314 



MAGNETISM 



represented by Fig. 233. There is also a small annual inequality 

 and probably a very small lunar inequality. 



If, instead of considering the actual value of an dement, the 

 amplitudes of its range are plotted, it is found that there i- a con- 

 nection between the range and sunspots, the range increa>ii 

 the area of the sunspots increases.* 



In addition to these changes there is a secular cha: 

 secular change in dip and declination is well represented 1 





method due to Bauer. Fig. J^JI. givei tin- rhan;e> in the declina- 

 tion at London since 1580.| 



These secular variations make it Decenary to take a particular 

 epoch when giving the value of the element- at a place. 



FIG. 235. 



Figs. 235 and 236 show the lines of horizontal force (Duperrey's 



* For an account of the inequalities see Chree, Studies in Terrestrial Magnetism 

 On the connection with sunspots and auroras, see Ency. lint., I.e. 



f A Table of the mean annual changes in the elements at different positions in 

 these islands is given by Thorpe and Ru'cker, Phil. Trans., A, clxxxi. (1890), p. 325. 



