lustrated by a very common
toy with which every boy is familiar. When a peg-top is set spinning, it
has, of course, a very rapid rotation around its axis; but besides this
rotation there is usually another motion, whereby the axis of the
peg-top does not remain in a constant direction, but moves in a conical
path around the vertical line. The adjoining figure (Fig. 101) gives a
view of the peg-top. It is, of course, rotating with great rapidity
around its axis, while the axis itself revolves around the vertical line
with a very deliberate motion. If we could imagine a vast peg-top which
rotated on its axis once a day, and if that axis were inclined at an
angle of twenty-three and a half degrees to the vertical, and if the
slow conical motion of the axis were such that the revolution of the
axis were completed in about 26,000 years, then the movements would
resemble those actually made by the earth. The illustration of the
peg-top comes, indeed, very close to the actual phenomenon of
precession. In each case the rotation about the axis is far more rapid
than that of the revolution of the axis itself; in each case also the
slow movement is due to an external interference. Looking at the figure
of the peg-top (Fig. 101) we may ask the question, Why does it not fall
down? The obvious effect of gravity would seem to say that it is
impossible for the peg-top to be in the position shown in the figure.
Yet everybody knows that this is possible so long as the top is
spinning. If the top were not spinning, it would, of course, fall. It
therefore follows that the effect of the rapid rotation of the top so
modifies the effect of gravitation that the latter, instead of producing
its apparently obvious consequence, causes the slow conical motion of
the axis of rotation. This is, no doubt, a dynamical question of some
difficulty, but it is easy to verify experimentally that it is the case.
If a top be constructed so that the point about which it is spinning
shall coincide with the centre of gravity, then there is no effect of
gravitation on the top, and there is no conical motion perceived.

[Illustration: Fig. 101.--Illustration of the Motion of Precession.]

If the earth were subject to no external interference, then the
direction of the axis about which it rotates must remain for ever
constant; but as the direction of the axis does not remain constant, it
is necessary to seek for a disturbing force adequate to the production
of th