science and invention induce us to believe we are far and away
the cleverest of all the living creatures in the great scheme of
Creation. And yet in the matter of flight the birds beat us;
what has taken us years of education, and vast efforts of
intelligence, foresight, and daring to accomplish, is known by
the tiny fledglings almost as soon as they come into the world.

It is easy to see why the story of aviation is of such romantic
interest. Man has been exercising his ingenuity, and
deliberately pursuing a certain train of thought, in an attempt
to harness the forces of Nature and compel them to act in what
seems to be the exact converse of Nature's own arrangements.

One of the mysteries of Nature is known as the FORCE OF GRAVITY.
It is not our purpose in this book to go deeply into a study of
gravitation; we may content ourselves with the statement, first
proved by Sir Isaac Newton, that there is an invisible force
which the Earth exerts on all bodies, by which it attracts or
draws them towards itself. This property does not belong to the
Earth alone, but to all matter--all matter attracts all other
matter. In discussing the problems of aviation we are concerned
mainly with the mutual attraction of The Earth and the bodies on
or near its surface; this is usually called TERRESTRIAL gravity.

It has been found that every body attracts very other body with a
force directly proportionate to its mass. Thus we see that, if
every particle in a mass exerts its attractive influence, the
more particles a body contains the greater will be the
attraction. If a mass of iron be dropped to the ground from the
roof of a building at the same time as a cork of similar size,