LINES OF EQUAL DIP AND EQUAL TOTAL FOECE. 93 



jection. It will be remarked that there is a little in- 

 accuracy near the south pole, arising from the circum- 



Fig. 37. 



BlU > OU ^ 



stance that it is impossible to include the north and the 

 south magnetic poles in the same geographical meridian. 

 The Dip and the Total Terrestrial Magnetic Force 

 at any place, like the elements of which we have treated 

 in Articles 24 and 34, are slowly changing. In 1843 

 the dip at Greenwich was about 69 1' ; it has diminish- 

 ed, with a rate continually accelerating, till in 1868 it 



