an altitude of 500 feet above sea-level.

The margin of lift decreases with altitude, owing to the decrease in the
density of the air, which adversely affects the engine. Provided the
engine maintained its impulse with altitude, then, if we ignore the
problem of the propeller, which I will go into later on, the margin of
lift would not disappear. Moreover, greater velocity for a given power
would be secured at a greater altitude, owing to the decreased density
of air to be overcome. After reading that you may like to light your
pipe and indulge in dreams of the wonderful possibilities which may
become realities if some brilliant genius shows us some day how to
secure a constant power with increasing altitude. I am afraid, however,
that will always remain impossible; but it is probable that some very
interesting steps may be taken in that direction.

THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift
ratio is highest. In modern aeroplanes it is that angle of incidence
possessed by the surface when the axis of the propeller is horizontal.

THE BEST CLIMBING ANGLE is approximately half-way between the maximum
and the optimum angles.

All present-day aeroplanes are a compromise between Climb and horizontal
Velocity. We will compare the essentials for two aeroplanes, one
designed for maximum climb, and the other for maximum velocity.

ESSENTIALS FOR MAXIMUM CLIMB:

1. _Low velocity_, in order to secure the best lift-drift ratio.

2. Having a low velocity, _a large surface_ will be necessary in order
to engage the necessary mass of air to secure the requisite lift.

[Illustration]

3. Since (1) such a climbing machine will move along an upward sloping
path, and (2) will climb with its propeller thrust horizontal, then a
_large angle relative to the direction of the thrust_ will be necessary
in order to secure the requisite angle relative to the direction of
motion.

The propeller thrust should be always horizontal, because the most
efficient flying-machine (having regard to climb or velocity) has, so
far, been found to be an arrangement of an inclined s