NATURAL PHILOSOPHY. 123 



variations to the sun as their primary source ; since we find in what- 

 ever part of the globe the phenomena are observed, the solstices and 

 equinoxes are the critical epochs of the variation whose period is a 

 year, whilst the diurnal variation follows in all meridians nearly the same 

 law of local solar hours. To these unquestionable evidences of solar 

 influence in the magnetic affections of the earth, we have now to add 

 the recently ascertained fact, that the magnetic storms, or disturbances, 

 which in the absence of more correct knowledge were supposed to be 

 wholly irregular in their occurrence, are strictly periodical pheno- 

 mena, conforming with systematic regularity to laws in which the 

 influence of local solar hours is distinctly traced. 



But whilst we recognise the sun as the primary cause of variations 

 whose periods attest the source from whence they derive their origin, 

 the mode or modes in which the effects are produced constitute a 

 question which has been and may still be open to a variety of 

 opinions : the direct action of the sun as being itself a magnet, its 

 calorific agency occasioning thermo-electric and galvanic currents, or 

 in alternately exalting and depressing the magnetic condition of sub- 

 stances near the earth, or in one of the constitutents of its atmosphere, 

 have been severally adduced as hypothesis affording plausible 

 explanations. Of each and all such hypothesis the facts are the only 

 true criterion ; but it is right that we should bear in mind that in the 

 present state of our knowledge, the evidence which may give a 

 decided countenance to one hypothesis in preference to others does 

 not preclude their possible co-existence. The analysis of the collected 

 materials and the disentanglement of the various effects which are 

 comprehended in them, is far from being yet complete. The corres- 

 pondence of the critical epochs of the annual variation with the solstices 

 and equinoxes rather than with the epochs of maxieujm and minimum 

 temperature, which at the surface of the earth, in the subsoil beneath 

 the surface, or in the atmosphere above the surface, or separated by 

 a wide interval from the solstitial epochs, appears to favor the 

 hypothesis of a direct action ; as does also the remarkable fact which 

 has been established, that the magnetic force is greater in both the 

 northern and southern hemispheres in the months of December, 

 January, and February, when the sun is nearest to the earth, than in 

 those of May, June, and July, when he is most distant from it ; 

 whereas if the effect were due to temperature, the two hemispheres 

 should be oppositely instead of similarly affected in each of the two 

 periods referred to. Still there are doubtless minor periodical irregular 

 variations which have yet to be made out by suitable analytical pro- 

 cesses, which, by their possible accordance with the epochs of maxi- 

 mum and minimum temperature, may support in a more limited 

 sense, not as a sole but as a co-ordinate cause, the hypothesis of calo- 

 rific agency so generally received, and so ably advocated of late in 

 connexion with the discovery by our great chemist and philosopher, 

 of the magnetic properties of oxygen, and of the manner in which 

 they are modified and affected by differences of temperature. It may 

 indeed be difficult to suppose that the magnetic phenomena which we 



