y resorted to, on account
of the varying declination of the moon, and the inconstancy of the
horizontal refractions, which are perpetually changing according to the
state the atmosphere is in at the time. For the moon continues but for
a short time in the equinoctial, and the refraction at a mean rate
elevates her apparent place near the horizon, half as much as her
parallax depresses it(54)."

(54) Astronomy, p. 265.

"It is well known that the parallax of the sun can never exceed nine
seconds, or the four-hundredth part of a degree(55)." "Observations,"
says Halley, "made upon the vibrations of a pendulum, to determine these
exceedingly small angles, are not sufficiently accurate to be depended
upon; for by this method of ascertaining the parallax, it will sometimes
come out to be nothing, or even negative; that is, the distance will
either be infinite, or greater than infinite, which is absurd. And, to
confess the truth, it is hardly possible for a person to distinguish
seconds with certainty by any instruments, however skilfully they may
be made; and therefore it is not to be wondered at, that the excessive
nicety of this matter should have eluded the many ingenious endeavours
of the most able opetators."(56).

(55) Ibid, p. 268.

(56) Phil. Transactions, Vol. XXIX, p. 456.

Such are the difficulties that beset the subject on every side. It is
for the impartial and dispassionate observers who have mastered all the
subtleties of the science, if such can be found, to determine
whether the remedies that have been resorted to to obviate the above
inaccuracies and their causes, have fulfilled their end, and are not
exposed to similar errors. But it would be vain to expect the persons,
who have "scorned delights, and lived laborious days" to possess
themselves of the mysteries of astronomy, should be impartial and
dispassionate, or be disposed to confess, even to their own minds, that
their researches were useless, and their labours ended in nothing.

It is further worthy of our attention, that the instruments with which
we measure the distance of the earth from the sun and the planets, are
the very instruments which have been pronounced upon as incompetent in
measuring the heights of mountains(57). In the latter case therefore we
have substituted a different mode for arriving at the truth, which
is supposed to be attended with greater precision: but we have no
substitute to which we can resort,