496 EDITOR'S NOTES. 



III. On the solar-diurnal variation of the Magnetic 

 Declination. 



UNDER the ordinary name of "diurnal variation" are 

 usually included several independent variations, distin- 

 guishable apart by their respective hours of maxima and 

 minima, and by the characters of their progressions, 

 but which unite in one common feature and are thereby 

 traceable to one common primary cause ; the common 

 feature being that, in all, the period of variation is a 

 mean solar day. Our first knowledge of these variations 

 is obtained by observing them in their conjoint or 

 accumulated effects, viz. in the changes of direction 

 which they produce in a freely suspended magnet at the 

 different hours of the day and night. It is the business 

 of the analyst, by various modes of grouping and com- 

 bining 'the observations, to distinguish from each other 

 the effects which proceed from different sources, and as 

 the first step towards a knowledge of the physical caus- 

 ation by which such dissimilar effects are made to pro- 

 ceed from the same primary source, to ascertain their 

 several phenomenal laws. The investigation which has 

 this purpose in view has already made some progress ; 

 and its advance may perhaps be aided by an endeavour 

 to classify the natural facts of which a knowledge has 

 thus been acquired. The endeavour must necessarily 

 be imperfect, because our knowledge is still but partial 

 and incomplete, as well as of very recent acquisition ; 

 and because we have as yet no true theoretical perception 



