August, 1902.] 



KNOWLEDGE. 



179 



Tliough the red sjiot is now very feebly visible, it ujiiy 

 still be iutaot aiul in no dan<,'er of disappearance. The 

 past history of the object is suijgestive that we may expect 

 a rej)etition of its formerly conspicuous aspect. The 

 hollow in the belt is as strongly pronounced as ever. If 

 it tilled up ami the south half of the double S. equatorial 

 belt Ix'came continuous (without bend or break) in one 

 and the same latitude throughout its circumference, then 

 we should conclude that the material of the spot had 

 really dispersed. But nothing of tiie kind seems imminent. 

 The past behaviour of the features iu this region indicates 

 that when the red spot regains its former prominence the 

 hollow and the southern half of the equatorial belt will 

 decay and probably fade away to invisibility. Possibly 

 the material evolved from Jupiter's heated surface is 

 responsible for the ]>roductii)n of the belts and spots on 

 the planet ; and the intensification of the bnlt north of tlit; 

 red sp(.)t me:\ns that the sustaining material of the latter 

 is being diverted to the belt, whereupon the sjiot necessarily 

 becomes very feeble. It is certain that the spot and the 

 hollow in the belt have been perceptible either alternately 

 or together ever since 1S31, September, a period of nearly 

 71 years, and that the objects can be distinctly traced on 

 a number of drawings made by various observers during 

 that lengthy interval. And it is quite possible that these 

 associated features of the i)lanet were in visible evidence in 

 the earlier years of last century though they do not appear 

 to have been recognised and to have occasioned special notice. 

 The following is a general summary of appearances 

 recorded at successive periods : — 



183I-I856. — A large and well-defined hollow in the 

 southern side of the southern equatorial belt. 

 — Schwabe. 

 1857, November. — An elliptical object of coQsiderable 



size resting upon the S. equatorial belt. — Dawes. 

 1859-1 8tJ0. — A large elliptic ring iu the southern hemi- 

 sphere and situated between the S. equatorial belt 

 and the S. temperate belt. — Huggins. 

 1860-1867. — A great depression or hollow in S. side of 

 the equatorial belt. — Baxendell, Huggins, Jacob, 

 Long, Noble and others. 

 1869-1871. — A large elliptic ring lying S. of the S. 



equatorial belt. — Gledhill and Mayer. 

 1872. ^A large oval cloudlike object in S. hemisphere, 

 between S. equatorial and S. temperate belts, aud 

 preceded by an oblique belt. — Birmingham, 

 Browning, Gledhill, Kuobel, Terby and others. 

 1873. — Great break in the S. side of the S. equatorial 

 belt.* A brick-red area filled up this break. It 

 was pointed at preceding end and rounded at 

 following end. It "extended over some 30 degrees 

 of longitude. — Lord Rosse and Dr. Copeland. 

 1876-1877. — Large pink spot often seen on S. side of 

 equatorial belt. Rounded at preceding end and 

 pointed at following end. — Russell and Bredichin. 

 1878-18S2. — Great elliptical spot of an intense brick 

 red colour, seen just south of the 8. equatorial 

 belt. Length 31 degrees. — Many observers. 

 1883-1886. — Large elliptical ring situated in a well- 

 marked dip or hollow on the S. side of the 

 equatorial belt. — Many observers. 

 1887-1902. — Larye faint dusky ellipse, varying slightly 

 in tint and tone from }'ear to year, and placed in a 

 hollow in the ecjuatorial belt. Darkest at following 

 end. Outlines not distinctly traceable except on a 

 night of good definition — Many observers. 



*Tho same object was figui-ed by Mr. E. B. Knobel in 1873, April 

 20,auil May 11, when it was on east limb (Monf/ilt/ Notices, XXWU.. 

 p. -170), but it cannot be satisfactorilv identified in 24 drawings of 

 Jupiter wliich he made in 1S74 (Monthly Notices, XXXIV., p. 4o3). 



The motion of the objects has been by no means uniforni 

 during the 71 years ; in fact, there have occurred some 

 curious oscillations, as the following brief summary will 

 prove. Throughout the whole time, however, the spot and 

 hollow appear to have exhibited identical rates of motion ; 

 a conclusion which is fully justified by the obsei^vations. 



In 1831-2 the rotation period was about 9h. 55m 33'5s., 

 rising very slowly during the next 24 years, at the rate of 

 about one-tenth of a second annually, until in 1855 it was 

 9h. 55m. 36"ls. Then the spot exhibited greater retai-da- 

 tion, so that in 1859 its period had augmented to 

 9h. 55m. 38'3s. It then accelerated its movements, aud 

 the period declined quickly, until in 1862 it amounted to 

 9h. 55m. 35'3s. Remaining for some 17 years at about 

 9h. 55m. 34'5s., another retardation set in in 1879, in- 

 creasing the period in 1885 to 9h. 55ni. 39-6s. From that 

 year the motion continued to exhibit a slight decrease 

 until 1900, when the period was 9h. 55m. 41'9s. In 1901 

 a marked acceleration affected the position of the object, 

 and the period decreased to 91i 55m. 409s. This was 

 continued in 1902, and it seems probable that the motion 

 will undergo a further acceleration in immediately ensuing 

 years. 



At the middle of 1894 the longitude of the spot coin- 

 cided with the position of the zero meridian System II. 

 (daily rate =9h. 65m. 40-63s.) of Mr. Marth's ephemerides. 

 But the retarded rate of velocity during the following sis 

 years caused it to drift eastwards, until its observed 

 longitude in 1900, September, was 44i degrees, and it 

 remained at this value in 1901 and first half of 1902. 

 Should the motion of the spot now continue to increase, 

 and the rotation period become less than 9h. 55m. 40'63s , 

 the longitude of the spot and hollow will, at some future 

 date, again exactly correspond with the zero meridian of 

 System II., as it did in 1894. 



In the chart the region of the great red spot is 

 delineated, as it successively appeared at various epochs in 

 and since 1831. It must be confessed that some of the 

 features, as represented, do not offer a very striking 

 similarity, but there seems little reason to question their 

 identity. It is well known that drawings by different 

 observers, owing to variety in method and individual con- 

 ception, sometimes appear strangely discordant, and that 

 the same details are represented under aspects which bear 

 very slight mutual resemblance. If we allow for this 

 personal equation in the matter of dra\ving, and for the 

 actual changes which undoubtedly occurred in the objects, 

 and virtually transformed them from time to time, the 

 apparent inconsistencies will disappear. The positions 

 and motion of the markings agree very well together. In 

 one drawing by Sir Win. Huggins, dated 1858, December 

 2, a large well-markeil hollow was figured a little E. of the 

 centre, but this would not accord with other results in 

 1857, 1859, and 18C0, so that doubts were thrown upon 

 the endeavour to satisfactorily trace back the history of 

 the feature, for the drawing alluded to required the 

 assumption that two hollows existed in the great southern 

 equatorial belt. But it was soon afterwards found that 

 by a clerical error the year 1858 had been assigned instead 

 of 1860, and this being rectified, the position of the 

 hollow was found to harmonize perfectly with other 

 observations. All the collated material exhibits in fact a 

 remarkaljle consistency, and forms a chain of evidence 

 which appears to be conclusive, as to the identity of the 

 features delineated by Schwabe and many other observers 

 since his time. 



In 1873 the red spot is described as pointed at its pre- 

 ceding end, while the opposite end was rounded. In 1877 

 this figure seems to have undergone reversal, for the sharj) 

 end was then on the following side. These alterations of 



