narrow blade effectively, it must be projected
through the air with the long margin forwardly.
Its sustaining power per square foot of surface
is much less if forced through the air lengthwise.

Experiments have shown why a narrow blade
has proportionally a greater lift, and this may
be more clearly understood by examining the
illustrations which show the movement of planes
through the air at appropriate angles.

_Fig. 22. Stream lines along a plane._

STREAM LINES ALONG A PLANE.--In Fig. 22, A
is a flat plane, which we will assume is 10 feet
from the front to the rear margin. For convenience
seven stream lines of air are shown,
which contact with this inclined surface. The first
line 1, after the contact at the forward end, is
driven downwardly along the surface, so that it
forms what we might term a moving film.

The second air stream 2, strikes the first stream,
followed successively by the other streams, 3, 4,
and so on, each succeeding stream being compelled
to ride over, or along on the preceding mass of
cushioned air, the last lines, near the lower end,
being, therefore, at such angles, and contacting
with such a rapidly-moving column, that it produces
but little lift in comparison with the 1st,