

of Terrestrial Magnetism. 449 



acute observation, that the coincidence of the solar spots with 

 the maximum of the perturbations demands a cosmical cause, 

 depending on that body*. 



We may be permitted to refer here to the hypothesis of Mairan 

 on the solar atmosphere, and on its relation to the zodiacal light 

 and the aurora borealis, and therefore to the magnetic per- 

 tui'bations. We are far from admitting the theory as proved, 

 since it appears impossible to admit that the solar atmosphere 

 extends so far as half the radius of the orbit of Mercury f, 

 whence it is rather to be inferred that the zodiacal light depends 

 on a nebulous ring circulating round the sun between Venus and 

 the Earth. 



But whatever hypothesis be adopted, there are various coin- 

 cidences which may be deserving of regard. Mairan had, even 

 in his day, remarked the greater frequency of the aurora borealis 

 at the equinoxes %, the epochs at which the zodiacal light is most 

 visible. Neither had the relation between the greater frequency 

 of the aurora borealis and the epochs of the greater solar spots 

 escaped him, a relation already remarked by Cassini§. 



The paucity of observations at that time permitted suggestions 

 to be made which have been since proved to be groundless ; 

 but in general such coincidences are worthy of consideration. 

 Modern observations of the eclipses of the sun, of the protu- 

 berances and of the corona, as also of the spots, of the tem- 

 perature of various parts of the disc, as well as photographic 

 impressions, have placed beyond doubt the existence of the 

 solar atmosphere even beyond the zodiacal light ||. In read- 



* It may be said, that as the spots are apertures in the atmosphere of 

 the sun, they give freer passage to the magnetic action of the solar nucleus. 



t See Mairan's Works, p. 199, in which for the months of the year the 

 following numbers are given relative to the frequency of the aurora, begin- 

 ning with January,— 21, 27, 22, 12, 1, 5, 7, 9, 34, 70, 26, 15. Mairan sur 

 I'Aurore Boreale, suite des Me'moires de I' Ac. des Sciences, 1731. 



X Vide Humboldt, vol. hi. p. 413, Sabine's edition. 



§ Vide Humboldt's Cosmos, vol. iii. et seq., Sabine's edition. 



|| In the past winter I have been engaged in taking the solar disc 

 on plates of glass, in order thus to confirm the discovery made in 1852 

 of the difference of temperature in different parts of the disc. The im- 

 pressions obtained were stronger in the centre than at the edges ; but I 

 have abstained from publication, because I do not consider this proof as 

 very decisive. In fact, the image cannot have an equal intensity in every 

 part of the field of the telescope, on account of the obliquity of the different 

 pencils of rays which compose it. It is otherwise in the thermo-electric 

 experiments, where the pile remains invariable in regard to the axis of the 

 lens, and therefore this obliquity hardly exists, or is equal for all points of 

 the disc from which rays fall upon it. I think it right to say this, became 

 I find photographic experiments recently brought forward as proofs of the 

 smaller intensity of the light at the edges than at the centre, and I think 

 it probable that they may be subject to the exceptions which I found in 

 my own. 



Phil. May. S. 4. Vol. 9. No. 61. June 1855. 2 G 



