180 ON MAGNETISM. 



small, the error of the compass will be large. To meet 

 the effects of this time-change it is very desirable that 

 the magnets by which the compass is corrected should 

 be so mounted that they can at any time be easily 

 adjusted to the distance at which they make the com- 

 pass correct. Gray's Binnacle is arranged expressly for 

 this purpose. 



The effect of change of geographical position is 

 sometimes very troublesome. If the subpermanent 

 magnetism is not completely neutralized by the mag- 

 nets, the remaining error, as above explained, produces 

 different deviations in different localities where the 

 horizontal force is different, and no table of errors 

 which applies to one place will apply to another place. 

 In the formulas of Article 68, and their explanation 

 in Article 69, it will be seen that induction produces 

 a term VN representing magnetism directed towards 

 the ship's head: and it is impossible to say, in the 

 operations for correcting the compass, whether this 

 term has a real value: and the compass must be 

 corrected as if it did not exist. But when the ship's 

 magnetic latitude is changed the value of V is changed ; 

 and when she goes into the south magnetic hemisphere 

 where the dip is reversed (Figures 21 and 37), the sign 

 of Fis changed; and these changes in the value of VN 

 produce a great change in the value of longitudinal 

 magnetism which was corrected as if it were constant ; 

 the correction therefore is no longer valid, and a con- 

 siderable error is produced in the compass. But upon 

 looking at the expression for N in Article 68, in which 



