he earth. His enormously rapid rotation has caused him to
bulge out at the equator to a remarkable extent.

The survey of our earth and the measurement of its dimensions having
been accomplished, the next operation for the astronomer is the
determination of its weight. Here, indeed, is a problem which taxes the
resources of science to the very uttermost. Of the interior of the earth
we know little--I might almost say we know nothing. No doubt we sink
deep mines into the earth. These mines enable us to penetrate half a
mile, or even a whole mile, into the depths of the interior. But this
is, after all, only a most insignificant attempt to explore the interior
of the earth. What is an advance of one mile in comparison with the
distance to the centre of the earth? It is only about one
four-thousandth part of the whole. Our knowledge of the earth merely
reaches to an utterly insignificant depth below the surface, and we have
not a conception of what may be the nature of our globe only a few miles
below where we are standing. Seeing, then, our almost complete ignorance
of the solid contents of the earth, does it not seem a hopeless task to
attempt to weigh the entire globe? Yet that problem has been solved, and
the result is known--not, indeed, with the accuracy attained in other
astronomical researches, but still with tolerable approximation.

It is needless to enunciate the weight of the earth in our ordinary
units. The enumeration of billions of tons does not convey any distinct
impression. It is a far more natural course to compare the mass of the
earth with that of an equal globe of water. We should be prepared to
find that our earth was heavier than a like volume of water. The rocks
which form its surface are heavier, bulk for bulk, than the oceans which
repose on those rocks. The abundance of metals in the earth, the gradual
increase in the density of the earth, which must arise from the enormous
pressure at great depths--all these considerations will prepare us to
learn that the earth is very much heavier than a globe of water of equal
size.

Newton supposed that the earth was between five and six times as heavy
as an equal bulk of water. Nor is it hard to see that such a suggestion
is plausible. The rocks and materials on the surface are usually about
two or three times as heavy as water, but the density of the interior
must be much greater. There is good reason to believe that down in the
remote depths of the ear