January 1, 1896.] 



KNOWLEDGE, 



world as Jupiter is a suflSciently striking circumstance, 

 and led at once to the suggestion that here we had a 

 glimpse of the real solid surface of Jupiter. But the 

 observations of three most careful and acute observers — 

 Mr. Denning, Mr. Stanley 'Williams, and Prof. Hough — 

 combined to prove that the red spot had a most remarkable 

 peculiarity : it changed its rate of rotation. 



The foUowing table by Mr. Denning {Xature, September 

 3rd, 1891, vol. sliv., p. 439) speaks for itself:— 



Rotation Pkkiods of the Gbeai Red Spot. 



The rotation period has remained fairly constant in more 

 recent years. 



Briefly stated, it imphes that in seven years the rotation 

 period of the spot lengthened by seven seconds, a statement 

 which is equivalent to saying that if the first rotation 

 period given above represents the true rotation period of 

 Jupiter, this vast formation, whatever its real character, 

 had travelled eastwards with a constantly accelerating 

 speed until it was moving at a rate which would bring it 

 back to its starting point in less than six years, and which 

 would suffice to make the tour of the earth's equator in 

 about seven months. If we could imagine Australia to 

 set oii" on its travels, and to quietly traverse the Southern 

 Ocean towards Africa at a speed of a little over four knots 

 an hour, we should have a very fair representation on the 

 earth of what appears to be taking place on Jupiter, for 

 Australia bears about the same relation as to area and 

 position to this planet as the great red spot to that. 



This fact seems to shut us off from adopting the 

 suggestion which the relative persistency of the spot in 

 shape and size so naturally suggests ; viz., that we have here 

 a ghmpse of the soUd surface of the planet, or, perhaps, a 

 partial crusting over a Hquid nucleus. The latter idea is 

 not absolutely irreconcilable with these observations of 

 drift, for it is quite conceivable that such a crusting might 

 become gradually dissolved away on the western side — 

 speaking now, as always, as from the point of view of an 

 inhabitant of Jupiter — and be extended at ubout the same 

 rate on the eastern. A slight variation in latitude which 

 has also been remarked might be explained in the same 

 way ; and the greater stability of rotation period at the 

 present time would imply that the crust had now attained 

 a greater consistency, and had ceased to undergo this 

 double process. Its present rotation time would then be 

 the rotation time of the planet itself. 



We cannot, however, adopt this suggestion, for if the 

 red spot is a surface marking, then the white region which 

 has surrounded it, and which, as a complete annulus 

 surrounding the spot, has been even more conspicuous on 

 photographs than to the eye, must be due to clouds — must 

 be, in effect, at a higher level. This transfers the marvel of 

 the persistency of the shape and size of the spot from the 

 spot itself to the overlying cloud ring. Of the true shape 

 of the actual spot itself we can know nothing ; the shape 



of that which we see of it is defined wholly and entirely by 

 the gap in the cloud regi dd abuve. 



When the spot was first observed in 1878 and 1879 

 its similarity in hue to the great ruddy equatorial belt was 

 frequently remarked upon ; and, as several observers 

 have remarked — Prof. Keeler in particular = {PuhUcatiom 

 of the Astronomical Societif of the Paciiic, Xo. 11) — the 

 appearance of the equatorial regions is strikingly like 

 what we should expect if the white cloud-like formations 

 were floating at various heights in a reddish fluid. It is 

 reasonable, therefore, to suppose that were the cloud masses 

 removed we should see the whole of Jupiter as of the same 

 deep hue as the red spot possessed in 1878 and 1879. 



This would be in perfect harmony with the opinion at 

 which almost every student of the planet has arrived, and 

 not by one course of reasoning alone, but by mmy, viz. : 

 that .Jupiter still retains a very considerable amount of 

 intrinsic heat, and the red tint might be reasonably 

 ascribed to the glow of heated material far below the 

 apparent surface, or possibly even to the high temperature 

 of the lower strata of the atmosphere itself. 



That the problem of the persistency of the outline of the 

 spot is really one of the persistency of the upper cloud 

 masses is, I think, shown by the definiteness of the 

 "shoulder," at one time very clearly an integral part of 

 the complete phenomenon, and its most stable and obvious 

 feature in recent years. Read by the light of some of the 

 photographs taken by the Brothers Henry, the red spot 

 was produced by a disturbance in a bright white belt. 

 The belt at a certain point divided into two arms, which, 

 arching round, the one to the north and the other to the 

 south, reunited again some forty degrees of longitude 

 further on ; the enclosed space being the celebrated spot, 

 and the commencement of the arch on the north foUowing 

 side the equally well-known " shoulder.'' 



This transference of the site of the problem does not, 

 however, help us to a solution. We may see in the 

 gradual diminution of the rotation period a subsidence of 

 the cloudy surroundings defining the spot, and in the fact 

 that the spot seems now to have adopted a steady rate, an 

 indication that the vapours have reached a level where they 

 are in more stable equilibrium. But the great mystery how 

 and why, amidst so many changes, the outline of the spot has 

 shown so little change remams a mystery still. Perhaps the 

 modeof dissolution of the spot, should it befated todisappear, 

 may give us a means of fathoming it that at present we lack. 



JUPITER. 



The drawing of Jupiter that accompanies this number 

 of Knowledge was made on the 17tb of April, 1885, at 10 

 p.m., with an 18-inch reflector and powers of 300 diameters. 

 The opposition of this year was particularly favourable for 

 obtaining views of Jupiter, the writer having made one hun- 

 dred and fifty-six drawings of the planet. The large red spot, 

 though not so clearly seen as in previous years, was still 

 distinctly visible, although the colour had faded. The 

 general details of the disc were well marked, and with their 

 various colours gave great interest to the study. The large 

 red spot was followed by two similar markings ; the first of 

 these was about the length of the spot distant from it, on 

 the same latitude. It was first observed in March of that 



• "The red bolts ]n'osenleil on nil oocasious the appearance of h 

 )).i9sive medium, in wliieli the phenouuMia of the stri-aiuers and other 

 forms shown in the drawings were manifested. These phenomena 

 would be exactly repriiduced by streamers of doudy-whito matter 

 lloatiug in a semi-transparent reddish lluid, sometimes submerged aud 

 sometimes rising to the surfai'c, and it is by no meaiu impouible that 

 such is actually their nature." 



