1891.] Problem of the Soaring Birds. 797 
close of experimenting in this line I hit upon a method not 
exhibited by the birds, which promises best results, but which 
waits further developments. 
Let us suppose a soaring bird of twenty-seven pounds weight, — 
with stretched wings on an angle of 5° from horizontal, translat- 
ing itself in calm air, or stationary when related to the earth in 
wind. Suppose, further, the initial impulse to have been made 
so that its motion is uniform. 
Then its weight is resolved into a normal and parallel compo- 
nent by the air plane of pressure beneath it. The normal com- 
ponent does work by driving the air out of place, thereby chang- 
ing its motion. The parallel component acts upon the bird, 
tending to drive it on the downward slant, backwards. The air 
in being condensed reacts in all directions, and reaction towards 
the front being resisted by the surface manipulations, the bird is 
driven against friction and the small component, on the upward 
slant, and, completing the diagram of velocities, the result is 
horizontal flight. 


b 
FIG, 2. 
If the parallel factor be supposed out of the way by neutraliza- 
tion arising from surface manipulation, and the air to be calm, 
then the normal factor would drive the plane against air pressure 
from a to å in any given unit of time, say one second. 
Simultaneously with this motion a small excess of parallel force 
over that required to balance the parallel factor—say one-half 
pound constant pressure—would drive the plane from a toc, 
against friction, in which case motion would occur on æ d, the 
resulting line of horizontal flight. 
I have said that direction, and not magnitude of force, was the 
_ vital point in the soaring activity. This fact must now be quite 


sho nue, - | 
. ORS ee As 

