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in the Western Arctic Sea. 77 LETTER XL Dear Thomas, Winter Harbour, May \st, 1820. In my last letter of the 1st March, I mentioned the hazard to which the expedition was exposed, by the conflagration of the instrument-house, erected on the shore. Had the astronomical clocks, the transit-instrument, the dipping-needle, or the dipping- sector been destroyed, or materially injured, several important purposes of our adventure must have been frustrated, several of its chief objects must have been unattainable. Without them we should not have been able to ascertain the position we occupied on the face of the globe with that accuracy required for the pur poses of science. With instruments of ordinary correctness of construction, the latitude of a place may be discovered with suffi cient precision. For such a purpose a well-constructed Hadley’s quadrant or sextant, answers very well. As you have frequently seen and examined that admirable instrument, I need say no more about it than, that by the combination of the principles of optics and geometry, the altitudes of the celestial bodies, and their re lative positions, and angular or apparent distances, may be deter mined on ship-board, notwithstanding the incessant motion of the vessel, with equal ease as upon solid ground. When, however, you have fixed a position in regard to its northing or southing, that is, its perpendicular distance N. or S. from the equinoctial line, or in a contrary direction from the terrestrial poles, you have obtained but one of the elements necessary for fixing its position on the globe. For all places situated on the parallel which encircles the earth at that distance from the line, will have the same latitude. To determine with strict precision the situa tion of any particular spot, you must also know its easting or westing, that is, its perpendicular distance E. or W. from some determined point on its own parallel; that is, you must fix its lon gitude. Longitude and latitude are strange terms to be employed in relation to a globe or sphere, for it is essential to that body that, in whatever direction you measure its circumference, the dimensions must be absolutely equal among themselves. But these terms have been conveyed down to us from very remote times : they have long been correctly understood; and now to introduce others strictly expressive of their value, would only savour of affectation, and produce extreme confusion. That our earth was a ball, globe, or sphere, was known to the philosophers of antiquity. They did not, however, conceive more than the