442 M. A. Secchi on the Periodical Variations 



or to the electricity developed in the meteorological changes of 

 which the sun is the principal cause. A single reflection seems 

 to exclude these from being principal causes of the magnetic 

 diurnal period. The characteristic fact, as we have already 

 noticed, is that the magnetic elements have a double period, 

 diurnal and nocturnal. 



Now temperature and the other causes suggested have a 

 simple period, with greater or less modifications it is true, but 

 not so constantly repeated when the sun is below the horizon as 

 the magnetic period is in all climates and in all seasons. This 

 appears to us a law peculiarly characteristic of magnetism, which 

 shows it to be essentially distinct in cause, and to have a sepa- 

 rate origin from meteorological phenomena ; in the same way 

 that the semi-diurnal lunar peiiod in the tides is a proof of a 

 special action of our satellite on the waters of the ocean, which 

 is universal gravitation. And just as the fact that the tide is 

 more or less retarded after the moon passes the meridian, is not 

 a sufficient objection to destroy belief in this cause, so some 

 irregularity of a like nature observed in the magnetic period will 

 not be sufficient to disprove the reality of the magnetic action, 

 if we believe the proofs adduced to be otherwise sufficient. We 

 remarked at the beginning of this work, that Arago's observa- 

 tions, well discussed, ought to lead to the same conclusions as 

 those of other observers ; and now that we have, at last, before 

 us an abstract of these observations, it appears that we were not 

 mistaken, and that they afford a striking proof of the existence 

 of the double period *. 



What has led some to consider the magnetic period as a 

 simple one, has been, seeing that in certain seasons the extreme 

 minima occur at night. The error arises from not distinguish- 

 ing the absolute from the relative maxima ; but these are the 

 true characteristics of the phenomenon, and ought to be looked 

 upon as decisive in the matter. 



To this proof in support of the solar magnetic theory, may be 

 added another, already noticed by Col. Sabine, and worked out 

 by us in § 1 of Part II. of this Memoir, viz. the opposite action 

 of the sun according to its declination, the inversion occurring 

 exactly at the epoch of the equinoxes ; and here another differ- 

 ence will be seen between the effects of thermical and me* 



* We received the fourth volume of Arago's works, the first of his 

 scientific works, after the present treatise was finished : we see with plea- 

 sure that our theories are not contradicted by the observations or the de- 

 ductions of this celebrated astronomer and physicist, so that, without our 

 knowing it, our opinions are almost coincident with his. We shall see 

 presently how his observations furnish an additional proof of a decennial 

 period in the magnetic changes which we expected to find there. For the 

 double period, see pp. 499, 540. 



