LINES OF EQUAL DIP AND EQUAL TOTAL FORCE. 91 



however, by observing the time of vibration of a dipping- 

 needle in the magnetic meridian : or by observing the 

 extent to which the the needle is displaced by a given 

 weight attached to a thread which is wrapped round the 

 axis of the needle. The theory of these is so simple 

 that there is no need to delay on them. 



The ring-shaped lines in Figures 20 and 21 represent 



Fig. 35. 



-08T 



(B is the primary and A the secondary northern pole of greatest magnetic intensity.) 



the lines of equal dip over the surface of the earth : and 

 the lines in Figures 35 and 36 represent the lines of 



