187 



leaving the harbour, on the 5th of November, they again returned, 

 and continued, with 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 to 

 the 18th, when the journal ceases, no signs of the above motions 

 could be perceived. 



Concerning the Differences 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,;?. 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 

 w r ere 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 

 bearings were taken are given ; also a specimen of the plan he fol- 

 lowed in protracting such bearings : 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 : 



1 st, 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- 



