SOLAR-DIURNAL VARIATION OF THE DECLINATION. 497 



of the agency by which any of the natural effects are 

 produced. It may tend also to a better appreciation of 

 the difficulties to be overcome, in obtaining a scientific 

 knowledge of the phenomena which present themselves 

 to the magnetic observer at the first outset, (viz. the 

 changes which he perceives to take place in the compass- 

 needle in the course of a single day), that it should be 

 shown what a variety of effects are combined in those 

 changes sometimes thought to be so simple. We may 

 commence the classification by describing the principal 

 characteristic features of the mean diurnal variation ; or 

 the result we obtain when we take an average of the 

 diurnal variation observed on every day of the year, 

 and thereby eliminate for the moment any systematic 

 differences which may belong to different portions of 

 the year. 



1. Mean diurnal variation. 



When proper precautions are taken to protect a decli- 

 nometer from casual and non-magnetic disturbances, 

 and when means are taken for every hour of all the 

 observations made at that hour throughout the year, it 

 is found that there are certain stations on the globe where 

 these means show scarcely any notable difference in the 

 direction of the magnet at any hour of the day or night. 

 In conformity with the terms frequently employed of 

 " north and south magnetic hemispheres," such stations 

 may be magnetically termed " equatorial stations." In 

 receding from them, either to the north or to the south, 

 indications begin to appear of a systematic horary dif- 

 ference, i. e. a diurnal variation, in the declination, and 



VOL. IV. K K 



