uscular power of
man, it is now generally accepted, is not sufficient to support his
weight in level flight on still air, but if the principles of flight had
been understood, there was no need to wait for the invention of the
powerful internal-combustion engine; a steam-engine in a well-designed
aeroplane might have performed very useful flights. It was knowledge
that lingered. Newton, when he saw an apple fall in his garden at
Woolsthorpe, 'began to think of gravity extending to the orb of the
moon'. If he had been in the habit of skimming flat stones on calm
water, he might have bent his mind to the problem of flight, and might
even have anticipated some of the discoveries in aerodynamics which were
reserved for the last century--in particular, the relations of speed and
angle of incidence to the reactions of air resistance on a moving plane.
The fact which is the basis of all aeroplane flight is that a perfectly
horizontal plane, free to fall through the air, has its time of falling
much retarded if it is in rapid horizontal motion. This is what makes
gliding possible. Now let the plane which is being propelled in a
horizontal direction be slightly tilted up, so that its front, or
leading edge, is higher than its back, or trailing edge. The reaction of
the air can then be resolved into two components, technically called
'lift' and 'drag'; lift, which tends to raise the plane, and drag, which
retards it in its forward motion. When the angle of incidence of the
plane is small, that is, when it is only slightly tilted from its
direction of motion, the greater part of the air reaction is converted
into lift. This is what makes flying possible. A moderate speed through
the air will enable the plane to lift much more than its own weight.

This is not a technical treatise, but some further facts of signal
importance in the theory and practice of flight are better explained at
once, in so far as the beautiful exactitude of mathematical
demonstration can be expressed in the crudities of popular speech. The
lift produced by the reaction of the air acts on the whole plane, but
not equally on all parts of it. At a flying angle, that is, when the
angle of incidence of the plane is small, the upward force is greatest
on those parts of the plane which are immediately behind the leading
edge. The wings of any soaring bird are long and narrow, and thus are
perfectly designed for their work. A square-winged bird would be a poor
soarer