78 ON MAGNETISM. 



sign of the vertical force has changed. This is so well 

 known that, in the best compasses, a sliding weight is 

 provided, which in north latitudes can be applied to the 

 blue end of the needle, and in south latitudes can be 

 applied to the red end of the needle. 



The instrument with which this vertical force is 

 most conspicuously exhibited and most accurately ex- 

 amined will be described in the next article. 



36. Description of the Dipping Needle. 



The function of this instrument is limited strictly to 

 the determination of the direction which a needle will 

 take under the action of the total terrestrial magnetic 

 force, when it is constrained to move in an arbitrary 

 vertical plane. This limitation permits the construction 

 of an instrument possessing great simplicity, and, in 

 consequence (viewing the nature of its action) great 

 accuracy. 



The needle must be carried by a horizontal axis, 

 passing as nearly as practicable through its center of 

 gravity. This condition, though convenient, is not neces- 

 sary : for, as will be shewn in the next Article, we can so 

 arrange the observations as perfectly to eliminate the 

 effects of error of position of the axis : and indeed, for 

 some observations, the place of the center of gravity is 

 purposely moved to a sensible distance from the axis. 

 The axis must terminate in two delicate pivots ; and it 

 is mainly in the formation of these that the utmost 

 skill of the artist is required. It is very difficult so to 

 arrange the observations that the injurious effect of an 



