*

first discovered, but is certainly not inferior in magnitude to the largest of Jupiter's moons. Cassini soon detected a singular phenomenon in this satellite; through nearly one half of its revolution about Saturn, it disappears regularly, even when sought with the same telescope in which, through the rest of its revolution, it is a conspicuous object. He concluded that one half of the surface of the satellite must be less capable of reflecting light than the other, and that, like our moon, it rotates once on its axis in each revolution about its primary. He subsequently abandoned these views; but they were confirmed by Newton and Herschel, the former showing that no explanation can be given of the regular disappearance of the satellite but that suggested by Cassini; the latter by a series of careful observations with his powerful reflectors, establishing the correctness of Cassini's observations. These, and similar observations by M. Bernard at Marseilles in 1787, and by later astronomers, seem to leave no doubt on the subject.† We have here, then, a secondary planet rotating on its axis in 2 months, while (as will presently appear) its primary, whose volume is 15,000 times as great, rotates on its axis in less than 10 hours. On December 23rd, 1672, Cassini discovered a third satellite

*The only satellites whose motions of rotation have been detected exhibit the same peculiar relation between rotation and revolution. They are six in number::-our moon, the four satellites of Jupiter, and the outer satellite of Saturn. Either the surface of the largest of Saturn's satellites is little marked with irregularities, or these are distributed with tolerable uniformity, since it presents no appreciable changes of brilliancy. Of the other six satellites of Saturn, the satellites (variously estimated at four, six, and eight) of Uranus, and Neptune's satellite, nothing is likely to be known till telescopes far more powerful than any now in use shall have been constructed.

In the year 1705, it was observed that this satellite was visible through a complete revolution, and it was hence concluded that the irregularities upon its surface are variable.. Far more probably, however, the phenomenon was due to the exceptional clearness and steadiness of the earth's atmosphere during the interval of two or three weeks occupied by the satellite in traversing the part of its orbit in which it usually disappears. Any one who is in the habit of using a telescope of even moderate power systematically, must soon become aware that there are occasionally brief intervals during which the power of the telescope seems increased, though the eye detects no corresponding change in the appearance of celestial objects. Unfortunately, such intervals occur but rarely in our latitudes, and seldom last more than two or three days. They generally occur in early spring and late autumn; winter and summer are seldom favourable seasons for astronomical observation, notwithstanding the brilliance of some of our winter nights, and the softer splendour of the nocturnal skies in summer.

whose orbit lies within those of the other two. He effected this discovery by means of a telescope of Campani's, 35 feet in focal length. This satellite revolves about Saturn in rather more than 4 days, at a mean distance of about 328,000 miles. Judged by its brightness, it is probably much smaller than either of the two satellites first discovered. It exceeds the outer satellite in brightness, however, when the latter is at or near its easterly elongation.

In March 1684, Cassini discovered two more satellites by means of Campani's object-glasses of 100 and 136 feet focal length.* These satellites revolve within the orbits of the first three, their mean distances from Saturn's centre being about 224,700 and 180,000 miles. Thus both are nearer Saturn's surface than our moon to the surface of the earth. They occupy about 2 days, and 13 days, respectively, in completing their revolutions about Saturn. They are about equal in brightness, being each slightly inferior in this respect, and therefore probably in magnitude, to the third satellite discovered.

Cassini found that the orbits of the five satellites hitherto discovered correspond with the laws of Kepler (see Table V.). He found also that the four inner satellites move in planes very nearly coincident with the plane of the rings, while the fifth moves in a

* Cassini also used object-glasses of 200 and 300 feet focal length, and Auzout constructed glasses having focal lengths of 600 feet. Of course, glasses of such enormous focal length were not fixed in tubes. They were attached to frames constructed to slide up and down tall uprights. The eye-glasses of such telescopes were simply connected with the object-glasses by wires of the proper length. Observation with such telescopes must have been wearisome work, and we cannot wonder that the invention of reflecting telescopes was gladly hailed as offering a relief from the use of such cumbrous and imperfect instruments. The reflector presented by Hadley to the Royal Society, in 1723, though it had a focal length of only 10 feet 5 inches, was fully equal in power to the refractor of 123 feet focal length given by Huygens to the same Society. Yet the difficulty of grinding the specula accurately, and of preserving them when ground from changes of form and loss of reflecting power, must always prevent reflecting telescopes from replacing refractors, now that the construction of achromatic object-glasses has attained such perfection. That absolute truth of form has been obtained in reflecting specula by the most ingenious systems of grinding may be doubted, when we remember that the Harvard refractor, with an object-glass of fifteen inches diameter, has clearly resolved nebula in which but doubtful indications of resolvability are afforded by the splendid 6-feet speculum of Lord Rosse's reflector.

It may be questioned whether, in certain applications of the telescope, tubeless telescopes might not be occasionally used with advantage, diminution of weight and consequent cheapness of construction compensating a slight loss of illuminating power.

plane inclined at an angle of about 15° to the plane of the ring. The younger Cassini investigated these relations more closely, and in 1717, published a table of the distances, mean motions, and inclinations of the orbits of these satellites. He determined also with considerable accuracy, the position of the ascending node of the rings' plane on the ecliptic, and on Saturn's orbit, and the position of the ascending node of the fifth satellite on the same circles. Halley corrected the results obtained by Huygens and the elder Cassini; and later, in 1720, published the elements of the orbits of the five satellites, corrected from a series of observations made by Pound. Halley also detected an eccentricity in the orbit of the largest satellite, and roughly determined its amount, and the position of the line of apsides.

Cassini called the four satellites he had discovered Sidera Lodoicea,' in honour of Louis XIV., under whose patronage his labours had been conducted.* This name has, however, long since been disused. The satellite discovered by Huygens has received the name of Titan; and the four discovered by Cassini have been called (in the order of their distances from Saturn), Tethys, Dione, Rhea, and Japetus. But the most convenient method of indicating these and the satellites since discovered, is by numbering them in the order of their distances from Saturn: thus the satellite discovered by Huygens is now known as the sixth satellite, while the satellites discovered by Cassini are known as the third, fourth, fifth, and eighth satellites. By referring to Plate I. the reader will be able to form an idea of the relative brightness of these bodies, and of the probable proportions they bear to each other, to the globe of Saturn, and to the other bodies represented in that engraving, all of which are on the same scale. In fig. 1, Plate VII., they are represented at their proper relative distances from Saturn; while in fig. 2, the dimensions of their orbits are represented on a smaller scale. The elements of the eight satellites are given in Table V., Appendix II.

For nearly a century after the discovery of Tethys and Dione no new features of importance were revealed by the tele

Cassini was naturalized in France in 1673. His son Jean Jacques Cassini, and his grandson César Francois Cassini, were both born in France. The family, however, originally came from Italy. Cassini himself was born at Périnaldo, in Nice.

scope in the Saturnian system. Several phenomena already suspected were verified, however, and others--not wanting in interest-detected. Hadley discovered that the outer part of the ring is thinner than the inner; he observed also the shadow of the ring on Saturn,* and the shadow of Saturn on the ring. He confirmed Huygens' observation of belts on Saturn's disc, and found that, like the belts of Jupiter, they vary in form and number. Halley, also, observed Saturn's belts, and concluded from their changes of form as Saturn traverses different parts of his orbit, that Saturn rotates on an axis perpendicular (to the sense) to the plane of the rings; in other words, that the plane of Saturn's equator coincides (to the sense) with the plane of the rings. In October, 1714, a few days before the disappearance of the rings, the earth being nearly in their plane, Maradi observed a singular phenomenon:-the narrowing ansæ of the ring appeared to be unequal in size, the eastern being the larger; yet after an interval of two nights the eastern ansa had disappeared, while the western was visible, though reduced to a faint line of light. From these observations he concluded that the rings are not of uniform thickness, and that they revolve about Saturn in their own plane.† In this conclusion may be traced the germ of the important discovery of the rotation of the ring afterwards made by Herschel. It may be noticed, however, that in arriving at this conclusion Maradi made two assumptions, neither of which (as will presently appear) is correct. He assumed, first, that the ring is a solid formation; and secondly, that it is a rigid solid. The first assumption was justified by the appearance of the ring, and was maintained, or rather never disputed, till the discoveries of the last few years led to a

* Cassini, in 1675, observed a dark belt on Saturn's body, parallel to the greater axis of the rings. This was probably either the shadow of the ring on Saturn, or the first indication of the existence of the dark inner ring lately discovered. In 1675, the rings were well opened, but not to their full extent; at such a time the outlines of the belts are elliptical. The outlines of the dark ring and of the shadow are, it is true, also elliptical, but they form parts of larger ellipses, and appear nearly straight and parallel to the greater axis of the ring.

† It will be seen from Table X. that the plane of the ring passed through the sun in February 1715, reappearing. After this the plane of the ring passed twice through the earth, disappearing at the first passage and reappearing at the second. These passages occurred in the summer of 1715, and within a few weeks of each other.

different view. The second assumption, on the other hand, is altogether unreasonable. It was not to be expected but that so vast a formation, subject to so many disturbing attractions, and whose thickness is obviously disproportionate to its other dimensions, should be subject to vast undulations; and these, for anything known to the contrary in Maradi's day, might sweep round the ring, altogether independently of any absolute motion of rotation in the system, and would thus sufficiently account for the phenomena observed by Maradi.

The motions and distances of the satellites, and the dimensions of the ring, were determined with considerable accuracy, by several astronomers, during the interval above mentioned. Some of these measurements will be made use of in a future chapter, but most of them have given place to the more exact determinations of the present century.

One or two observations, rather curious than valuable, were also made in the interval named. Thus, Whiston records that his father had seen a star through one of the openings between the planet and the ring. Such an occurrence, though uncommon with the telescopes in use in his day, is not infrequent with modern telescopes, especially when Saturn is traversing the constellations Taurus and Gemini in one part, Scorpio and Sagittarius in the opposite part of the Zodiac. No star of the first four or five magnitudes has ever, I believe, been seen through these openings. Again, Cassini has recorded that in 1692 he saw a fixed star occulted by Saturn's largest satellite, an occurrence that must be exceedingly rare even with the most powerful telescopes, and when Saturn is traversing those parts of the Zodiac in which stars of all magnitudes are most profusely scattered.

During the last fifteen years of the eighteenth century, many important discoveries were made by the elder Herschel in the Saturnian system. When the northern side of the ring was visible before the disappearance of the ring in 1789, he carefully examined the black line discovered by Ball. He appears during this time to have been strongly opposed to the idea that the ring is divided, even where this line is seen; still less was he willing to accept the hypothesis of the multiple division of the ring. Four observations