Observations on the Deviation of the Compass. 31 



In an ordinary way, a vessel sailing up the English Channel, 

 steering E., or E. by S., will probably have only 25° or 26° of 

 westerly variation, instead of 27° or 28° ; the difference of 2° or 

 3°, or about a quarter of a point, being the effect of the local at- 

 traction, which, in thick weather, or during the night, must pro- 

 duce a serious error in the reckoning. In going down the Chan- 

 nel, on the contrary, the actual variation of the compass on 

 board the vessel will probably be 29° or 30°, instead of 27° or 

 28°, the variation on shore, which difference, if unknown to the 

 captain or pilot, must throw the vessel considerably to the south- 

 ward of her position. Even more than this quantity of error 

 was fully proved by Mr Bain, whose " Essay on the Deviation" 

 contains a number of useful practical observations on the im*. 

 portant subject on which it treats * ; and a still larger quantity 

 of error, amounting, when at a maximum, to 6° or 7° of devia- 

 tion, or even more, has subsequently been discovered in very 

 many of our ships of war under the magnetic dip and condition 

 of our own coasts. 



Bain also found, in navigating the river St Lawrence, that it 

 was necessary to steer a different course coming down from the 

 opposite one he steered on going up. Owing to this circum- 

 stance (the local attraction), one of our ships of war, the Zea- 

 lous, had a very narrow escape in going up that river. During 

 a fog, this vessel ran so near the shore, not far from Cape Chat, 

 that she was in nineteen fathoms water ; and had not the fog 

 fortunately cleared at the moment, she would probably have 

 been wrecked. Many of the losses that have occurred in the 

 St Lawrence are, he reasonably concludes, attributable to the 

 local attraction. 



In crossing the Atlantic to the W. or S.W., vessels will al- 

 most always be found to the southward and eastward of their 

 , reckoning ; and in an equal degree if passing to the north-east- 

 ward or eastward, the error being still towards the S.E. 



In the voyage to Greenland, I have invariably found the de- 

 viation acting with the most marked effects ; so much so, indeed, 

 for some years before I knew any thing about the cause, that 

 I found it necessary to allow only two points westerly variation 



• Mr Barlow's admirable investigations on the Laws of the Magnetic 

 Deviation were not in my hands when this Lecture was deli»^ered. 



