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APT. N" IX.] MAGNKTICAL OBSERVATIONS. 549 



(c.) Although the after-part of the quarter-deck appears to 

 be the best situation for the binnacle, yet it is desirable 

 to have a compass, for reference, placed as far as pos- 

 sible, out of the attraction of the iron in the ship. A 

 compass at the jib-boom end, where the travellers and 

 hoops are of copper, or at the top-gallant-mast head, 

 which is better, is found to be scarcely at all influenced 

 by the attraction of the ship. Hence, I have been 

 in the habit of carrying a compass, occasionally, in the 

 crow's nest, fixed at the mast head, where it was found 

 to be free from those anomalies which are so sensible 

 in a compass on deck. 



7. The greatest anomaly with the compass in the position 

 last described, being ascertained by observation, the error 

 on every other point of the compass may be easily calcula- 

 ted ; the anomalies produced by the attraction of the iron 

 in the ship, being found to be proportionate to the sines of 

 the angles between the ship's head and the point of no ano- 

 maly, which point is most commonly the magnetic meridian, 

 (a.) On the supposition that the point of no anomaly occurs 

 when the position of the ship corresponds with the 

 magnetic meridian, that is, when her head lies north or 

 south. Captain Flinders has given thp following rule : 

 — As the sine of eight points (or radius) is to the sine 

 of the angle between the ship's head and the magnetic 

 meridian, (or sine of the course reckoned from south 

 or north), so is the anomaly found at east or west by 

 observation, to the anomaly on the course steered ; or, 

 the anomaly on any other course being found by ob- 

 servation, the error on that position of the ship's head 

 ** would be to the error at east or west, at the same 

 dip, as the sine of the angle between the ship's head 

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

 or radius." 



(6.) As, however, the line of greatest error is not always 

 north and south, the above rule has been modified by 



