phenomena, which theory _is_ competent to remove. Some of these we shall
notice presently; and, it may be remarked, that it is in those minute
quantities which, in astronomy, are usually attributed to errors of
observation, that this theory will eventually find the surest evidence
of its truth.

KEPLER'S THIRD LAW ONLY APPROXIMATELY TRUE.

But it may be asked: If there be a modifying force in astronomy derived
from another source than that of gravitation, why is it that the
elements of the various members of the system derived solely from
gravitation should be so perfect? To this it may be answered, that
although astronomers have endeavored to derive every movement in the
heavens from that great principle, they have but partially succeeded.
Let us not surrender our right of examining Nature to the authority of a
great name, nor call any man master, either in moral or physical
science. It is well known that Kepler's law of the planetary distances
and periods, is a direct consequence of the Newtonian Law of
gravitation, and that the squares of the periodic times ought to be
proportional to the cubes of the mean distances. These times are given
accurately by the planets themselves, by the interval elapsing between
two consecutive passages of the node, and as in the case of the ancient
planets we have observations for more than two thousand years past,
these times are known to the fraction of the second. The determination
of the distances however, depends on the astronomer, and a tyro in the
science might suppose that these distances were actually measured; and
so they are roughly; but the astronomer does not depend on his
instruments, he trusts to _analogy_, and the mathematical perfection of
a law, which in the abstract is true; but which he does not know is
rigidly exact when applied to physical phenomena. From the immense
distance of the planets and the smallness of the earth, man is unable to
command a base line sufficiently long, to make the horizontal parallax a
sensible angle for the more distant planets; and there are difficulties
of no small magnitude to contend with, with those that are the nearest.
In the occasional transit of Venus across the sun, however, he is
presented with a means of measuring on an enlarged scale, from which the
distance of the sun is determined; and by _analogy_ the distance of all
the planets. Even the parallax of the sun itself is only correct, by
supposing that the square of the p