e position of the terrestrial axis during each successive
year, would cause it to describe an entire circle of nearly 50 deg. in
diameter, in a period of about 26,000 years.

Newton conjectured that this force arose from the action of the sun and
moon upon the redundant matter accumulated in the equatorial regions of
the earth: thus he made the precession of the equinoxes depend upon the
spheroidal figure of the earth; he declared that upon a round planet no
precession would exist.

All this was quite true, but Newton did not succeed in establishing it
by a mathematical process. Now this great man had introduced into
philosophy the severe and just rule: Consider as certain only what has
been demonstrated. The demonstration of the Newtonian conception of the
precession of the equinoxes was, then, a great discovery, and it is to
D'Alembert that the glory of it is due.[27] The illustrious geometer
gave a complete explanation of the general movement, in virtue of which
the terrestrial axis returns to the same stars in a period of about
26,000 years. He also connected with the theory of gravitation the
perturbation of precession discovered by Bradley, that remarkable
oscillation which the earth's axis experiences continually during its
movement of progression, and the period of which, amounting to about
eighteen years, is exactly equal to the time which the intersection of
the moon's orbit with the ecliptic employs in describing the 360 deg. of the
entire circumference.

Geometers and astronomers are justly occupied as much with the figure
and physical constitution which the earth might have had in remote ages
as with its present figure and constitution.

As soon as our countryman Richer discovered that a body, whatever be its
nature, weighs less when it is transported nearer the equatorial
regions, everybody perceived that the earth, if it was originally
fluid, ought to bulge out at the equator. Huyghens and Newton did more;
they calculated the difference between the greatest and least axes, the
excess of the equatorial diameter over the line of the poles.[28]

The calculation of Huyghens was founded upon hypothetic properties of
the attractive force which were wholly inadmissible; that of Newton upon
a theorem which he ought to have demonstrated; the theory of the latter
was characterized by a defect of a still more serious nature: it
supposed the density of the earth during the original state of fluidity,
to be homog