June, 1903.] 



KNOWLEDGE, 



127 



The greater lumiuosity of the centre of the disk indicates 

 absorjJtiou of light, prol >ably due to au extensive atmosphere. 

 . . . I (Prof. Hough) assume that the visible boundary 

 lias a density of about one-half that of water. This 

 medium is in the nature of a liquid ; in it are located the 

 Ureat Red Spot and the egg-shaped white spots. In sucb a 

 medium, all motions in longitude and latitude woidd be 

 slow and gradual, and the shape and size of the object 

 would have great permanency. . . . The great bay in 

 the south edge of the equatorial l>elt may be accounted 

 for l>y assuming that the great red spot is at a lower 

 temperature than the medium in which it floats, and I'y 

 its lower temperature condensing a portion of the vapour 

 composing the belt. ... in 1883 1 stated that the 

 spot seemed to have a repelling infltience on the belt." 



Jf the action of the sim is largely instrumental in causing 

 the phenomena of Jupiter's spots and belts, then any 

 periodicity due to revolution round the sun should recur 

 every twelve years. There is no such cycle conspicuous, 

 and any slight periodicity due to this cause — either in the 

 forms or the rotations of the markings — has escaped notice 

 if it e.xists. Jupiter has also a fairly large family of his 

 own, but we cannot lay to the account of his satelhtes the 

 credit of originating his disturbances any more than we 

 can attribute to the planets, the spots, iaculaj, and promi- 

 nences on the sun. But in both cases there may be a 

 modifying influence — a Jovian observer of the very tirst 

 raulc has told me that in his transits of the Great Red Spot 

 during the last years, he has detected a period, corre- 

 spondiug to a conjunction of the two largest satellites — a 

 swing or vibration in the Spot as the satellites pulled on 

 it this way or that. 



Here, then, on Jupiter we seem to get some analogy lo 

 the sun. The planet is itself at a higli temperature, but 

 is scarcely at all self-luminous ; its photosphere, if it, may 

 be so termed, is of the nature of a liquid ; and its maculie 

 have increased and become more permanent as its radiative 

 output decreased. 



lu a 3'et minor degree still, the condition of things ju'e- 

 vailing in the sun must also prevail in the interior of the 

 earth. From the solid crust, radiation is nil ; in the centre 

 the pressure and temperature must be such as have no 

 name. 



In the year 1(300 Dr. William Grilbert wrotxj that the 

 "earth is a great magnet"; thuit its "magnetic virtue" 

 conies from witliin the earth, and not from the heavens 

 above. This was an inspiration on his part rather tliana 

 valid deduction from facts at his disposal, and it was not 

 until the nineteenth century that it was shown that only 

 about five per cent, of the earth's magnetic force is to be 

 referred to electric currents, either circulating round the 

 earth or passing to and fro betv»een the earth and the 

 air. The remainder is derived partly from permanently 

 magnetised substances in the earth's crust, but chietiy 

 from a system of electric currents embedded deep in the 

 interior of the eartli and connected with the earth's 

 rotation. The diurnal variations, annual variations (not 

 secular change), and magnetic perturbations are to be 

 ascribed to electric currents in the upper regions ; and it 

 is these which vary in intensity with the suuspot cycle. 

 But for tlie rest there are. 1 think, thnio points to lie noted. 

 First, in the " IVlagnetic Declination Tables and Isogonic 

 Charts for l!t02,' Mr. L. A. Bauer statics that. ' accordiiig 

 to tlie researches of two Russians, Leyst and I'assalski], 

 the diurnal variation is different over locally disturbed 

 areas, e.ij.. in regions of iron mines, from what it would be 



if (he (listurliaiices did not exis( Whether 



the secuhir change is likewise different over locally dis- 

 turbed regions troin what it would lie if the local 

 disturbance were not present, is not yet known." Next, 



magnetic storms are most frequent and violent during the 

 periods of sunspot activity, and are in very many instances 

 coincident with the passage of a disturbed region across 

 the central meridian of the sun. During the prevalen<^ 

 of these storms, according to Mr. Bauer, strongly marked 

 variations in the electric currents within the earth's crust 

 manifest themselves along with the variations of the 

 magnetic needle. And, thirdly, it may be a mere coinci- 

 dence, but it seems a significant one, that the commence- 

 ment of the prolonged sunspot minimum in the 

 seventeenth century synchronized with the vanishing of 

 the iii;i'4iii'tir .ji'cliuation. It would not be wise to press 

 this ii>in(i(lcuce too far, seeing that we cannot be certain 

 when the long solar calm really commenced. It was 

 marked and commented upon from 1660 onwards. Neither 

 have we frequent observations of the magnetic declination, 

 nor at many places, and the time at which it disappeared 

 is not the same for all the places of observation. But, taken 

 in conjuaction with each other, these three phenomena 

 suggest that the deep-seated magnetism of the earth is 

 not wholly unaffected by the solar changes, and the study 

 of its variation may show evidence of change in the sun. 



The chief source of the earth's magnetism is due to 

 a system of electric currents in its interior, and connected 

 in some manner with its rotation. It is believed that the 

 seetilar change of the earth's magnetism is to be referred 

 primarily to the eft'cct of secondary electric currents 

 generated within the earth by its rotation around an axis 

 not coincident with its magnetic axis. The diurnal 

 variation is taken as due to a system of currents in the 

 earth's upper atmosphere, which are connected with the 

 earth's rotation, in that different parts of its surface are 

 exposed to the action of the sun's rays in a period of 

 twenty-four hours. The variation itself varies in sympathy 

 with several solar phenomena, and is thereby seen to be 

 influenced, directly or indirectly, by the sun's rotation. 

 The solar phenomena which change concurrently with the 

 diurnal variation of the earth's magnetic force are, the 

 corona, the prominences (eruptive and quiescent), facuhe 

 and spots. 



The variation of sunspots, both in niiniljtn-, area, and 

 distribution during the progress of the solar cycle, is too 

 well known to need niucti detailed description. Briefly, at 

 maxima, the spot-zones occupy mean positions in about l-S"^ 

 of N. and S. latitude ; as the minima approach, the zones 

 sink and die out near the equator ; and about tiie same time 

 a new series of spot-zones have come into existence in high 

 solar latitudes, aud gradually with the advancing cycle 

 approach again the maximum position at about 15" N. and 

 «. There is tlius indicated an intimate relation between 

 the intensity of the whole solar disturliance and its dis- 

 tribution in solar latitude. But the individual spots and 

 streams of spots show a closer connection still with the 

 rotation of tne sun on his axis. A stream of spots is, or 

 always tends to become, iiarallel to the sun's equator. 

 Where there is comparatively large initial inclination, the 

 "pull "is so strong that 1 have noticed a slight swing 

 across the line of 'latitude before the stream settled down 

 to steaily parallelism. So too with a single spot whose 

 umbra was oval, and inclined to the equator. 



Spots are rarely seen in higher latitudes than 30'^ ; never 

 1 believe above 50'. Facuhv. like spots, wax and wane 

 with the solar I'vcle, but have two zones of maximum 

 activity in each hemisphere. Their principal maxima are 

 disiilaceil with llie spot-zones ; secondary maxima occur in 

 higii latitudes beyond the regions frequented by spots. 

 Tiicy are only distinguishable near the suns limb, where 

 the greater absorption of photosphoric liglit by the sun's 

 •• dusky veil " throws them into bright relief. They are, 

 however, closely connected with, if not actually identical — 



