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The peculiar character of the magnetic storms (or disturbances as 

 they are sometimes called), and the periodical laws exhibited in their 

 mean effects, have been the subject of frequent investigations since 

 1845. It is not necessary to notice on this occasion the results 

 of these further than as they are connected with the explanation of 

 the phenomena of the diurnal variation, which forms the subject 

 of this paper. It has been shown, by abundant evidence, that 

 though apparently casual in the times of their occurrence, the mag- 

 netic storms nevertheless produce mean effects, which, when the ob- 

 servations of more than a very few days are combined, are seen to be 

 of a highly systematic character in all parts of the globe where their 

 effects have been examined : that the mean deflections which they 

 occasion have always their particular hours of extreme elongation, 

 with continuous intermediate progression : that these hours are dif- 

 ferent in different parts of the globe, exhibiting apparently every 

 possible variety : that the disturbance diurnal variation, as for di- 

 stinction's sake it may be called, constitutes everywhere a sensible 

 portion of the diurnal variation shown by the mean of the hourly 

 observations from which no elimination of disturbed observations has 

 been made : that the diurnal variation so obtained is in fact a result- 

 ant of two diurnal variations superposed, both referable to the sun 

 as their primary cause, but manifesting by the difference in the cha- 

 racter of the effects produced, a distinction in the mode of operation 

 to which they are severally due. The disturbance variation is caused 

 by deflections which are only of occasional occurrence ; the more 

 regular solar diurnal variation is distinguished, on the other hand, 

 by the regularity of its daily occurrence ; and its hours of extreme 

 elongation, or (as they may be more familiarly termed) its turning 

 hours are the same, or nearly the same hours of local solar time in all 

 parts of the globe, whilst those of the disturbance variation show 

 almost every possible variety. The relative magnitudes or proportions 

 of the two components differ also very greatly at different stations ; 

 and thus, by the operation of causes which as yet are but very im- 

 perfectly known, at localities where the magnetic storms are ex- 

 cessive, the disproportion of the components becomes excessive also, 

 and the phases of the regular variation are rendered altogether 

 subordinate to those of the disturbance variation. Until therefore 

 the extension of observations shall give rise to and establish some 



