KNOWLEDGE 



[Januaby, 1903. 



throughout an entire solar cycle, but only some preliminary 

 sets were executed, and the project was left in abeyance. 

 The subject has been quite lately— since the appearance 

 of Dr. Halm's discussion — resumed by Prof. Very in 

 America.* By means of spectro-bolometric measures at 

 various points on the sun's disc, he proposes to ascertain 

 whether the curve of diminishing radiant energy from 

 centre to limb remains constant, or alters in form from 

 time to time or progressively; and this, as Mr. Wilson and 

 Dr. Rambaut had pointed out, would serve to test the 

 occurrence of changes in the quality or depth of the solar 

 atmosphere. Tbe quantities concerned would in any case. 

 Prof. Very remarks, be of a very small order ; still, he 

 regards their discrimination as possible by an extension of 

 the method practically exemplified in his paper, the 

 determinations being made predominantly at epochs of 

 spot-maxima and minima. Only at a great public 

 observatory, he adds, could the grasp and continuity 

 needed to give substantial value to the work be secured. 



Meanwhile no proof — no suspicion even of a proof — is 

 as yet at hand of fluctuations in the effectiveness of solar 

 heat-conservation. That it has a secular tendency to 

 augment is, however, rendered probable by the fact that 

 stars, on the whole, redden with antiquity, and upon this 

 slow process interruptions may possibly supervene. The 

 question remains open. The affirmative answer to it 

 suj^plied by Dr. Halm is based upon theoretical considera- 

 tions. He assumes that the sun is a cooling body, and 

 hence that the absorption due to its waste products is, on 

 the whole, in course of intensification. Nevertheless, 

 vicissitudes have to be reckoned with, and their conse- 

 quences are supposed to occasion and prescribe all the 

 complex phenomena connected with the periodicity of the 

 sun. For intensifying absorption brings in its train an 

 excess of heat-reteutiou. The protective envelope becomes 

 over-protective. It more than redresses the balance 

 between thermal loss and gravitational restoration. Super- 

 heating sets in, eruptions ensue, and the atmosphere is 

 temporarily cleared. Then the swing of change begins 

 anew. The eleven-year cycle is in fact established. 



There is much in this speculation that allures thought. 

 It is novel, it is reasonable, it smoothes away some out- 

 standing difficulties, it affords a prospect of escape from 

 the weary round of abortive hypotheses. It seems, above 

 all, comprehensive enough to include the problem of light- 

 variability in red stars. Yet, when we come to details, the 

 way out appears, after all, to be hopelessly blocked. Like 

 many other solar theories, this new and highly ingenious 

 one promises moi-e than it can perform. Its merits are 

 undeniable ; but it does not meet the full exigencies of 

 the situation. 



The cyclonic hyjjothesis of spot-formation does duty 

 once more in Dr. Halm's solar scheme. Eruptions take 

 the initiative ; their subsiding materials diffuse into polar 

 and equatorial currents ; aud the encounters of those 

 oppositely directed, and issuing naturally from different 

 sources of disturbance, produce vortices showing to the 

 eye as spots. But these are affected by no such conspicuous 

 aud systematic internal movements as should appear in 

 them if they in truth originated after the supposed fashion. 

 Spots fitfully gyrating do indeed occur, but they attract 

 attention as rarities. No more than two or three per cent, 

 of the whole thus distinguish themselves. And even in 

 these the movements are capricious and unmethodical. 

 Opposite rotations are sometimes perceived to proceed 

 simultaneously in different members of a single spot-group; 

 nay, one of the aggregated umbrae may wheel by turns in 

 contrary directions. Effects so casual cannot dej^end upon 



* AstrophysicaL Journal, Vol. XVI., p. 73. 



a fundamental cause ; and they indeed necessarily arise 

 when matter drawn towards a centre deviates, however 

 slightly, from a radial course. Dr. Halm tries to reconcile . 

 the contradiction between what his theory demands and 

 what observation attests by suggesting that the gyrating 

 portions of a spot are mainly those that are invisible ; but 

 the plea is inadmissible. A genuinely spiral structure 

 should throughout bear the imprint of certain characteristic 

 features. The " thatched edges," for instance, which so 

 often constitute penumbrse, could no more subsist as the 

 garniture of a whirling umbra than the ship of Ul} sses 

 could have escaped engulfment by Charybdis, Nor are 

 the uinbral cavities uniformly dark. Secchi frequently 

 saw them overspread with a sort of floccular haze ; Mr. 

 Maw has noted in them delicate, vein-like traceries; and the 

 '■ black holes " first detected by Dawes are unmistakably 

 chasms of irregular shapes and stationary behaviour, not 

 foci of swirling movements. The claims to acceptance of 

 the cyclonic theory of sunspots are in fact less plausible 

 now than they seemed thirty years ago, when less evidence 

 had been accumulated by which to try their vabdity. 



This, fatal though it be, is not the only objection to Dr. 

 Halm's views. They are also in glaring disaccord with 

 observation as regards the connection of spots with 

 prominences. They involve the production of spots, 

 cei'tainly as a consequence, but far from the neighbourhood 

 of a primarv outburst of metallic flames ; while for the 

 avowed object of harmonising this state of things with 

 what is actually seen, a further expedient is resorted to. 



If a spot be a vortex, there must be an indraught of the 

 surrounding atmosphere towards its centre. The screening 

 action accordingly, owing to which heat collects and finally 

 occasions eruptions, should be particularly effective over a 

 maculated region. Hence, it is alleged, the close associa- 

 tion of prominences with spots. But what are the facts ? 

 Par from being obscured by extra-absoi-ption. spot-bearing 

 tracts of the photosphere are usually of dazzling brilliancy. 

 And for this precise reason that they are often measurably 

 humped up as if by the relief of pressure, thus j>artially 

 di.scarding the veil uniformly spread over the unbroken 

 disc. The escape of heat pent up by an atmospheric 

 cloak has then assuredly nothing to do with the genesis of 

 prominences. 



There are indeed many more reasons than can be here 

 adverted to in detail for rejecting the opinion that spots 

 are mere eddies in the drifting luminous material of the 

 photosphere. Thus Mr. Maunder has expressly dwelt, in 

 the pages of this journal, upon peculiarities in their 

 movements and mode of development indicating their deep- 

 seated nature.* And the strange frequency of antipodal 

 outbreaks, both photospheric and chromospheric, intimate 

 as their precedent conditions, disturbances reaching to the 

 very core of the globe. 



Dr. Halm's interpretation of solar phenomena cannot, 

 it is clear, be accepted in its entirety ; yet it includes much 

 that is instructive and suggestive. Especially noteworthy 

 is the relation elicited by him from Spoerer's and 

 Carrington's observations, between the rate of solar 

 rotation and the condition of the solar surface. When 

 spots are numerous, he finds reason to conclude that the 

 equatorial drift is quicker by 750 miles a day than when 

 spots are scarce. The figures tabulated certainly show 

 a cyclical variation of the indicated kind ; and the 

 result is in itself plausible. It needs, however, to be 

 verified, since figures are apt to show illusory coincidences 

 which vanish with the repetition of the series. But the 

 enigma of the sun's rotation has hitherto proved so baffling 

 that any hint towards its solution is welcome; and should 



* Knowleboe, Vol. XVII., p. 200. 



