roughness, which in- 
creases as the pressure of the fingers becomes greater. If a 
small tube be taken into the mouth and the breath blown 
quite slanting towards the tip, the feather will remain smooth and 
the air slip over it easily. In the reverse direction the blast will 
tend to open the ribs and increase the roughness. The feather is 
a little like a cat’s back ; a rub toward the rear is pleasant to all 
parties, while one the other way tends to cause motion of the ani- 
mal to the front. Notice also that the tip is very thin and elastic, 
yielding in a graceful curve to slight pressure. The surface of 
the wing is composed mainly of these feathers, having their general 
i direction across, with the tips to the rear. They aré held by the 
integument covering the bones of the wing, which are located on 
the front edge, and while on the upper front side the curve is- 
smooth and regularly rounded, beneath there is a projection 
downwards of the skin, which forms a ledge reaching along the 
humerus to the elbow and thence to the junction of the ulna and 
_ Tadius with the bones of the wrist, being greatest at the elbow. 
Tn the Soaring birds this ledge is largely developed, projecting — 
Wards in the frigate birds one and one-half inches at the- 
elbow, tapering to the wrist and body. This ledge, at the front — 
edge of the wing, seems at first view to be an obstruction to- 
flight, as it makes considerable thickness where the surface first — 
‘Meets the air, but, as we shall presently see, it has an important — 
