Solar- diurnal Variation of the Magnetic Declination at Pekin. 473 



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 

 general theory whereby the influence of the disturbances in different 

 parts of the globe may be predicated, their particular laws at every 

 station must be sought by a special investigation; and no con- 

 clusion in regard to either of the components of the diurnal variation 

 is entitled to be viewed as final which has not been preceded by 

 such an investigation. 



It has appeared desirable to enter more at length into this pre- 

 liminary statement than may at first sight be thought to be required 

 by those who have followed the different stages of the inquiries re- 

 ferred to, because the interpretation, which was given so far back as 

 1845, of the diurnal variation at Toronto and Hobarton, has scarcely 

 received the consideration which might seem due to a laborious and 

 apparently successful analysis of the phenomena ; and there are some 

 eminent physicists who have framed or adopted theories for the ex- 

 planation of the diurnal variation, in which theories the existence of 

 a double progression as a universal and necessary phase is essentially 

 implied. Amongst these, the most prominent perhaps, and the one 

 which has obtained the widest circulation, is the theory of the 

 R. P. A. Secchi, Director of the Observatory of the Collegio Romano, 

 published originally in Italian in 1854 in the ' Correspondenza Sci- 

 entifica' in Rome, translated into English in the edition of 1857 of 

 the late Dr. NichoPs ' Cyclopaedia of the Physical Sciences/ and 

 more recently adopted in the third volume of M. de la Rive's ' Traite 

 d'ElectiiciteV In M. Secchi's memoir, the diurnal variation, with its 

 double movement in the day and night, is ascribed to the direct 

 action of the sun as a distant and powerful magnet, influencing the 

 magnetic needle at different stations on the globe in a manner con- 



Phil Mag, S. 4. Vol. 20. No. 135. Dec. 1860. 2 I 



