Concerning the differences in the magnetic needle, on board the investigator, arising from an alteration in the direction of the ship’s head. By Matthew Flinders, Esq. Commander of His Majesty’s Ship Investigator. In a letter to the Right Hon. Sir Joseph Banks, K. B. P. R. S

Abstract

In the years 1801 and 1802, while Capt. Flinders, on board the Investigator, was surveying the south coast of New Holland, he observed a difference in the direction of the magnetic needle, for which there appeared no other cause than that of the ship’s head being in a different direction. The compasses made use of on board the above mentioned ship were of Walker’s construction, one excepted, which was made by Adams; and it appears, from a table of observations given by Capt. Flinders, that some of the variations here treated of were 4° less, and others 4° greater than the truth. It also appears, that when this error was to the west, the ship’s head was to the east, or nearly so; when the error was eastward, the ship’s head was in a contrary direction; and when the observations agree best with those taken on shore, which may be considered as having the true variation, the ship’s head was nearly north or south. A minute inspection of the table seems to favour the opinion, that the excess or diminution of the variation was generally in proportion to the inclination of the ship’s head, from the magnetic meridian, on either side. Capt. Flinders, having ascertained the certainty of a difference in the compass, arising from an alteration in the point steered, thought it necessary, when he wanted a set of bearings from a point where the ship tacked, to take one set just before and another immediately after that operation. Several specimens of the manner in which these bearings were taken are given; also a specimen of the plan he followed in protracting such bearings: these specimens are in the form of tables, and are not of a nature to be abridged.