r and the equatorial axes. In this way the equatorial
diameter has been found equal to 7,927 miles, while the polar diameter
is 27 miles shorter.

The polar axis of the earth may be defined as the diameter about which
the earth rotates. This axis intersects the surface at the north and
south poles. The time which the earth occupies in making a complete
rotation around this axis is called a sidereal day. The sidereal day is
a little shorter than the ordinary day, being only 23 hours, 56 minutes,
and 4 seconds. The rotation is performed just as if a rigid axis passed
through the centre of the earth; or, to use the old and homely
illustration, the earth rotates just as a ball of worsted may be made to
rotate around a knitting-needle thrust through its centre.

It is a noteworthy circumstance that the axis about which the earth
rotates occupies a position identical with that of the shortest diameter
of the earth as found by actual surveying. This is a coincidence which
would be utterly inconceivable if the shape of the earth was not in some
way physically connected with the fact that the earth is rotating. What
connection can then be traced? Let us enquire into the subject, and we
shall find that the shape of the earth is a consequence of its rotation.

The earth at the present time is subject, at various localities, to
occasional volcanic outbreaks. The phenomena of such eruptions, the
allied occurrence of earthquakes, the well-known fact that the heat
increases the deeper we descend into the earth, the existence of hot
springs, the geysers found in Iceland and elsewhere, all testify to the
fact that heat exists in the interior of the earth. Whether that heat
be, as some suppose, universal in the interior of the earth, or whether
it be merely local at the several places where its manifestations are
felt, is a matter which need not now concern us. All that is necessary
for our present purpose is the admission that heat is present to some
extent.

This internal heat, be it much or little, has obviously a different
origin from the heat which we know on the surface. The heat we enjoy is
derived from the sun. The internal heat cannot have been derived from
the sun; its intensity is far too great, and there are other insuperable
difficulties attending the supposition that it has come from the sun.
Where, then, has this heat come from? This is a question which at
present we can hardly answer--nor, indeed, does it much concern