tions. But he should have considered, that at sea, nay even on land,
the results of the most accurate observations will not always be the
same. Different compasses will give different variations; and even the
same compass will differ from itself two degrees, without our being able
to discover, much less to remove, the cause.

[Footnote 91: Nichelson.]

[Footnote 92: Mr Dun.]

Whoever imagines he can find the variation within a degree, will very
often see himself much deceived. For, besides the imperfection which may
be in the construction of the instrument, or in the power of the needle,
it is certain that the motion of the ship, or attraction of the
iron-work, or some other cause not yet discovered, will frequently
occasion far greater errors than this. That the variation may be found,
with a share of accuracy more than sufficient to determine the ship's
course, is allowed; but that it can be found so exactly as to fix the
longitude within a degree, or sixty miles, I absolutely deny.[93]

[Footnote 93: Few readers, it is presumed, require to be informed, that
the mode of endeavouring to ascertain the longitude by the variation of
the compass is no longer in use. In a work already referred to, Clerke's
Prog. of Mar. Disc., a singular enough communication is inserted
respecting the effect of tallow on the compass. It is subscribed by
Lieutenant Mason of the marines; but whether the experiments it relates
have been repeated by others, or if the inference it maintains has been
otherwise confirmed, the writer has yet to learn. He thought it right,
however, to notice it, as the more extensively hints are spread which
concern the advancement of useful discovery, the greater chance we have
of correcting errors, and perfecting science, The same reason justifies
his remarking, that the most important observations respecting the
variation of the compass made of late years, are those of Captain
Flinders, as to the effect of the ship's course upon it. The reader will
find them in the appendix to the account of his voyage lately published,
2d volume. Similar observations have still more recently been made by an
officer on board his majesty's ship Sibyl, while in the North Sea
protecting our Greenland fishery. They form an appendix to the Account
of a Voyage to Spitzbergen, by Mr John Laing, Surgeon, published at
Edinburgh, 1815. Of their importance and accuracy, notwithstanding the
small scale on which they were made, and the mea