o suspect that the lift of curved
surfaces very little exceeded that of planes of the same size, but
further investigation and experiment led to the opinion that (1) the
anemometer used by us over-recorded the true velocity of the wind by
nearly 15 per cent; (2) that the well-known Smeaton co-efficient of.005
V squared for the wind pressure at 90 degrees is probably too great by
at least 20 per cent; (3) that Lilienthal's estimate that the pressure
on a curved surface having an angle of incidence of 3 degrees equals.545
of the pressure at go degrees is too large, being nearly 50 per
cent greater than very recent experiments of our own with a pressure
testing-machine indicate; (4) that the superposition of the surfaces
somewhat reduced the lift per square foot, as compared with a single
surface of equal area.

'In gliding experiments, however, the amount of lift is of less relative
importance than the ratio of lift to drift, as this alone decides
the angle of gliding descent. In a plane the pressure is always
perpendicular to the surface, and the ratio of lift to drift is
therefore the same as that of the cosine to the sine of the angle of
incidence. But in curved surfaces a very remarkable situation is found.
The pressure, instead of being uniformly normal to the chord of the
arc, is usually inclined considerably in front of the perpendicular.
The result is that the lift is greater and the drift less than if
the pressure were normal. Lilienthal was the first to discover this
exceedingly important fact, which is fully set forth in his book, Bird
Flight the Basis of the Flying Art, but owing to some errors in the
methods he used in making measurements, question was raised by other
investigators not only as to the accuracy of his figures, but even as
to the existence of any tangential force at all. Our experiments confirm
the existence of this force, though our measurements differ considerably
from those of Lilienthal. While at Kitty Hawk we spent much time in
measuring the horizontal pressure on our unloaded machine at various
angles of incidence. We found that at 13 degrees the horizontal pressure
was about 23 lbs. This included not only the drift proper, or horizontal
component of the pressure on the side of the surface, but also the head
resistance of the framing as well. The weight of the machine at the time
of this test was about 108 lbs. Now, if the pressure had been normal to
the chord of the surface, the drift pro