548 MAGNETICAL OBSERVATIONS. [aPP. n" IX. 



is about west or east ; because the focus of attraction then 

 operates at right angles to the position of the compass- 

 needle ; but the anomaly generally disappears when the course 

 h about north or south, because the focus of attraction is 

 then in a line with, or parallel to, the compass-needle, and 

 consequently has no power to deflect it from its direct posi- 

 tion. (See Observations in the prefixed table, Nos. 4, 5. 10. 

 11. and 12.) 



(L This situation for the binnacle is deemed one of the best 

 in the ship, and is very properly preferred. Being 

 abaft the focus of attraction, the north point of the 

 compass, in this magnetic hemisphere, is always at- 

 tracted forward, and the errors at equal distances from 

 the magnetic meridian, in the same dip, are alike in 

 quantity both on easterly and westerly courses, and 

 always (excepting in rases where any large mass of 

 iron is placed near the compass) towards the north ; 

 the correction, when applied to the apparent course, 

 must therefore be towards the south, to give the true 

 course steered. Thus, the ship's head being west by 

 the compass, where there is 10" of local attraction or 

 anomaly on each side of the magnetic meridian, the 

 true course will be W. 10° S., and her head being 

 east per compass, the course corrected will be E. lO*' S. ; 

 and hence, in high northern latitudes, where the ano- 

 maly is thus great, a ship steering west by the com- 

 pass 100 leagues, and then east 100 leagues, Instead 

 of coming to the place from whence she started, will 

 be 104 miles to the southward of it ! 



(h.) Where the distribution of iron in a ship is unequal, more 

 being on one side of the midship line than on the other, 

 the course of no anomaly will not be nortli and south, 

 but something to the right or left of it, accordingly as 

 the greater mass of iron may be on the starboard or 

 larboard side. Hence, in the Isabella, when under 

 the command of Captain Ross, the line of no deviation 

 was found to be 17° from the magnetic meridian; or 

 when the course was N. 17° E. and S. 17° W. 



