June 2, 1881] 



NATURE 



115 



as almost to prove that there i> no other kaowa power capable 

 of producing such a stupendous result. 



According to this hypothesis we may imagine the particles of 

 matter, when originally produced, to have been at a great dis- 

 tance from each other, all however being endowed with the 

 power of gravitation — forming in fact a chaotic mass. As 

 these particles gradually came together in virtue of their mutual 

 attraction, heat would be generated in the condensing mass, and 

 it hai been calculated that this cause, by storing up a vast amount 

 of heat in the sun, is sufficient to account for its wonderful out- 

 pouring of heat and light throughout a long series of ages. 



But the wh^le of the riddle is not thus solved. A man may 

 have vast resources and yet a totatab ence of ready money. Or 

 a nation may have plenty of food and yet not be able to bring it 

 fast enough into a famine-stricken district. And so the sun may 

 possess in its interior abund.ince of high-class energy and yet be 

 unable to bring it quickly to the surface — indeed it has been 

 calculated by Sir William Thomson that if the sun were an in- 

 candescent solid body its surface would probably cool in a few 

 minutes of time. The perplexing fact about the sun and stars is 

 not so much that they have somehow obtained a vast store of 

 energy, as that they are able to bring it to the surface w ith an 

 astonishing regularity. Neveitheless this regularity, great as it 

 is, is not apparently perfect. There are a good many examples 

 of variable stars of which some few suffer sudden and extreme 

 changes of brilliancy, while in others the variation is much less 

 conspicuous. In these orbs the transport service by which the 

 heat is brought to the surface appears to work unequally, and 

 even in some cases to break down altogether. Were we much 

 nearer to them than we are we 'might study these inequalities 

 with advantage, and perhaps gain some insight thereby into the 

 nature of the wonderful machinery that brings the heat to the 

 surface. 



As it is however we must chiefly confine ourselves to a study 

 of the sun. Can we therefore hope to find out the nature of the 

 machinery by which the light and heat of our sun are brought to 

 the surface ? and is this machinery unequal in its action? Is the 

 sun, in fine, a variable star ? First of all, let us have a clear 

 conception of the precise meaning of this question. No doubt 

 the clouds by day and the earth itself by night interpose them- 

 selves between us and our luminary so as to render its direct 

 influence exceedingly variable ; but this is not the point. Fre- 

 quently in passing along the streets of an evening we see into 

 the interior of some room which has just been lighted up ; but 

 immediately the blind is jiuUed down, and we see it no longer. 

 The gas may however be all the while burning behind the 

 blind with a constant lustre ; or it may be that from water in the 

 pipe or some other cause the flame is intermittent. Now this is 

 the point which we wish to determine about our sun. Is sun- 

 light intrinsically constant, or is it subject to variations ? and if 

 so, can we determine the extent and the periods of these varia- 

 tions? Now at first sight it seems exceedingly strange that we 

 are compelled to ask this question. 



It might naturally be im.agined that astronomers, who can give 

 us the light variations of Beta Lyrce or some other variable star 

 with the greatest precision, must certainly be able to give 

 us similar information about the sun. That they are totally 

 unable to do so is unquestionably very strange. When however 

 we begin to examine we find several reasons for this curious 

 failure. In the first place we must all be glad to think that 

 within historic times at least the variations of the sun's light- 

 giving power can never have amounted to a large proportion of 

 the whole. Had this been otherwise none of us could have 

 been alive at this moment to speculate on solar vari.ability. 



Nevertheless these suspected differences, although not exceed- 

 ingly great, may still be large enough to enable astronomers in 

 some remote part of the universe to pronounce our sun to be a 

 variable star. How is it then that we who are mainly concerned 

 in this variability are yet unable at first sight to decide upon the 

 fundamental question of its existence? 



We have not far to seek for an answer to this enigma. The 

 fact is we are too near and too deeply concerned in the issues to 

 be able easily to detect the variation. We have never the 

 opportunity of comparing the sun's light with the pure light of the 

 stars in the way in which we can compare the light of one star with 

 that of another. We must therefore resort to means by which 

 the direct light and heat of the sun may be accurately measured. 

 Now it cannot be said that instruments for this purpose do not 

 exist, but they have not been systematically made use of to 

 determine this important point, and indeed there almost appears 



to be a reluctance in humanity to face the fact of the sun's 

 variability. 



When, in process of time, the telescope came to be invented, 

 by its means Fabricius and Galileo speedily discovered that the 

 face of our luminary was not altogether frc from spots. This 

 fact had been previously known to the Chinese, who in the 

 course of their long and peculiar civilisation had recorded many 

 instances where such spots were large enough to be visible to 

 the naked eye. But at present we have to do with the progress 

 of European thought. The first accurate observer of these 

 phenomena was Hofrath Schwabe of Dessau, a distinguished 

 German astronomer. More than fifty years ago he set himself 

 to the task of taking frequent sketches of the disk of the sim, 

 which mi^ht record approximately the positions and areas of the 

 various groups of spots. For forty years he continued to 

 labour at this somewhat monotonous task with great perse- 

 verance, until at length his unwearied libours were crowned by 

 a singular and unlooked-for discovery. This consisted in the 

 evident periodicity of these phenomena. Daring some years 

 Schwabe found the sun to be almost entirely free from spots, 

 while on other occasions the solar disk was mottled over with 

 very frequent groups, the period from maximum to maximum, 

 or from minimum to minimum, being nearly eleven years. 

 From the observations of Schwabe and otliers it would appear 

 that 1828, 1837, 184S, i860, and 1S70 were years of maximum- 

 spot frequency, while 1833, 1843, 1856, 1867, and 1877 were 

 characterise i by a nearly total absence of sp its. Carrington, of 

 this country, followed in the steps of Schwabe, and gave the 

 world a very accurate record of the spots which appeared from 

 1854 to 1S60 inclusive. 



In 1858 De La Rue introduced the application of photography 

 to solar research, and since then photnheliographs have been 

 at work at Kew, Ely, and Greenwich, in this country, at 

 Wilna and Moscow in Russia, at Mauritius, Melbourne, 

 India, and Cambridge, U.S., more or less continuously up to 

 the present time. 



I can only allude to the magnificent solar pictures produced 

 by Langley at the Alleghany Observatory, and more recently by 

 M. Tanssen, the distinguished French astronomer, as forming a 

 new" point of departure in the history of solar debneation. 

 Janssen's pictures are more than a foot in diameter, and in them 

 every minute detail of the sjn's structure is accurately repre- 

 sented. 



But it is time to tell you what a sun-spot really is. Prof. Wilson 

 of Glasgow made in 1774 an observation which greatly startled the 

 scientific world. He found that sun-spots behaved exactly as if 

 they were caverns with sloping sides dug into the body of the 

 sun. The bottom of these caverns is generally black, while 

 the sloping sides are less so. The black portion is therefore 

 called the umbra, while the less black sloping sides are 

 called the penumbra of the spot. It is easy to explain the 

 nature of Wilson's reasoning. The sun, it is well known, 

 revolves on its axis about once in twenty-six days from east to 

 west, so that a spot will take about thirteen days to travel across 

 the visible disk or hemisphere. It will come on at the left-hand 

 border or limb and disappear at the right, provided it remains 

 so long. Now Wilson noticed that when a spot is near the 

 limb the penumbra on the side nearest the sun's visual centre is 

 hidden from our view, on the same principle by which, when look- 

 ing into a silver jug, for instance, from one side of it, that interior 

 which is nearest the eye is hidden from the view. In fine, he 

 concluded, with perfect justice, that spots were pits or hollows 

 with sloping sides, and we are justified in adding that they are 

 cloud pits, and not caverns of solid matter. 



These conclusions of Wilson have been abundantly confirmed 

 by the Kew observers, Mr. De La Rue and his colleagues, and 

 also by the spectroscopists who have devoted themselves to the 

 sun. 



It has furthermore been sho^\n by these observers why the 

 bottoms and sides, but more especially the bottoms, of such 

 caverns should be blacker than the sun's ordinary surface. 

 They are blacker because they are colder, end they are colder 

 because they represent a down-rush of matter from the high and 

 comparatively cold regions of the solar atmosphere— cf some 

 kind of celestial hail, we may perhaps imagine. So magnificent 

 is the scale of operations that fifty or sixty of our own earths 

 might be dropped into the cloud-cavern formed by the down- 

 rush— at least in the case of large spots. 



But a down-rush implies an up-rash, and we may add that a 

 down-rush of matter comparatively cold implies an uprush of 



