not realize
it, maintains the correct attitude of the aeroplane by observing its
position relative to the horizon. Flying into a fog or cloud the horizon
is lost to view, and he must then rely upon his instruments--(1) the
compass for direction; (2) an inclinometer (arched spirit-level) mounted
transversely to the longitudinal axis, for lateral stability; and (3) an
inclinometer mounted parallel to the longitudinal axis, or the airspeed
indicator, which will indicate a nose-down position by increase in air
speed, and a tail-down position by decrease in air speed.

The pilot is then under the necessity of watching three instruments
and manipulating his three controls to keep the instruments indicating
longitudinal, lateral, and directional stability. That is a feat beyond
the capacity of the ordinary man. If, however, by the simple movement
of throttling down the power and thrust, he can be relieved of looking
after the longitudinal stability, he then has only two instruments to
watch. That is no small job in itself, but it is, at any rate, fairly
practicable.

Aeroplanes are, then, designed, or should be, so that the centre of
gravity is slightly forward of centre of lift. The aeroplane is then, as
a glider, nose-heavy--and the distance the C.G. is placed in advance
of the C.L. should be such as to ensure a gliding angle producing a
velocity the same as the normal flying speed (for which the strength of
construction has been designed).

In order that this nose-heavy tendency should not exist when the thrust
is working and descent not required, the centre of thrust is placed a
little below the centre of drift or resistance, and thus tends to pull
up the nose of the aeroplane.

The distance the centre of thrust is placed below the centre of drift
should be such as to produce a force equal and opposite to that due to
the C.G. being forward of the C.L.

LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired, it is best to
throttle the engine down at point A. The C.G. being forward of the C.P.,
then causes the aeroplane to nose-down, and assists the pilot in making
a reasonably small loop along the course C and in securing a quick
recovery. If the engine is not throttled down, then the aeroplane may
be expected to follow the course D, which results in a longer nose dive
than in the case of the course C.

A steady, gentle movement of the elevator is necessary. A jerky movement
may change the direction of motion so sudd