Page:Proceedings of the Royal Society of London Vol 1.djvu/207

Rh leaving the harbour. on the 5th of November, they again returned, and continued, vu'th their usual regularity, until the arrival of the ship at the entrance of Hoogly river, on the 3rd of December. While the ship continued in the lower part of that river, a slight tendency to the equatropical motions might be perceived; but up the river, at Diamond harbour. the mercury was nearly stationary the whole twenty-four hours.

On the 13th of January 1804, after clearing Hoogly river, the equatropical motions again returned, and continued until the arrival of the ship at Bombay, on the 12th of February; from which day t6 the 18th, when the journal ceases, no signs of the above motions could be perceived.

Concerning the Diﬁ‘erences in the magnetic Needle, on Board the Inves- tigator, 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. Read March 28, 1805. [PhiL Trans. 1805, p. 186.]

In the years 1801 and 1802, while Capt. Flinders, on board the Investigator, was surveying the south coast of New Holland, he ob- served 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 in- spection of the table seems to favour the opinion, that the excess or diminution of the variation was generally in proportion to the incli- nation 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 hearings were taken are given; also a specimen of the plan he fol- lowed in protracting such hearings: these specimens are in. the form of tables, and are not of a nature to be abridged.

With respect to the cause of the differences here treated of, Capt. Flinders offers the following conjectures :—

1st, That the attractive power of the different substances in a ship, which are capable of affecting the compass, is brought into a sort of focal point nearly in the centre of the ship, where the shot are de-