existence is concerned, for a particular account of it would be altogether foreign to the purposes of this work. The period of each of these planets is subject to a continuous change owing to the mutual influence exerted by each on the orbit of the other and the time required for this change to go through its various stages is the Period of the Great Inequality. It amounts to 918 years.

The Tables of Jupiter used till recently were those of A. Bouvard, published in 1821, but the new and far superior Tables of Le Verrier have superseded them. For the satellites, Damoiseau's Tables (published in 1836) are employed. As regards the satellites there is room for much improvement in the Tables at present employed. They fail to give results characterised by the precision which modern science demands.

* See Sir J. Herschel's Outlines, p. 502.

d These tables were employed for the

first time in England in the preparation of the Nautical Almanac for 1878.

CHAPTER XII.

Period, &c.-Figure and Colour of Saturn.-Belts and Spots.-Observations of the Belts by Holden.—By Ranyard.--Bright spot recorded by Hall.-Probable atmosphere.— Observations of Galileo, and the perplexity they caused.—Logogriph sent by him to Kepler.-Huygens's discovery of the Ring.—His logogriph.-The bisection of the Ring discovered by the brothers Ball.-Sir W. Herschel's Doubts.-Historical epitome of the progress of discovery.-The Dusky" Ring.- Facts relating to the Rings.-Appearances presented by them under different circumstances.-Rotation of the Ring.-Secchi's inquiries into this.-The Ring not concentric with the Ball.-Measurements by W. Struve.— Other measurements.—Miscellaneous particulars.—Theory of the Ring being fluid. Now thought to consist of an aggregation of Satellites.—The “Beaded” appearance of the Ring.-O. Struve's surmise about its contraction.—Irregularities in the appearances of the ansa.-Rings not bounded by plane surfaces.-Mountains suspected on them.-An atmosphere suspected.—Physical observations between 1872 and 1876 by Trouvelot.—Observations by MM. Henry.— By Keeler.-Brightness of Rings and Ball.-Bessel's investigations into the Mass of the Rings.-Saturn attended by 8 Satellites.-Table of them.— Physical data relating to each.-Elements by Jacob.-Coincidences in the Rotation-periods of certain of them.—Transits of Titan.—Celestial phenomena on Saturn.-Lockyer's summary of the appearances presented by the Rings.— Peculiarity relative to the illumination of Iapetus.-Mass of Saturn.—Ancient observations.-Saturnian Astronomy.

INFERI

NFERIOR in size to Jupiter only, Saturn may fairly be pronounced to be the most interesting member of the Solar System. It revolves round the Sun in 10759-2 or 29'45' at a

For drawings, &c. of Saturn, see Annals of Harvard Coll. Obs., vol. ii. (120 drawings by the Bonds); Ast. Nach., vol. xxviii. No. 650, Nov. 1848 (J. F. J. Schmidt); Ibid., vol. xxxix. No. 929, Jan. 8, 1855 (Secchi); Mem. R.A.S., vol. iv. p. 383 (Kater); Ibid., vol. xxi. p. 151 (8 figs. by Lassell); Month. Not., vol. xi. p. 23 (Dawes and Lassell); Ibid., vol. xiii. p. 16 (Dawes); Ibid., vol. xiv. p. 17

(Dawes); Ibid., xv. p. 79 (Dawes); Ibid., vol. xvi. p. 120 (one fig. by Jacob); Ibid., vol. xviii. p. 75 (abstract of Harvard Obs.); and vol. xxii. p. 89 (two figs. by Jacob); Student, vol. ii. p. 240 (Browning). Month. Not., vol. xliv. p. 407 (Pratt); Ibid., vol. xlv. p. 401 (Green); Ast. Nach., vol. cxii. No. 2682 (Lamp); Month. Not., vol. xlvii. p. 514 (Elger); L'Astronomie, vol. vi. p. 208 (Stuyvaert).

SATURN. March 27 and 29, 1856. (De La Rue.)

The dark ring C is very decidedly too narrow as indicated in the engraving.

mean distance of 886,065,000 miles, which an orbital eccentricity of 0.056 may increase to 931,033,000 or diminish to 841,097,000 miles. Its apparent diameter varies between 15'1" in conjunction, and 20.7" in opposition, and its real (equatorial) diameter may be taken at 75,036 miles. Its polar compression is larger than that of any other planet, Jupiter not excepted: but it is usually less noticeable than that of Jupiter because the ring distracts the eye. Sir W. Herschel's value of the compression is 103; Bessel's To; the Rev. R. Main's; and Hind's.

Saturn has no perceptible phases. The maximum defalcation of light under extreme circumstances is so small that the maximum breadth of the shaded area can hardly be of a second of arc -a quantity inappreciable.

The figure of Saturn is now quite understood to be that of an oblate spheroid, but at one time considerable doubt existed about the matter in consequence of Sir W. Herschel having advanced the opinion, from observations made in April 1805, that the planet was compressed at the equator as well as at the poles ; or, as it is generally phrased, that it resembles a parallelogram with the corners rounded off, so as to leave both the equatorial and the polar regions flatter than they would be in a regular spheroidal figure. This opinion, never received with much favour (though not entirely unconfirmed by later observers), is now almost universally repudiated, chiefly owing to the micrometrical measurements performed by Bessel in 1833 and by Main in 1848. Some optical illusion was probably at the foundation of it, though it is right to say that the notion is believed in to this day by some persons, and ascribed to an actual upheaval of the planet's surface recurring from time to time and due to quasi-volcanic causes. It must also be added, that (as in the case of Jupiter) we only see the outline of Saturn's atmosphere and not that of the solid (or fluid) body of the planet itself.

Belts exist on Saturn resembling those of Jupiter, but they

See Month. Not., vol. xiii. p. 79, Jan. 1853, for others, and same vol., p. 152, for a note by the Rev. R. Main: an important

memoir by the same observer appears in Mem. R.A.S., vol. xviii. p. 27, 1850.