It is remarkable as a mechanical fact, that nothing is so permanent in
nature as the Axis of Rotation of any thing which is rapidly whirled.
We have examples of this in every-day practice. The first is the
motion of _a boy’s hoop_. What keeps the hoop from falling?--It is its
rotation, which is one of the most complicated subjects in mechanics.
Another thing pertinent to this question is, _the motion of a quoit_.
Every body who ever threw a quoit knows that to make it preserve its
position as it goes through the air, it is necessary to give it a
whirling motion. It will be seen that while whirling, it preserves its
plane, whatever the position of the plane may be, and however it may
be inclined to the direction in which the quoit travels. Now, this has
greater analogy with the motion of the earth than any thing else.
Another illustration is _the motion of a spinning top_. The greatest
mathematician of the last century, the celebrated Euler, has written
a whole book on the motion of a top, and his Latin treatise _De motu
Turbinis_ is one of the most remarkable books on mechanics. The motion
of a top is a matter of the greatest importance; it is applicable
to the elucidation of some of the greatest phenomena of nature. In
all these instances there is this wonderful tendency in rotation to
preserve the axis of rotation unaltered.--_Prof. Airy’s Lect. on
Astronomy._