it should here be mentioned that the
above figures are taken from Professor F.R. Moulton's _Introduction to
Astronomy_ (1906). They are there stated to be given on the authority of
"Barnard's many measures at the Lick Observatory."

CHAPTER IV

CELESTIAL MECHANISM

As soon as we begin to inquire closely into the actual condition of the
various members of the solar system we are struck with a certain
distinction. We find that there are two quite different points of view
from which these bodies can be regarded. For instance, we may make our
estimates of them either as regards _volume_--that is to say, the mere
room which they take up; or as regards _mass_--that is to say, the
amount of matter which they contain.

Let us imagine two globes of equal volume; in other words, which take up
an equal amount of space. One of these globes, however, may be composed
of material much more tightly put together than in the other; or of
greater _density_, as the term goes. That globe is said to be the
greater of the two in mass. Were such a pair of globes to be weighed in
scales, one globe in each pan, we should see at once, by its weighing
down the other, which of the two was composed of the more tightly packed
materials; and we should, in astronomical parlance, say of this one that
it had the greater mass.

Volume being merely another word for size, the order of the members of
the solar system, with regard to their volumes, will be as follows,
beginning with the greatest:--the Sun, Jupiter, Saturn, Uranus,
Neptune, the Earth, Venus, Mars, and Mercury.

With regard to mass the same order strangely enough holds good. The
actual densities of the bodies in question are, however, very different.
The densest or closest packed body of all is the Earth, which is about
five and a half times as dense as if it were composed entirely of water.
Venus follows next, then Mars, and then Mercury. The remaining bodies,
on the other hand, are relatively loose in structure. Saturn is the
least dense of all, less so than water. The density of the Sun is a
little greater than that of water.

This method of estimating is, however, subject to a qualification. It
must be remembered that in speaking of the Sun, for instance, as being
only a little denser than water, we are merely treating the question
from the point of view of an average. Certain parts of it in fact will
be ever so much denser than water: those are the parts in the centre.
Ot