204 



PHYSIOGRAPHY 



declination. All places west of this agonic line have east declina- 

 tion. In general, declination increases with increasing distance 

 from the agonic line. At New York City the declination is about 

 10 W., and in Maine it is more than 20 W. at a maximum. At 

 Chicago the declination is about 3 E., at Denver about 13 E., 



Fig. 200. Lines of equal magnetic declination for the United States, 1902. 

 The heavy line is an agonic line, or line of no decimation. (U. S. C. and 

 G. Surv.) 



at San Francisco about 16 E., and in the State of Washington 

 more than 20 E. (Fig. 200). 



It will be seen that it is necessary to know the magnetic declina- 

 tion of a region, if the compass is to be used there for determining 

 directions accurately. The declination being known, the compass is 

 of great value to travelers through forests and other tracts which 

 have no roads or trails. By its help directions may be known 

 accurately. The compass, too, is of the greatest help on the sea, 

 where vessels are guided by it. 



The declination at a given place does not remain quite constant. 

 The declination at Chicago, for example, has shifted more than 2 

 since 1820. 



