CHAP, ii.] ELECTRICITY AND MAGNETISM. 



119 



diminished to its present value of 67 39'. The period in 

 which the cycle is completed is not known, but the rate 

 of variation of the dip is less at the present time than it 

 was fifty years ago. In all parts of the earth both declin- 

 ation and inclination are changing similarly. The follow- 

 ing table gives the data of the secular changes at London. 



TABLE OF SECULAR MAGNETIC VARIATIONS. 



The Total Magnetic force, or " Intensity," also 

 slowly changes in value. As measured near London it 

 was equal to '4791 dynes in 1848, and '474 m 1866. 

 Its value at the beginning of 1880 was -4736 dynes. 1 

 Owing to the steady decrease of the angle at which the 

 needle dips, the horizontal component of this force (i.e. 

 the "Horizontal Intensity") is slightly increasing. It 

 was '1716 dynes in 1848, and '1797 dynes at the 

 beginning of 1880. 



1 That is to say, a north magnet pole of unit strength is urged in the line 

 of dip, with a mechanical force of a little less than half a dyne. 



