10 



KNOWLEDGE 



[jANUARy, 1904. 



that in 1879 the period was only about Oh. 50m., but, 

 subsequently, increased steadily to 9h. 50m. 3(:ls. in 1896. 

 Since then "the value has again declined, a sudden drop 

 liaving been followed by somewhat irregular variations. 



The following are the periods which I have found from 

 a discussion of my own observations during the past few 

 years : — 



ITear. 



1898 

 1899 

 1900 

 1901 

 1902 



A remarkable feature of the Great Equatorial Current is 

 found in the peculiar wanderings or oscillations of the 

 spots on each side of their mean or com]nited positions ; 

 and it frequently happens that a whole grou]i of spots will 

 share these vagaries of motion together. Despite these 

 wanderings, however, there can be no doubt that many of 

 the equatorial spots remain visible for long periods of 

 time, but the fact that the planet is lost in the sun's rays, 

 so far as satisfactorv oliservations are concerned, for at 

 least three months about the time of his conjunction — to 

 say nothing of the difficulties caused by irregularities of 

 motion and changes of form in the spots themselves — 

 mal;e their correct identification from year to year almost 

 impossilile. 



No. 6. — A bright rift is usually seen to divide the 

 S. equatorial belt into two separate components. During 

 the last few years Captain Molesworth lias followed a large 

 number of bright spots in this rift, wliich appears to form 

 a kind of transition stage between tiie two well-knowu 

 periods of 9h. 50m. + and 9h. 55m. +. His rates for 

 1900 and 1901 are 9h. 51m. 37-3s. (from 17 spots) and 

 9h. 51m. 32-2s. (from 20 spots) respectively. 



No. 7. — On the occasions when markings on the S. com- 

 ponent of the S. equatorial belt have been observed and 

 followed, it has been found that their period differs but 

 little from the contemporary period of the Great Red Spot. 



No. 8. — This can scarcely be called a current, as the 

 surface material referred to under this heading is con- 

 fined within the limits of the Great Red Spot. This 

 remarkable object was detected in 1878 by M. O. Lohse, 

 of Potsdam (who appears to have been the first to 

 draw it), and by Professor Pritchett, of Missouri, and 

 Mr. Dennett, of Southampton (whose observations seem to 

 have been the earliest j^ublished), and quickly attracted 

 general notice. Nearly every telescope was directed to 

 its observation, and its behaviour carefully watched. 

 It is elliptical in shape ; its dimensions being about 

 27,000 miles in length, and nearly 9000 in breadth. 

 What the nature of the spot may be it is impossible at 

 present to say. Certainly it cannot be regarded as a solid 

 feature of the planet's globe, since it is by no means stable 

 in position ; but, on the other hand, there can be no doubt 

 that it is the product of forces which have considerable 

 permanence, and, judging from the very definite and 

 regular appearance of the well-known hollow or bay on 

 the S. side of the S. equatorial belt in which the Red Spot 

 lies (see Pig. 6), despite the present faintuess of the spot 

 itself, as yet show no signs of declining energy. A very 

 interesting account of the early history of the Red Spot 

 will be found in two valuable papers by Mr. Denning in 

 the supplementary numbers of Monthly Notices, R. A. S., 

 1898 and 1899, and also in his article in this journal for 

 August, 1902. In these papers Mr. Denning connects the 

 present spot and hollow in which it lies with the ellipse 

 seen by Mr. Gledhill in 18()9, and with numerous similar 

 objects which have appeared iu the southern hemisphere 



at intervals since 1831. Indeed it is quite possible that 

 the Red Spot of to-day may he identical with the remark- 

 able object discovered by Dr. Hooke.-so long ago as 1664. 

 The determinations of the rotation period have been very 

 numerous. Mr Denning, from a careful examination of 

 existing material, and assuming his identifications to be 

 correct, finds that in 1831 the period was 9h. 55m. 33-3s., 

 that it increased to 9h. 55m. 38'3s. in 1859, again declined 

 to 9h. 55ni. 334s. in 1877, and once again increased to 

 9h. 56m. 41-9s. in 1899. In 1900 the rotational velocity 

 exhibited a slight increase; in 1901 the spot remained 

 almost statiouarv in longitude (as based on the period of 

 9h. 56m. 4063s., adopted by Messi's. Marth and Crommelin 

 as the value of their zero meridian of System II ) ; and 

 iu 1902 — from a discussion of about 100 transit observa- 

 tions of the spot and hollow secured by various observers — 

 I find the period of the object to have been r-edueed to 

 9h. 66m. 39-3s. It should be added that the spot, in addition 

 to its oscillations in longitude, like so many of the markings 

 on the planet, has also a motion in latitude — the extreme 

 drift being about 4000 miles. The deep red tone which 

 distinguished the spot at the time of its appearance in 

 1878 soon proved evanescent, and the object is now but a 

 ghost of its former self. In some years it has appeared 

 merely as a faint elliptical ring ; at others, the whole has 

 just been visible as a feeble dusky stain on the bright zone 

 in whi(_'h it lies. Possilily it may lie dimmed by the over- 

 lying vapoiu's, but. as already stated, there is no reason to 

 sujipose that the forces which produce it are on the wane, 

 and we may yet hope that at some future time it will 

 reassume its former glory. 



No. 9. — This is unquestionably the steadiest and most 

 uniform of all the .Jovian currents. It was detected by 

 Schriiter so long ago as 1787, since which time it has 

 shown practically no variation. It extends over quite a 

 broad zone, emliracing the region between the S. edge of the 

 S. equatorial belt, and the N. edge of the S.S. temperate 

 belt. Observers of Jupiter will remember the remarkable 

 S. tropical mass of dark material — extending eventually 

 over about 90° of longitude — wliich swept round the S. 

 side of the Red Spot during the summer of 1902. 



No. 10. — This rapid current so far south is remarkable. 

 It appears to be fairly constant and uniform, but has 

 nothing in a similar latitude to correspond with it in the 

 northern hemisphere. 



No. 11. — In 1901 Captain Molesworth detected a number 

 of dark objects at the edge of the N. polar shading. These 

 did not share in the rapid drift of the Great Southern 

 Current (No. 10), but moved appi'oximately at the 

 tabulated rate. More observations are needed to establish 

 the constancy of this current. 



But interesting as is the investigation of these surface 

 currents, the real nature of Jupiter's physical condition is 

 the problem which students of the planet must endeavour 

 to solve. It has generally lieen agreed that the belts and 

 spots of Jupiter are of the nature of clouds and atmospheric 

 vapours, that the true globe of the planet has never been 

 seen ; and that its real rotation period is consecjuently 

 unknown. But whatever view may be adopted as to the 

 vaporous character or otherwise of the visible features of 

 the disc, it is pi\>bal)le that the internal liody of the planet 

 rotates iu a period somewhat longer than any markings we 

 can observe — possibly in a period just a minute or so less 

 than 10 hours. As regards the relative altitudes of the 

 various markings, there seems good reason to suppose that 

 the more swiftly moving objects are situated at a greater 

 height than those which move more slowly. Of course, it 

 must be remembered that the planet may have no solid or 

 definite surface divided off from the vapours which form 

 its belts and spots. It is highly probable^beariug iu 



