ogether, will be equal to the
longer diameter itself. Now let us conceive such an ellipse. At one of
the points mentioned, which are the _foci_, let us fasten an orange. By
an elastic thread let us connect this orange with a pea; and let us
place this latter on the circumference of the ellipse. Let us now move
the pea continuously around the orange--keeping always on the
circumference of the ellipse. The elastic thread, which, of course,
varies in length as we move the pea, will form what in geometry is
called a _radius vector_. Now, if the orange be understood as the Sun,
and the pea as a planet revolving about it, then the revolution should
be made at such a rate--with a velocity so varying--that the _radius
vector_ may pass over _equal areas of space in equal times_. The
progress of the pea _should be_--in other words, the progress of the
planet _is_, of course,--slow in proportion to its distance from the
Sun--swift in proportion to its proximity. Those planets, moreover, move
the more slowly which are the farther from the Sun; _the squares of
their periods of revolution having the same proportion to each other, as
have to each other the cubes of their mean distances from the Sun_.

The wonderfully complex laws of revolution here described, however, are
not to be understood as obtaining in our system alone. They _everywhere_
prevail where Attraction prevails. They control _the Universe_. Every
shining speck in the firmament is, no doubt, a luminous sun, resembling
our own, at least in its general features, and having in attendance upon
it a greater or less number of planets, greater or less, whose still
lingering luminosity is not sufficient to render them visible to us at
so vast a distance, but which, nevertheless, revolve, moon-attended,
about their starry centres, in obedience to the principles just
detailed--in obedience to the three omniprevalent laws of revolution--the
three immortal laws _guessed_ by the imaginative Kepler, and but
subsequently demonstrated and accounted for by the patient and
mathematical Newton. Among a tribe of philosophers who pride themselves
excessively upon matter-of-fact, it is far too fashionable to sneer at
all speculation under the comprehensive _sobriquet_, "guess-work." The
point to be considered is, _who_ guesses. In guessing with Plato, we
spend our time to better purpose, now and then, than in hearkening to a
demonstration by Alcmaeon.

In many works on Astronomy I find it d