122 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, 1904. 



a uniform temperature. At the surface, however, there 

 will be a wide difference ; between the rapid loss by 

 radiation and the rapid restoring currents from below, 

 no permanent or equable balance can be maintained ; the 

 slow currents from within, which tend to make their way 

 outwards under the tendency of the bulk to settle down 

 with greater condensation in its outward parts, here burst 

 forth with a violence which all can see, and having 

 parted with their energy, return nearly as precipitately. 

 I Radiation, then, is the dominating factor in the distri- 

 bution of temperature and density within the Sun, and 

 it will be noticed that in showing it to be so no assump- 

 tion has been made as to the law by which it proceeds. 

 If we wish to put our conclusions in a numerical shape, 

 such assumptions cannot be escaped. For example, 

 Stefan's well-established law of radiation, according to 

 the fourth power of the absolute temperature from the 

 surface of a " black body," does not apparently permit 

 any conclusions to be drawn as to the law by which a 

 gas would radiate. Between imperfect physical know- 

 ledge on the one hand, and mathematical difficulties on 

 the other, nothing can be done except to produce a more 

 precise illustration of the foregoing argument, and to 

 show that the conclusions will stand scrutiny. This I 

 have done in a paper published some nine years ago."" 



There are two other general problems presented by the 

 Sun which appear to invite solutions upon general 

 mechanical principles. The first of these is the eleven- 

 year period in solar activity. But as to an efficient cause 

 for it, or even any calculable phenomenon which could 

 follow its phases in a similar period, we seem to be still 

 quite in the dark. An attempt has been made to repro- 

 duce such a period by a combination of tidal effects pro- 

 duced by Jupiter and Saturn ; but the result is uncon- 

 vincing, because the tide produced must be at most very 

 minute, and the coincidence of period is dependent upon 

 a hypothesis for which no reason can be assigned as to 

 the relative intensity of effect of the two planets. In 

 fact, we know as yet too little of the phases of this cycle 

 to hope to theorise upon it successfully. Any real ex- 

 planation must cover the more detailed description which 

 Lockyer has given, to which allusion has been made 

 above. 



The second problem to which I refer is the law of rotation 

 of the surface, by which the equator of the Sun rotates most 

 vapidly, and parts in lower latitudes rotate more rapidly 

 than parts in higher latitudes. The law was discovered 

 by Carrington from motions of the spots, and was at 

 first believed to refer to the spots, but in the hands of 

 M. Dun&r, the spectroscope has proved that the property 

 belongs to the whole photosphere. If we do not mark 

 off the photosphere from the rest of the body of the Sun, 

 this law contains, I believe, no mystery. If we suppose 

 that in the course of its condensation in the past the 

 inner strata of the Sun were to be found rotating faster 

 than those outside them, it can be proved that as soon as 

 the body had condensed to a compact Huid consistency 

 so that the internal friction of its relative motions came 

 into play, a law of rotation identical with that exhibited 

 in the vSun woulil deselop. But perhaps more striking, 

 though less complete than a mathematical proof, is an 

 illustrative experiment that was carried out some years 

 ago by M. Beloposky, who filled a glass globe with water, 

 carrying powdered stearin in suspension, and whirled it 

 on a whirling machine until a uniform rate of rotation was 

 taken up by the whole. The glass was then stopped and 

 the motion of the water as exhibited by the particles in 

 suspension was watched. The circumstances were now 



•Memoirs Royal Astronomical Society. Vol. LI. 



in substance just such as I have sketched above, and the 

 apparatus exhibited just such relative motions as the 

 Sun displays, individual particles travelling spirally from 

 the equator towards either pole, with an angular motion 

 which was less for greater latitude, ultimately passing 

 inwards radially into the body. This last detail seems to 

 convey also a suggestion of activities limited to special 

 zones that may prove fruitful. 



Photograph of the Solar Granula-tions. 



In the accompanying plate we give a reproduction on a 

 reduced scale of part of one of the magnificent photo- 

 graphs of the solar surface, recently published by 

 M. lanssen in the " Atlas," which we noticed in the 

 April issue. The wonderful manner in which the 

 minute structure of the solar photosphere is brought out 

 in M. Janssen's superb photographs is due principally to 

 the care which he has taken to secure two points — the 

 one that the photograph shall be taken by light which is 

 practically monochromatic, so that the image is as sharp 

 as it is possible to obtain it ; the other that the exposure 

 shall be extremely short, so as to accentuate minute 

 differences of brightness in the most luminous portions 

 of the disc. It will be noted that the photograph is so 

 under-exposed that in the present reproduction the penum- 

 bra;' of the spots are perfectly black. They were not 

 absolutely featureless in the original, but were exceedingly 

 faint, the darker portions of the sun being thus sacrificed 

 in order to secure the maximum of detail in the more 

 brilliant parts. The intensely granular nature of the 

 disc and the thatch-like structure between the spots are 

 very clearly seen. This particular region of the sun 

 does not show any strongly developed instance of the 

 blurring of the granules ; but here and there small 

 smudged regions show themselves. 



The original of this photograph was taken on April i, 

 1884, at loh 46"^ G.M.T. The group of spots in the 

 centre of the field is the one numbered 1343 in the 

 Greenwich series. It was a sudden outburst, the day 

 of the photograph being only the second of its exist- 

 ence. Its area at the time was 177 millionths of the 

 sun's visible hemisphere, or slightly over 200 millions of 

 square miles. The group increased in size with great 

 rapidity. On April 2 its area was nearly five times as 

 great as on April i, and l)y April 6 the group was one of 

 the largest seen during the entire 1882-1884 maximum. 

 It returned to the visible hemisphere on April 21. I )uring 

 the thirteen days that it was under oliservation at this 

 return, it was gradually diminishing in area; the leader 

 spot being as usual a circular spot of regular structure, 

 and much more stable than the rest of the group. Before 

 the group disappeared at the west limb on May 3, the 

 leader was the only survivor. The leader was seen again, 

 still as a well marked circular spot, during two further 

 returns. It slowly diminished in size, and was last seen 

 on July 12, when it had shrunk to an area of no more 

 than six millionths of the solar hemisphere. The entire 

 life of the .group was thus 103 days. 



The scale of the accompanying photograph is one of 

 ^^ inches to the diameter of the sun. 



Mr. J. \V. Jarvis, l'\r,.S., St. Mark's College, Chelse.i, S.W., 

 has been appointi-d Cl.iss Secretary and Class Treasurer to 

 the London C.eolosical Field Class. The excursions this 

 season are to Mcrstham on April 30, and to I'urley, Henley, 

 Wimbledon, Aylesford, Leighton, liedford, Chislehurst on 

 succeeding Saturdays. 



