THE EXACT MEASUREMENT OF PHENOMENA. 327 



9. If so, is it a secular, i. e. a continually growing 

 change, or does it give evidence of a greater period \ 



Already the periodic changes of a certain number of 

 stars have been determined with accuracy, and the lengths 

 of the periods vary from less than three days up to in- 

 tervals of time at least 250 times as great. Periods 

 within periods have also been detected . 



There is, perhaps, no subject in which more complicated 

 quantitative conditions have to be determined than ter- 

 restrial magnetism. Since the time when the declination 

 of ' the compass was first noticed, as some suppose by 

 Columbus, we have had successive discoveries from time 

 to time of the progressive change of declination from 

 century to century ; of the periodic character of this 

 change ; of the difference of the declination in various 

 parts of the earth's surface ; of the varying laws of the 

 change of declination ; of the dip or inclination of the 

 needle, and the corresponding laws of its periodic changes ; 

 the horizontal and perpendicular intensities have also 

 been the subject of exact measurement, and have been 

 found to vary by place and time, like the directions of the 

 needle ; daily and yearly periodic changes have also been 

 detected, and all the elements are found to be subject to 

 occasional storms or abnormal perturbations, in which the 

 eleven year period, now known to be common to many 

 planetary relations, is apparent. The complete solution 

 of these motions of the compass needle involves nothing 

 less than a determination of its position and oscillations in 

 every part of the world at any epoch, the like deter- 

 mination for another epoch, and so on, time after time, 

 until the periods of all changes are ascertained, and the 

 character of the variations determined. This one subject 

 offers to men of science an almost inexhaustible field for 



Humboldt's 'Cosmos,' translated by Otte, vol. iii. p. 228. 



