distances and other observations taken in the Investigator's
voyage having been ordered by the Commissioners of the Board of Longitude
to be recalculated by a professed astronomer, with every degree of
correctness which science has hitherto been able to point out as
necessary, this delicate, but laborious task was assigned to Mr. John
Crosley, formerly assistant at the Royal Observatory at Greenwich; a
gentleman who formed part of the expedition as far as the Cape of Good
Hope, but whose ill health had then made it necessary to relinquish the
voyage and return to England. The data and results of all the
observations will probably be made public, by order of the Commissioners;
but in the mean time, for the satisfaction of the geographer, and more
especially for that of the seaman, whose life and property may be
connected with the accuracy of the charts., the _results of the lunar
distances_ observed upon each coast are added in the form of an Appendix
to the volume wherein that coast is described. It is by these results
that the time keepers have been regulated; and the longitudes used in the
construction of the charts are taken from the time keepers.

To appreciate the degree of confidence to which these results may be
entitled, it is necessary to know under what circumstances the
observations were taken; also the method used in the calculations, and
the corrections which have been applied beyond what is usual in the
common practice at sea: of these the following is a general statement.

1st. The instruments used in taking the distances, were a nine-inch
sextant by Ramsden, and three sextants of eight inches radius by
Troughton, the latter being made in 1801, expressly for the voyage. On
board the ship, the sextant was necessarily held in the hand, and the
distances were sometimes so taken on shore; but in most of the latter
cases, it was fixed on a stand admitting of the sextant being turned
easily in any direction. The telescopes were of the largest magnifying
powers which the motion of the ship, or state of the atmosphere could
admit, and each longitude is the result of a set of observations, most
generally consisting of six independent sights. They were taken either by
lieutenant Flinders or by myself; those by him being designated in the
column of _Observers_ by the letter F, the others by C.

2nd. Preparatory to the reduction of the apparent to the true distance,
the four following corrections have been applied.