plane of the ring 1° 33'. Sir John Herschel, about the same time, found the λ to be 313° 43′, the longitude of the peri-saturnium to be 53° 40', the eccentricity 004217, and the orbit to be precisely in the plane of the ring. The serious differences in these two results are to be ascribed to errors in the observations arising from the difficulty attending them, but such differences naturally make us distrust the entire batch of figures.

Dione. Sir John Herschel in 1836 found the λ to be 327° 40′, the longitude of the peri-saturnium 42° 30′, the eccentricity 0·0206, and the orbit to be precisely in the plane of the ring.

Rhea. Sir John Herschel in 1835-7 found the λ to be 353° 44′, the longitude of the peri-saturnium 95°, and the eccentricity 0.02269. The inclination is very small.

Titan", as the satellite most easily seen, has naturally received most attention. Bessel's determination of its orbit is reputed to be the most complete. For the epoch of 1830'0 he gave the λ at 137° 21′, the longitude of the peri-saturnium at 256° 38′, and the eccentricity 0.029314. The line of apsides has a direct motion on the ecliptic of 30′ 28′′ annually, completing a revolution in 718 years, the nodes completing a revolution in 3600 years.

Hyperion has been so recently discovered that its orbit has not been very fully investigated. From Washington observations made in 1875 Hall found the λ to be 120° 12', the longitude of the peri-saturnium 173°, the eccentricity o118, and the inclination of the orbit 6° 12'. Lassell's observations made at Malta in 1852 and 1853 agree with these conclusions in part, but Hall remarks that neither Lassell's observations nor those at Washington "fix the position of the satellite in its orbit with much certainty, since

"When Huygens discovered this satellite in 1655, he was imprudent enough to predict that there were no others, because Titan being the 6th secondary planet, and there being only 6 primary planets known, Nature's (supposed) laws of symmetry were satisfied. The danger of prediction in matters of this kind is well illustrated in the case of Mr. John Harris, F.R.S. That learned gentleman published a book in 1729, in which he

says: ""Tis highly probable that there may be more than 5 moons revolving round this remote planet [the number of satellites which Saturn was then known to possess]; but their distance is so great as that they have hitherto escaped our eyes, and perhaps may continue to do so for ever; for I do not think that our telescopes will be much further im proved!!"

they were made when the plane of the orbit was nearly edgewise to the observer." He adds:-" If we examine the elements we shall see that Hyperion moves nearly in the plane of the orbit of Titan, and considering the values of the eccentricities it will be seen that these satellites can approach very near each other." Hyperion was seen by Bond on Sept. 16, 1847, and by Lassell on Sept. 18, but it was not till the date given in the table that its character was determined.

Iapetus. Lalande for the epoch of 1790 gave the λ at 269° 37′, and the at 150° 27', reckoned on the orbit.

The following elements are by Captain Jacob3:

A. Hall considers that the orbits of the 5 inner satellites are sensibly circular and that they move in the plane of the ring or nearly so, but it will be readily understood that the small apparent size of most of these satellites, and the consequently limited number of telescopes and observers which can be brought to bear on them, materially retards the attainment of any more perfect acquaintance with their motions, though it is reasonable to hope that the multiplication of large instruments and experienced observers now taking place will ere long lead to a development of our knowledge of the orbits of these satellites.

• Ast. Nach., vol. xcv. No. 2263. June 17, 1879.

P Month. Not., vol. xviii. p. 1. Nov. 1857.

Sir J. Herschel pointed out the curious circumstance that the period of Mimas is that of Tethys, and the period of Enceladus that of Dione. Monck puts these facts in the shape that the ratio of these 4 periods are 2, 3, 4 and 6, adding that the period of Iapetus is very nearly 5 times that of Titan. D'Arrest further called attention to the commensurability within, or 2, of 274 revolutions of Mimas, 170 of Enceladus, and 85 of Dione1.

Kirkwood has discovered a still more complicated relationship, which may be thus enunciated: To 5 times the daily angular motion in its orbit of Mimas add the daily motion of Tethys, and 4 times the daily motion of Dione, and the sum total will be equal to 10 times the daily motion of Enceladus.

The disappearance of the ring, in 1862, was taken advantage of by various observers for watching the rare phenomenon of a transit of the shadow of Titan across the planet. The satellite itself was not seen on any occasion, but Dawes and others obtained several good views of the shadow. Again in 1877 the shadow of Titan was seen by Common, and others. The only observation of this kind prior to 1862 appears to have been made by Sir W. Herschel on Nov. 2, 1789. Dawes on May 25, 1862, saw an eclipse of this satellite in the shadow of Saturnthe only instance on record.

It must not be supposed that Titan is the only satellite of which an eclipse, transit, or occultation is possible, for all the satellites are occasionally subject to these effects. This is especially true of the two innermost ones, but the small apparent size of all except Titan hinders observation of them.

Celestial phenomena on Saturn must possess extreme grandeur and magnificence, the rings forming a remarkable series of arches stretched across the Saturnian heavens. The nearest satellite, Mimas, traverses its orbit at the rate of 16' of arc in a minute of time, so that, as viewed from Saturn, it moves in 2 minutes over a space equal to the apparent diameter of the Moon. Considering

Month. Not., vol. vii. p. 24. Dec. 1845. r Ast. Nach., lvii. No. 1364. June 14.

s Month. Not., vol. xxii. pp. 264, 297, &c. May and June 1862.