474 



NA TURE 



[JiNt i6, 1910 



set of features registered on the plates, no less corroborative 

 of the drawings made of him at F"lagstafT, but utterly 

 unlike those of Mars. Their symmetry is immediately 

 striking, and then no less is its purely latitudinal character. 

 They are belts, bright and dark, banding the disc half- 

 way to the poles. Their behaviour, however, indicates 

 in them no regard for the sun, as they are quite oblivious 

 both to the planet's day and to his year. They last 

 indifferently through both, and disappear at their own 

 good time. That the brighter are clouds and the darker 

 the gaps between seems inferable ; but they are not as 

 our clouds. With us the heat that causes cloud comes 

 from without, with Jupiter from within. Sun-occasioned 

 the one, self-evolved the other. We have visual evidence 

 of this internal heat of Jupiter in the cherry-red that tinges 

 his darker belts| as if we there looked down into the 

 seething cauldron below. We have theoretic proof of it, 

 too, in the oblateness the disc presents taken in connec- 

 tion with what we know to be the planet's mean density. 

 In two articles shortly to appear in the Philosophical 

 Magazine, those who care for mathematics will find that 

 his own fire alone enables Jupiter to keep his youthful 

 figure, and, furthermore, that his shape shows him to 

 consist of a comparatively small kernel wrapped in a 

 huge husk of cloud. Even those who do not care for the 

 oldest of the sciences must admit a certain grandeur in it 

 when theory can thus plumb depths experiment may never 

 fathom. 



These belts have another peculiarity. Their several 

 parts are travelling at idiosyncratic rates. With them it 

 is a go-as-you-please race, in which each outruns or falls 

 behind its neighbour. On this interesting subject we owe 

 most to your fellow countryman, Mr. Stanley Williams, 

 who for some years has acted as timekeeper and referee 

 of this Jovian family contest. In future he will have no 

 mean rival in the photographic plate. Not that it sees 

 as well, but that it may be measured at leisure by any 

 investigator who likes. 



There is one feature in the photographs which has had 

 a long and eventful history. I refer to the Great Red 

 Spot. Detected in 1879, it lasted as such to within a few 

 years. Rather a long life for a hole in the clouds ! Now, 

 properly speaking, we see only the grave in which it lies 

 buried, the oval shell it once occupied ; but these same 

 photographs were, in a sense, the means of bringing its 

 cradle also to light. Sixty years ago, a cycle of Cathay, 

 Sir Wiliam Huggins made a fine series of drawings of 

 the planet, and on receiving the present pictures was 

 struck by the resemblance of the two. In consequence 

 he sent me prints of his. On scanning them my eye was 

 caught by an oval placed as the present one lies. Clearly 

 it was the cradle prepared already for the Great Red Spot 

 twenty years in advance. He had been present before its 

 birth, as he is still, happily, present after its demise. 



In the next set of images we envisage a Jovian event of 

 some interest, in spite of the frequency with which it 

 occurs — the transit both of a satellite and of its shadow 

 across the planet's disc. For this means to Jupiter the 

 occasion of a total eclipse of the sun, an impressive pheno- 

 menon were there anyone there to see. In the left-hand 

 images the satellite itself may be descried just passing 

 off the disc, while in their complete procession the shadow, 

 which is the eclipse, may successively be followed in its 

 travels from one side toward the other of the planet's 

 face. The swiftness of its traverse may be marked in 

 the displacement it undergoes, not only directly, but with 

 regard to the Jovian cloud-belts, which are themselves 

 whirling round at the rate of 25,000 miles an hour. To 

 witness thus the progress of a total solar eclipse on the 

 great planet gives one, perhaps, his most vivid experience 

 of Jovian affairs. 



The third point we may mention in these photographs 

 is their revelation of the equatorial wisps. Some years 

 ago Mr. Scriven Bolton detected a most curious set of 

 markings lacing Jupiter's bright equatorial belt. His dis- 

 covery met with the usual approved disapprobation which 

 has been the orthodox reception of astronomical advance 

 since Galileo's time. Were a discovery to be hospitably 

 hailed it would prove disconcerting to the discoverer, who 

 would instantly suspect something wrong. Eventually the 

 subject was referred to us for corroboration. This we 

 were able, fortunately, to secure. A singular phenomenon 

 NO. 2120, VOL. 83] 



they proved to be, criss-cross filaments of shading 

 traversing the belt from triangular spots at its edges, for 

 all the world like the lacings of a sail that hold the bolt- 

 rope to its spar. Though perfectly evident to the eye, we 

 hardly hoped to catch them on a plate. Nevertheless, Mr. 

 E. C. Slipher did, and innumerable other images of them 

 have since been got by us ; their pictures you will presently 

 see for yourselves upon the screen. Why such peculiar 

 rents should be torn in the planet's great cloud envelope 

 we cannot yet explain, but further news about them has 

 still more lately come to us from the planet to which we 

 now pass in our journey outward from the sun, the great 

 ringed planet Saturn. 



In some respects Saturn is the most difficult of the three 

 planets to photograph, certainly the most tiring. So faintly 

 is it illuminated that what takes but two seconds for Mars 

 takes twent}' or more for Saturn. To keep the image 

 of the planet upon its guiding cross-wires for that length 

 of time, with the nervous knowledge that any slip will be 



Rei-roduclion of a photograph of the planet Saturn taken on 

 November 4, 1909, by Prof. Lowell. 



fatal, seems an eternity. Since sensations measure exist- 

 ence, it may be commended as a sure, though not happy, 

 way to prolong one's life. 



On the resultant images may be seen abundant detail. 

 Cassini's division is there as large as life, and somewhat 

 broader, due to the difficulty of keeping it still ; so also 

 is the shading of the inner side of ring B, and the tones 

 of the several portions of ring A. The ball appears finely, 

 its belts standing out even more than to the eye, and the 

 duskiness of its polar hoods being peculiarly pronounced. 

 The shadow of the ball upon the rings is, of course, salient, 

 and so is the shadow of the rings upon the ball. This 

 much is evident at a glance, but there is more to be made 

 out by him who examines closely. 



If we consider the images of November 4, which happen 

 to be mine, we shall notice a dark band below the rings 

 where thev cross the ball, and one which is but dusky 

 above them. Now at this date both the sun and the earth 

 were above the plane of the rings, as we see the image, 

 the sun the higher, the sun's relative latitude being 

 -12° 18', that of earth -11° 4'. 



We saw, in consequence, the shadow of the rings A and 

 B underneath the rings themselves. This accounts for the 

 dark band below. What, then, was the dusky band above? 

 It could not be the shadow of these rings, for the shadow- 

 could not fall on both sides of them at once, nor could it 

 be seen above. A little consideration will reveal to us 

 what this band was. Inside of ring B toward the planet 

 lies the crepe-ring C. It is a semi-transparent ring, be- 

 cause its particles are widely scattered, instead of seeming 

 solid like the outer rings, vi^here the particles lie closer 

 together. Their constitution we owe to perhaps the 

 greatest mind of the last century, your own Clerk-Maxwell. 

 This, then, was the explanation : in the dusky band we 

 were looking through the crepe-ring on to its own shadow 

 thrown upon the ball. Thus the crepe-ring revealed its 

 presence unmistakably, not by being seen, but by being 

 seen through. 



When we compare these images of November 4 with 

 those taken bv Mr. E. C. Slipher on September q, we note 



