Every aeroplane follows the type set by nature
in the particular that the body is caused to oscillate
on a vertical fore and aft plane while in
flight. The bird has one important advantage,
however, in structure. Its wing has a flexure at
the joint, so that its body can so oscillate independently
of the angle of the wings.

The aeroplane has the wing firmly fixed to the
body, hence the only way in which it is possible
to effect a change in the angle of the wing is by
changing the angle of the body. To be consistent
the aeroplane should be so constructed that the
angle of the supporting surfaces should be movable,
and not controllable by the body.

The bird, in initiating flight from a perch, darts
downwardly, and changes the angle of the body to
correspond with the direction of the flying start.
When it alights the body is thrown so that its
breast banks against the air, but in ordinary flight
its wings only are used to change the angle of
flight.

ANGLE AND DIRECTION OF FLIGHT.--In order to
become familiar with terms which will be frequently
used throughout the book, care should be
taken to distinguish between the terms angle and
direction of flight. The former has reference to
the up and down movement of an aeroplane,