HEAD RESISTANCE.--Fig. 5 shows such a plane,
the only resistance being the thickness of the
plane as at A. This is called head resistance,
and on this subject there has been much controversy,
and many theories, which will be considered
under the proper headings.

If a plane is placed at an angle of 45 degrees
the lift and the drift are the same, assumedly, because,
if we were to measure the power required
to drive it forwardly, it would be found to equal
the weight necessary to lift it. That is, suppose
we should hold a plane at that angle with a heavy
wind blowing against it, and attach two pairs of
scales to the plane, both would show the same
pull.

_Fig. 6. Measuring Lift and Drift_

MEASURING LIFT AND DRIFT.--In Fig. 6, A is the
plane, B the horizontal line which attaches the
plane to a scale C, and D the line attaching it to
the scale E. When the wind is of sufficient force
to hold up the plane, the scales will show the same
pull, neglecting, of course, the weight of the
plane itself.

PRESSURE AT DIFFERENT ANGLES.--What every
one wants to know, and a subject on which a
great deal of experiment and time have been expended,