At the lower turn, position 4, the tail is turned
similar to the angle of position 2, which throws
the rear end of the machine down, and as the
horizontal line of flight is resumed, in an inverted
position, as in position 4, the tail has the same
angle, with relation to the frame, as the supporting
planes.

During this evolution the engine is running, and
the downward plunge develops a tremendous
speed, and the great momentum thus acquired,
together with the pulling power of the propeller
while thus in flight, is sufficient to propel it along
horizontally, whatever the plane surface curve, or
formation may be.

It is the momentum which sustains it in space,
not the air pressure beneath the wings, for
reasons which we have heretofore explained.
Flights of sufficient duration have thus been made
to prove that convex, as well as concave surfaces
are efficient; nevertheless, in its proper place we
have given an exposition of the reasoning which
led to the adoption of the concaved supporting
surfaces.

WHEN CONCAVED PLANES ARE DESIRABLE.--
Unquestionably, for slow speeds the concaved wing
is desirable, as will be explained, but for high