Page:A Voyage to Terra Australis Volume 1.djvu/5

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PREFACE.





The publication in 1814 of a voyage commenced in 1801, and of which all the essential parts were concluded within three years, requires some explanation. Shipwreck and a long imprisonment prevented my arrival in England until the latter end of 1810; much had then been done to forward the account, and the charts in particular were nearly prepared for the engraver; but it was desirable that the astronomical observations, upon which so much depended, should undergo a re-calculation, and the lunar distances have the advantage of being compared with the observations made at the same time at Greenwich; and in July 1811, the necessary authority was obtained from the Board of Longitude. A considerable delay hence arose, and it was prolonged by the Greenwich observations being found to differ so much from the calculated places of the sun and moon, given in the Nautical Almanacks of 1801, 2 and 3, as to make considerable alterations in the longitudes of places settled during the voyage; and a reconstruction of all the charts becoming thence indispensable to accuracy, I wished also to employ in it corrections of another kind, which before had been adopted only in some particular instances.

A variety of observations with the compass had shown the magnetic needle to differ from itself sometimes as much as six, and even seven degrees, in or very near the same place, and the differences appeared to be subject to regular laws; but it was so extraordinary