Page:A Voyage to Terra Australis Volume 1.djvu/487

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Lunar observations.]

Burg. He deduces from thence the longitude of Sydney Cove to be 151° 12' 45"; and from forty-four sets of lunar distances by lieutenant Flinders, it would be 151° 11' 49" east.

At Port Louis in the Isle Mauritius, the Abbé de la Caille observed an eclipse of the sun, the transit of mercury over the sun's disk, and various occultations of Jupiter's satellites; M. d'Après also observed several occultations; and this place should therefore be well determined. Its longitude in the Requisite Tables is 57° 29' 15" east; and from twenty-seven sets of distances taken whilst a prisoner there, I made it, when corrected for the errors of the tables, 57° 29' 57" east.

In appreciating the degrees of accuracy to which a small or larger number of lunar distances may be expected to give the longitude, I suppose the observer to be moderately well practised, his sextant or circle, and time keeper to be good, and his calculations to be carefully made; and it is also supposed, that the distances in the nautical almanack are perfectly correct. As, however, there may still be some errors, notwithstanding the science and the labour employed to obviate them, it cannot be too much recommended to sea officers to preserve all the data of their observations; more especially of such as may be used in fixing the longitudes of places but little, or imperfectly known. The observations may then be recalculated, if requisite; the corrections found to be necessary may be applied; and the observer may have the satisfaction of forwarding the progress of geography and navigation, after having contributed to the safety of the ship, and benefit of the particular service in which he may happen to have been engaged.