January 2, 1899.] 



KNOWLEDGE 



11 



or two observers of great repntation have challenged the 

 reality of this duplicity, and in this they were for a short 

 time confirmed by the writer, at a time when his eyes had 

 not the training necessary for the detection of such delicate 

 details. It was subsequently found, however, that the 

 broad appearance of this belt is merely an imperfect view 

 of its really double structure. And as this conclusion 

 has been subsequently confirmed by photography, dissent 

 on this subject is necessarily sQenced. 



The equatorial zone is the brightest region of the planet, 

 though its brightness falls far below that of the inner 

 bright ring. A narrow band often marks the equator. 

 The colour of the zone is a strong yellow, in which adequate 

 aperture shows, as in the case of -Jupiter, a decided "wool- 

 pack " structui'e. At times, some of these cumuli become 

 so brilliant as to ofier easy marks for any rotational dis- 

 placement of spots. 



The rotation period of Saturn was discovered by the 

 illustrious W. Herschel in 1793 from changes in the 

 appearance of the famous " quintuple belt," in the planets 

 south hemisphere. From one hundred and fifty-four 

 rotations, Herschel gave the value of lOh. 16m. O'ls., 

 " exact to much less than two minutes either way."* 



For a long time Herschel's results remained unconfirmed, 

 when, in 1876, Prof. Asaph Hall detected a bright spot in 

 the equatorial zone. A careful discussion of his observa- 

 tions gave him the value of lOh. Um. 23-83. ± 2-303.1 



The subject was attacked with the most severe consistency 

 in 1891 by Mr. Stanley Williams. Having obtained, 

 between 1887 and 1891, nearly four thousand observations 

 of transits of spots across the central meridian of Jupiter, 

 this great observer was better prepared than anybody else 

 for the investigation of the arduous problem. Mr. Stanley 

 Williams saw not only the white spots in the tropical 

 regions of Saturn, but, further, had the good fortune of 

 seeing his persevering efforts crowned by the discovery of 

 a series of dark spots on the great double belt, south of the 

 planet's equator. The observations of four bright spots 

 and a dark one, discussed by the late Dr. Marth, gave the 

 value of lOh. llm. 21-84s. In 1892, the bright equatorial 

 spots gave to Mr. WUliams the period of lOh. 13m. 38-4s. 

 In 1893, the Brighton observer saw easily both the bright 

 and the dark spots of the northern hemisphere, and con- 

 firmed, by observation. Proctor's suggestion to the elfect 

 that " the equatorial zone of Saturn has a rotation period 

 measurably shorter than that of the non-equatorial zones. " ; 

 Mr. Williams further found that the surface material of 

 the self-same latitude did not rotate at a uniform period in 

 difi'erent values of kronocentric longitude. § 



These remarkable results have been confirmed in 1891. 

 The comparison of the annual rotation periods determined 

 from 1891 to 1891 showed a decrease in the period of 

 rotation amounting to Im. 4Gs. " This dift'erence " says 

 Mr. WiUiams, " means a very considerable increase in the 

 velocity of motion of the surface material, amounting to 

 sixty-six mUes an hour. In other words, the great equa- 

 torial atmospheric current of Saturn was blowing sixty-six 

 miles an hour more quickly in 1891 than it was in 1891." 

 The same sceptics whom we have seen denying the 

 duplicity of the belts, questioned the reality of the spotted 

 appearance of Saturn.* But, inasmuch as their ob3erva- 

 tions have invariably dealt with "the most piomLnent 



* Philosophical Transactions, 1794, p. 66. 



t "Saturn and its Eing," App. II. to Washington Obs., 1885, p. 15. 



I " Old and New Astronomy," p. 624. 



§ Monthly Notices, E. A. S., llai-ch, 1894. [ Ibid, May, 1895. 



T Mee'a " Obserrational Astronomy," App. II. to Second Edition. 



features," it would seem that, in throwing doubt on 

 the reality of details whose delicacy is beyond their 

 reach, they are going beyond their own department, 

 and their opinion is no longer of weight. The spots of 

 Saturn have been seen by observers of great practical 

 skill, among whom we might mention Messrs. H. MacEwen, 

 at Glasgow, A. Henderson, at Liverpool, G. L. Brown, at 

 Stirling, the Rev. P.H. Kempthorne, of Wellington College, 

 and the late Rev. A. Freeman. At Juvisy the dark spots 

 have been held steadily, and their rotation followed, by M. 

 Camille Flammarion ; also by persons who knew nothing 

 about their existence. It was only after two years' 

 fruitless attempts with the nine and three-qaarter inch 

 equatorial that the writer saw them in 1896, when the 

 observation of three spots gave him lOh. ll-3m. for the 

 rotation period. 



This powerful array of evidence establishes the objective 

 reality of the spots of Saturn on an indestructible basis. 

 Theur discovery is one of the most remarkable achieve- 

 ments in planetary work of our times. And when we 

 recoUect that the instrument used in their detection was 

 only a six and a-half inch reflector, we deem it impossible 

 to do adequate justice to the steady perseverance and 

 unrivalled skill of Mr. Stanley Williams, whose name we 

 feel assured will pass down to posterity in company with 

 those of the greatest observers who have ever lived. 



The question of the nature of the ring system, whose 

 eccentricity to the globe is manifest at present* (see the 

 Plate accompanying this paper), has been attacked by 

 Laplace, to whose theory very valuable contributions were 

 brought by two distinguished lady astronomers, Madame 

 Sophie Kowalewski and Miss Dorothea Klumpke.t Bond 

 and Pierce also investigated the subject, but it is to the 

 genius of Edouard Roche and J. Clerk Maxwell that we 

 owe our knowledge of the constitution of that marvellous 

 configuration. As a result of his studies on the figure of 

 a fluid mass submitted to the attraction of a distant point, 

 Roche found, in 1819, that when a satellite, supposed of 

 the same density as its primary, is situated at a distance 

 of less than 2-11 radii, any ellipsoidal figure of equilibrium 

 becomes impossible through the tidal action of the primary. 

 The semi-axis major of the orbit of Mimas, the first 

 satellite, being 3-10 radii, this moon could form and 

 subsist ; but the ring, 

 being entirely within 

 the theoretical limit 

 (1-18 to 2-30), has 

 actually remained in a 

 state of cosmical dust. 

 Thus, for Roche, as 

 well as for Clerk Max- 

 well, the only possible 

 system of rings would 

 be composed of a num- 

 ber of discrete particles 

 revolving round Saturn 

 with varying velocities 

 — a theoretical con- 

 clusion, which the 

 application by Keeler 

 of the Doppler-Fizeau 

 principle to the revolu- 

 tion of the ring has confirmed in such a striking manner. 

 These particles are either united into a series of narrow 



* Since 1895 the writer invariably found that the eastern vacuity 

 is greater than the western— a somewhat normal appearance. 



t Theses prJsentees a, la Faeulte des Sciences de Farit ipour obtenir 

 le grade de Docteur 'h Sciences Mathematiques, par Mdlle. D. 

 Klumpke. Paris, 1893. 



Fig. 3. — Amphitheatrical Gradations 

 separated by Rarefaction Zones, seen on 

 the Inner Bright Eing on 1896, April 18 

 (9|in. O.a., power 300). 



A 



