the Direction of the Compass Needles, b'c. 279 



The accordance in so many ships gave him reason to believe 

 that in compasses placed near the binnacle, and amidships, the 

 points of no ^rror would be most commonly those of the mag- 

 netic meridian. Considering, however, that tliis must depend 

 altogether on the distribution of iron, and may be therefore hable 

 to great diversity, he recommends that in every ship, as soon as 

 a fixed position has been selected for a compass, the points of no 

 error should be determined by repeated observation. The me- 

 thod that was adopted for this purjiose in the late voyage appear- 

 ing l)oth simple and effectual, it may be useful to exemplify it by 

 an instance or two. 



The Isabella being at anchor in Brassa Sound, Shetland, her 

 head was placed, by means of warps, on each point of the com- 

 pass successively, and the bearing of a pile of stones on the sum- 

 mit of a distant hill noted by her compass at each point : at the 

 same time that these observations were made on board, her bear- 

 ing from the hill was also observed by a compass placed on the 

 pile of stones : the agreement m bearing showed the points of 

 no error, and the differences the errors in each point, without 

 the calculation which azimuths involve. — [See PL IV. fig. 1.] 



The Alexander being along side a floe of ice in Baffin's Bay, 

 the true magnetic bearing from the ship, of a very distant and 

 well defined object on the main land, was found by carrying a 

 compass on the ice in an opposite direction, to a distance which 

 ensured its being perfectly free from local influence. The ship's 

 head being then warped round to each point of the compass suc- 

 cessively, the errors in each were determined by the difference in 

 bearing, as in the last instance. — [See PI. IV. fig. 2.] 



The regularity in the above results is the best testimony that 

 the method is a satisfactory one. Certain precautions must be 

 attended to : thus, the object must be sufficiently distant, that the 

 space occupied in warping the ship round may not subtend any 

 sensible parallax. The direction of the ship's head should be 

 noted by the compass by which the bearings are taken. A short 

 time must be allowed to elapse after the ship is steady on any 

 point, to ensure the traversing of the cards : this is particularly 

 necessary in high latitudes when the compasses move very slug- 

 gishly. And lastly, the observations should be rejjcated. 



It will be observed by the above results, in the Isabella and 

 Alexander, that the points of no error were not coincident in either 

 ship with those of the magnetic meridian ; in the Alexander es- 

 pecially, they were more nearly at right angles to it. That this 

 sliip should have differed so materially from all the instances on 

 record, may be attril)utcd to her compass being so near the level 

 of the deck, and therefore being more affected by the influence 

 of a considerable (juunlity of iron articles (such as ice anchors, 



S 4 ice 



