t is certainly yielding, and although the solid earth might
decline to bulge at the equator in deference to the diurnal rotation,
that would not prevent the ocean from flowing from the poles to the
equator and piling itself up as an equatorial ocean fourteen miles deep,
leaving dry land everywhere near either pole. Nothing of this sort is
observed: the distribution of land and water is not thus regulated.
Hence, whatever the earth may be now, it must once have been plastic
enough to accommodate itself perfectly to the centrifugal forces, and to
take the shape appropriate to a perfectly plastic body. In all
probability it was once molten, and for long afterwards pasty.

Thus, then, the shape of the earth can be calculated from the length of
its day and the intensity of its gravity. The calculation is not
difficult: it consists in imagining a couple of holes bored to the
centre of the earth, one from a pole and one from the equator; filling
these both with water, and calculating how much higher the water will
stand in one leg of the gigantic V tube so formed than in the other. The
answer comes out about fourteen miles.

The shape of the earth can now be observed geodetically, and it accords
with calculation, but the observations are extremely delicate; in
Newton's time the _size_ was only barely known, the _shape_ was not
observed till long after; but on the principles of mechanics, combined
with a little common-sense reasoning, it could be calculated with
certainty and accuracy.

No. 11. From the observed shape of Jupiter or any planet the length of
its day could be estimated.

Jupiter is much more oblate than the earth. Its two diameters are to one
another as 17 is to 16; the ellipticity of its disk is manifest to
simple inspection. Hence we perceive that its whirling action must be
more violent--it must rotate quicker. As a matter of fact its day is ten

[Illustration: FIG. 72.--Jupiter.]

hours long--five hours daylight and five hours night. The times of
rotation of other bodies in the solar system are recorded in a table
above.

No. 12. The so-calculated shape of the earth, in combination with
centrifugal force, causes the weight of bodies to vary with latitude;
and Newton calculated the amount of this variation. 194 lbs. at pole
balance 195 lbs. at equator.

But following from the calculated shape of the earth follow several
interesting consequences. First of all, the intensity of gravity will
not be the s