896 M. A. Secchi on the Periodical Variations 



than the other. If the two places which we are considering are in 

 opposite hemispheres, this new opposition will have to be taken 

 into account, that is to say, we shall have to make the product 

 of the algeliraical signs relatively to their positions and to their 

 names. This rule will be useful to us presently. It is a conse- 

 quence of the complete antagonism which exists in the two hemi- 

 spheres relatively to magnetic phsenomena. 



Corollary III. — A consequence of the dependence on the 

 magnetic meridian is the advancement or retardation of the 

 phases with the seasons, as in the course of the year the sun 

 arrives at the same azimuth from the geographical meridian by 

 describing a different horary angle, greater in winter and less in 

 summer ; but the discussion of this point will be more suitably 

 placed in the consideration of the elementary movements of the 

 needle. 



We will not however omit to notice here one important point. 

 The needle, in its nocturnal oscillation, and especially in winter, 

 makes an excursion which sometimes exceeds the diui-nal one. 

 This has sometimes caused it to be believed, that the maximum 

 of deviation, especially of western deviation, was subject to great 

 displacement. But the case is otherwise. The proper maxima 

 of the semi-diurnal excursions always remain at nearly the same 

 hours ; but if it should happen that the nocturnal should exceed 

 the diurnal, we are not therefore to say, without qualification, 

 that the maximum occurs in the evening ; the times and the 

 periods are to be distinguished, and all will be clear ; for if the 

 absolute maximum may happen at night, the relative maxima 

 however (eliminating the perturbations) follow constantly the 

 period above enounced; and it is these relative maxima and 

 minima which constitute the characteristic properties of the 

 variations of the magnetic as distinguished from the meteorolo- 

 gical period. 



Remark. — The two laws hitherto enounced are themselves no 

 other than corollaries of another more general law, which we 

 will now proceed to expose ; but I have thought it well to pre- 

 mise them, and to enunciate them separately, in order to pro- 

 ceed afterwards with greater clearness. 



Third Law, — " The diurnal excursion of the needle is the sum 

 of two distinct excursions, of which the first depends solely on the 

 horary angle, and the second depends besides on the sun's decli- 

 nation. These two fluctuations being variously superimposed 

 upon each other, produce by their interferences all the phenomena 

 of the ordinary diurnal and annual variations/' 



Declaration. — Nothing is in appearance more bizarre than the 

 curve traced by the magnetic needle in a single day; but as 

 there is no real iiTCgularity in natm-e, it is obvious to presume 



