1821.] Capt. Parry's Voyage. 67 



changed from 128° 58' W. in the longitude of 91° 48', where 

 the last observations on shore had been made, to 165° 50' 09" 

 E. at this station, so that in sailing over the space included 

 between these two meridians, they crossed immediately to the 

 northward of the magnetic pole over one of those spots upon the 

 globe, where the needle would have been found to vary to 180°; 

 or, in other words, where its north pole would have pointed due 

 south. This spot would, in all probability, at this time be some 

 where not far from the meridian of 100° W. of Greenwich. Capt. 

 Sabine remarked, in obtaining the observations for the variation, 

 that the compasses, which were those of Capt. Kater's construc- 

 tion, required somewhat more tapping with the hand to make 

 them traverse than they did at the place of observation in Prince 

 Regent's Inlet on August 7, where the magnetic dip was very 

 nearly the same; but that, when they had settled, they indicated 

 the meridian with more precision. 



The azimuth compasses used in these observations were of 

 Capt. Kater's improved construction. These compasses were 

 originally constructed for the voyage of discovery in 1818, and 

 are described in the Philosophical Transactions for 1819, a fur- 

 ther improvement having been made in them during the equip- 

 ment of this expedition. 



In the course of these observations, two objects were designed 

 to be kept in view ; it was requisite, first, for the purposes of 

 navigation, that the amount of the variation on the courses 

 steered by the ship should be known, that her true direction 

 might be deduced from that indicated by the compass ; and, 

 secondly, to these necessary observations, it was desired to add 

 such as could be made without material inconvenience or delay 

 with the ship's head placed on other points than those of her 

 immediate courses, for the purposes of exemplifying more 

 extensively than had been done heretofore the irregularities 

 which take place in the direction of compass needles in conse- 

 quence of the attraction of the iron contained in ships. 



It was evident from a course of experiments that the common 

 centre of attraction of the ship's iron was forward and very 

 nearly a-midships ; and that, consequently, when the ship's head 

 was north or south bv the compass, the direction of the earth's 

 magnetism and of the local attraction coinciding, the compass 

 indicated the true magnetic bearing of objects. 



The true variation of the needle, therefore, could be at all 

 times ascertained by azimuths observed with the ship's head, or 

 either of those points ; when the error with which the result 

 might be affected from local attraction might be reasonably 

 expected not to exceed the other incidental errors to which such 

 observations are necessarily liable. The irregularities in the 

 direction of the compass proceeding from the ship's iron occa- 

 sioned no other practical inconvenience in her navigation than a 

 little additional trouble in computing the day's works. 



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