1820.] Philosophical IVansactiuns for 18\9, Part I. 49 



-would differ very materially from itself on being removed from 

 one part of a ship to another. Hence he was led to confine his 

 ' compass to one particular spot. He selected the binnacle for 

 conveniency. It was exactly midships. This observation of 

 Capt. Flinders was fully corroborated in the Isabella and 

 Alexander. 



Secondly, Capt. Flinders found that in his compass, perma- 

 nently fixed as described, no error took place when the ship's 

 headVas on the magnetic north or south points ; showing that 

 at such times the attraction of the ship and of magnetism was m 

 the same line of direction. The maximum of error also took 

 place v.'hen the ship's head was at right angles to these points ; 

 namely, at east or west ; being, however, in opposite directions, 

 in excess of the true variation on the one side, and in defect on the 

 other ; so that the extreme difference occasioned by altering the 

 course from east to west, or the reverse, would be twice the error 

 fit either. On the intermediate points, the ratio of the error to 

 its maximum was as the sine of the angle between the ship's- 

 head and the magnetic meridian to the sine of eight points, or 

 radius, or sufficiently near to admit of corrections being calcu- 

 lated for every course, when the errors on a single one v/ere known 

 by observation. 



Though Capt. Flinders was induced from his observations to 

 conclude that the point of no error was when the ship's head 

 coincided with the magnetic meridian, yet this did not hold 

 either in the Isabella or the Alexander. Indeed in the Alexander 

 it was nearly at right angles to that meridian. The point of no 

 error did not coincide in the two ships. The reason was, that 

 in the Isabeha the compass of observation was raised a consider- 

 able height above the deck, while in the Alexander it was upon 

 the deck. Capt. Sabine, in consequence of this, proposes to 

 alter Capt. Fhnders's rule to the following : 



" The error produced in any direction of the ship's head will 

 be to the error at the point of the greatest irregularity, as the 

 sine of the angle between the ship's head and the points of no 

 error to the sine of eight points or radius." 



Thirdly, Capt. Flinders's experience in the Investigator show'ed 

 that the maximum of error in the same compass v/ould be different 

 in different parts of the world, although the use of the compass 

 was confined to one particular spot in the ship, and every pre- 

 caution taken to avoid an interference v.'ith the distribution of 

 the ship's iron. 



Capt. Flinders observed that the error increased with the dip, 

 and he conceived that the amount of the error under any one dip 

 being known, the amount may be calculated for any other dip by 

 using as a multiplier the decimal expression of the proportion 

 •whicli the error in the one ascertained instance may have borne 

 to the dip. But it is obvious from the observations made iuthe 



Vol. XV. N° I. D 



