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HISTORY OF PHYSICAL ASTRONOMY.

“ The spots, in this view of the subject,” says the illustrious astronomerreferred to, “ would come to be assimilated to those regions on the earth’ssurface in which, for the moment, hurricanes and tornadoes prevail—theupper stratum being temporarily carried downwards, displacing by its im-petus the two strata of luminous matter beneath (which may be conceived asforming an habitually tranquil limit between the opposite upper and undercurrents), the upper, of course, to a greater extent than the lower, andthus wholly or partially denuding the opaque surface of the sun below.Such processes cannot be unaccompanied with vorticose motions, which,left to themselves, die away by degrees and dissipate, with this peculiarity,that their lower portions come to rest more speedily than their upper, byreason of the greater resistance below, as well as the remoteness from thepoint of action, which lies in a higher region, so that their centre (as seenin our water-spouts, which are nothing but small tornadoes) appears toretreat upwards. Now, this agrees perfectly well with what is observedduring the obliteration of the solar spots, which appear as if filled in bythe collapse of their sides, the penumbra closing in upon the spot and dis-appearing after it.”*

The views contained in the foregoing passage are eminently worthyof attention. It is remarkable that no preceding astronomer attempted toestablish a connexion between the origin of the solar spots and the rotationof the sun on his axis, although the intimate relation which appeared tosubsist between the situation of the spots and the solar equator, pointed outvery unequivocally the mutual dependence of the two phenomena. Dr.Wilson, in a paper already cited, alludes to the invariable appearance of thespots in the regions near the solar equator, as a remarkable fact of whichhe was unable to offer any explanation f. It is curious, however, to findamong a series of conjectures thrown out by him respecting the origin ofthese phenomena, an idea analogous to that propounded in the above passage j.His allusion to their possible generation by means of whirlpools in thesolar atmosphere, commencing from the surface, in common with his otherconjectures on the subject, shews that he possessed no inconsiderableshare of the imaginative faculty which is so necessary in all physical specu-lations ; but, whatever degree of truth may eventually turn out to be in-volved in this surmise, he cannot be entitled to any credit on its account,since he was unable to discover any physical connexion between it and thesubject of investigation. The explanation proposed by Sir John Herschel is valuable, inasmuch as it exhibits the solar spots in the character of dy-namical consequences flowing from the operation of established principlesin physics. It is one of those conceptions of genius which, being formed bya comprehensive and luminous view of the mutual relations of facts accuratelyobserved, rather than suddenly suggested by the sallies of a lively butwayward imagination, is found not unfrequently to constitute the germ ofa theory of indisputable rigour. The distinguishing character of all thosespeculations in science which have facts for their basis consists in theirtendency to promote further research. By a comparison of their resultswith those deduced from actual observation, the fundamental principlesare modified, and a further advance is made towards a true explanation ofthe phenomena which form the groundwork of inquiry. In the presentinstance, the question whether the spots recur periodically in the sameprecise localities of the solar regions, acquires additional interest from the

* Results of Astronomical Observations at the Cape of Good Hope , p. 434.

t Phil. Trans., 1774. f Seep. 222.