is would be what the astronomers might take as the mass of
the planet.

With these explanations, let us see how the weight of the earth is
found. The principle we apply is that round bodies of the same specific
gravity attract small objects on their surface with a force
proportional to the diameter of the attracting body. For example, a
body two feet in diameter attracts twice as strongly as one of a foot,
one of three feet three times as strongly, and so on. Now, our earth is
about 40,000,000 feet in diameter; that is 10,000,000 times four feet.
It follows that if we made a little model of the earth four feet in
diameter, having the average specific gravity of the earth, it would
attract a particle with one ten-millionth part of the attraction of the
earth. The attraction of such a model has actually been measured. Since
we do not know the average specific gravity of the earth--that being in
fact what we want to find out--we take a globe of lead, four feet in
diameter, let us suppose. By means of a balance of the most exquisite
construction it is found that such a globe does exert a minute
attraction on small bodies around it, and that this attraction is a
little more than the ten-millionth part of that of the earth. This
shows that the specific gravity of the lead is a little greater than
that of the average of the whole earth. All the minute calculations
made, it is found that the earth, in order to attract with the force it
does, must be about five and one-half times as heavy as its bulk of
water, or perhaps a little more. Different experimenters find different
results; the best between 5.5 and 5.6, so that 5.5 is, perhaps, as near
the number as we can now get. This is much more than the average
specific gravity of the materials which compose that part of the earth
which we can reach by digging mines. The difference arises from the
fact that, at the depth of many miles, the matter composing the earth
is compressed into a smaller space by the enormous weight of the
portions lying above it. Thus, at the depth of 1000 miles, the pressure
on every cubic inch is more than 2000 tons, a weight which would
greatly condense the hardest metal.

We come now to the planets. I have said that the mass or weight of a
heavenly body is determined by its attraction on some other body. There
are two ways in which the attraction of a planet may be measured. One
is by its attraction on the planets next to it. If these bodies did not
a