


1891.] Problem of the Soaring Birds. 789 
ward to any desired angle of obliquity with the horizon. This 
wire must in all cases be kept perpendicular, and the other two 
parallel to the plane. ‘ 
It is obvious that the plane can be pulled backwards through © 
all degrees of the quadrant, and suspended at any position by the 
wires and balances in a state of rest. While the plane hangs 
vertically all its weight will be on the parallel wires, half on each. 
As it is pulled to the rear less and less weight will be on the 
parallel wires, and more and more on the normal wire, until at a 
horizontal position, 90° from the starting point, all weight will be 
on the normal wire, and none on the parallel. 
If we assume the soaring inclination of a bird to be 5° from 
horizontal, and pull the plane back to that angle, the normal 
scale will mark twenty-seven pounds and the parallel one pound 
each, small fractions of weight being neglected. If anangle of 
45° be used, the normal scale will mark about nineteen pounds 
and the two others 9% pounds each. 

Fic. I. 
To operate this device, a time of calm air is chosen, and the 
plane is pulled back to any desired angle, say 45°, the normal 
scale fastened in position, and the weight indications of each 
scale noted. When wind arises, rotate the platform to face it, so 
that the plane will stand squarely across its direction. This is 
not done to expose the plane to any particular direction of wind, 
but to get all the resistance possible out of it. Then observe the 
scales to see what effect wind has on the normal and parallel 
gravity stresses. Any perceptible breeze will be shown by the 
normal scale, which will indicate less and less weight as wind 
increases. If the air moves with sufficient force, all weight- will 
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