l rotating, commences to pass from the liquid
to the solid state. The form which the earth would assume on
consolidation would, no doubt, be very irregular on the surface; it
would be irregular in consequence of the upheavals and the outbursts
incident to the transformation of so mighty a mass of matter; but
irregular though it be, we can be sure that, on the whole, the form of
the earth's surface would coincide with the shape which it had assumed
by the movement of rotation. Hence we can explain the protuberant form
of the equator of the earth, and we can appeal to that form in
corroboration of the view that this globe was once in a soft or molten
condition.

The argument may be supported and illustrated by comparing the shape of
our earth with the shapes of some of the other celestial bodies. The
sun, for instance, seems to be almost a perfect globe. No measures that
we can make show that the polar diameter of the sun is shorter than the
equatorial diameter. But this is what we might have expected. No doubt
the sun is rotating on its axis, and, as it is the rotation that causes
the protuberance, why should not the rotation have deformed the sun like
the earth? The probability is that a difference really does exist
between the two diameters of the sun, but that the difference is too
small for us to measure. It is impossible not to connect this with the
_slowness_ of the sun's rotation. The sun takes twenty-five days to
complete a rotation, and the protuberance appropriate to so low a
velocity is not appreciable.

On the other hand, when we look at one of the quickly-rotating planets,
we obtain a very different result. Let us take the very striking
instance which is presented in the great planet Jupiter. Viewed in the
telescope, Jupiter is at once seen not to be a globe. The difference is
so conspicuous that accurate measures are not necessary to show that the
polar diameter of Jupiter is shorter than the equatorial diameter. The
departure of Jupiter from the truly spherical shape is indeed much
greater than the departure of the earth. It is impossible not to connect
this with the much more rapid rotation of Jupiter. We shall presently
have to devote a chapter to the consideration of this splendid orb. We
may, however, so far anticipate what we shall then say as to state that
the time of Jupiter's rotation is under ten hours, and this
notwithstanding the fact that Jupiter is more than one thousand times
greater than t