
” x891.] Problem of the Soaring Birds. 795 
No single force, however great, could drive a plane obliquely 
through air. There must of necessity be not less than two 
stresses, derived either from the application of two forces, or the 
resolution into two components of one. 
I need go no further in this direction, but will add a few words 
regarding my work during the past year and a half, in the way of 
direct flight. 
It is sufficiently obvious that a force of about two pounds’ con- 
stant pressure applied to the plane, either in wind or calm air, in 
the above experiments, to take the place of the cross-arm and 
rotating arm, neutralizing the parallel component of gravity, 
would, if obtained from the constant flow of twenty-seven pounds 
pressure escaping from beneath the surface, produce a true soar- 
ing plane in calm and wind. 
A smooth plane has no tendency to throw two pounds of this 
twenty-seven pounds (over 90°) against the obstructive component. 
The true problem of flight consists in so manipulating the 
surface as to perform this function. I have hitherto taken the 
wing of a soaring bird as a model, and have had unexpected and 
most gratifying success. 
I can produce true soaring flight in natural wind with a plane 
exceeding two pounds to a square foot of surface whenever I 
wish to do so and can obtain wind strong enough for the pur- 
pose. During the above-mentioned period of time I have made 
about fifty planes of various shapes and sizes, and from 25 
' to 400 pounds in weight. These planes are not set free in wind, 
but used as in the experimental cases above, with rigid rods in 
place of the parallel wires. These rods run in large rings and 
have a cross head at their outer ends, allowing the plane to run 
to the front until its edge rests against the rings. In the best 
trials the parallel component is neutralized at 10° from horizon- 
tal, far exceeding my expectations derived from observations of 
the birds; their angle of obliquity being rarely over 5°. 
On a few occasions these planes accidentally escaped me in 
time of highest wind, and were ruined at once for all purposes 
excepting fire-wood ; in each case being a loss of two or three 
months’ work, and playing havoc with my finances. One that I 
