ame, _i. e._ its
different positions in space are all nearly parallel to each other. This
parallelism, however, is not accurately preserved, so that the axis,
instead of coming exactly into the position which it occupied a year
before, becomes inclined to it at a very small angle, but always
retaining very nearly the same inclination to the plane of the earth's
orbit. This motion of the pole changes the position of the equinoxes by
about fifty seconds annually, and always in the same direction. Thus the
pole-star, after a certain time, will entirely lose its claim to that
appellation, until in the course of somewhat more than 25,000 years the
earth's axis shall again occupy its present angular position, and again
point very nearly as now to the pole-star. This motion of the axis is
called _precession_. It is caused by the attraction of the sun and moon,
and principally the moon, on the protuberant parts of the earth's
equator; and if these parts were solid to a great depth, the motion thus
produced would differ considerably from that which would exist if they
were perfectly fluid, and incrusted over with a thin shell only a few
miles thick. In other words, the disturbing action of the moon will not
be the same upon a globe all solid and upon one nearly all fluid, or it
will not be the same upon a globe in which the solid shell forms
one-half of the mass, and another in which it forms only one-tenth.

Mr. Hopkins has, therefore, calculated the amount of precessional motion
which would result if we assume the earth to be constituted as above
stated; _i. e._ fluid internally, and enveloped by a solid shell; and he
finds that the amount will not agree with the observed motion, unless
the crust of the earth be of a certain thickness. In calculating the
exact amount some ambiguity arises in consequence of our ignorance of
the effect of pressure in promoting the solidification of matter at high
temperatures. The hypothesis least favorable for a great thickness is
found to be that which assumes the pressure to produce no effect on the
process of solidification. Even on this extreme assumption the thickness
of the solid crust must be nearly _four hundred miles_, and this would
lead to the remarkable result that the proportion of the solid to the
fluid part would be as 49 to 51, or, to speak in round numbers, there
would be nearly as much solid as fluid matter in the globe. The
conclusion, however, which Mr. Hopkins announces as t