294 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



are given. These three things therefore constitute what are 

 called the magnetic elements at a given place.* 



5. Magnetic maps. Figure 3 is a map of the world showing 

 the lines of equal magnetic declination for the year 1905. The 

 heavy black curves pass through the regions where the compass 

 needle points true north, and the numbers attached to the fine- 



Fig. 3. 

 Lines of equal magnetic declination. 



line curves are the values of the declination. Thus, in England 

 the compass points 20 to the east of north and at San Francisco 

 the compass points about 17 west of north. Figure 4 is a map 

 of the world showing the lines of equal magnetic dip or incli- 

 nation for the year 1905. Thus, the dip needle stands in a 

 horizontal position at all places on the heavy curve which is 

 marked zero, the magnetic dip in England is about 70 (north 

 pole of dip needle down), and the dip at Cape Town, South 

 Africa, is about 55 (south pole of dip needle down). Figure 5 

 is a map of the world showing the lines of equal horizontal inten- 



* The methods in use in the Magnetic Observatory at Kew, England, for determin- 

 ing the magnetic elements are fully described in Stewart and Gee, Elementary Practi- 

 cal Physics, Vol. II, pages 274-313. 



