these orbits are exhibited on smaller scales in Plate VII. The distance of the moon from the earth, on the scale of Plate I., is 6.6 inches, corresponding very nearly to the distance of the figure representing Saturn's eighth satellite from the figure of the earth. On the same scale the diameters of Uranus and Neptune would each be rather less than 1 inch, and the diameter of Jupiter rather less than 2 inches, while the diameter of the sun would fall short of 2ft. by less than half an inch. Again, on the same scale the mean distance of the earth from the sun would be nearly half a mile, and the mean distance of Saturn from the sun more than 4 miles. From these relations the reader will see:-first, how insignificant are the dimensions of our earth compared with those of the larger members of our system; secondly, how small even these globes appear when compared with the sun; and lastly, how minute are the proportions even of that gigantic globe when compared with the distances at which his attendant orbs revolve around him.* Only the most powerful telescopes exhibit (under favourable atmospheric conditions) the complete series of phenomena described in this chapter. The two inner and the seventh satellites of Saturn are especially difficult objects. Mr. Wray records, however, that Mimas and Enceladus were visible in December, 1861, (when the dark side of the ring was turned towards the earth,) with his achromatic of only 7 inches clear aperture. The third, fourth, and fifth satellites are not very difficult objects. The fifth is slightly brighter than the other two; but all three are visible with a good achromatic of 4 inches aperture, the atmosphere being clear and steady.† The sixth and eighth can be readily detected with telescopes of

* When we compare the dimensions and orbits of the satellites with the diameters of their primaries, we do not find the same uniformity of disproportion. Thus, while the satellites of Jupiter and Saturn are mere atoms compared with their primaries, the volume of the earth does not exceed that of the moon more than fifty-five times; on the other hand, while the three outer satellites of Saturn, the outer satellite of Jupiter, and our own moon, revolve at distances from their respective primaries compared with which the diameters of those primaries appear very small, the same is not true of the other satellites of Saturn and Jupiter. The three inner satellites of Saturn revolve in orbits of very moderate dimensions, the mean distances of all falling within five and a half semi-diameters of Saturn.

† Wargentin relates that he saw the five brightest satellites with an achromatic of 10 feet focal length. On December 10th, 1793, Sir Wm. Herschel saw them with a

moderate power, Japetus requiring a power of 100, and Titan being easily visible with a power of 80. In general, Japetus must be sought for at a considerable distance from the disc of his primary.

The lowest power with which the rings become visible (as such) is about 50.* A power of 150 is required to exhibit them with distinctness. The division between the rings can be seen with a power of 200, when the rings are open to nearly their full extent and viewed under favourable conditions. Under the same circumstances, the dark ring may be seen, as already mentioned, with a good achromatic of 4 inches aperture. The division in the outer ring and the variable divisions are only visible with a few of the finest reflectors and refractors in the world.

The belts on the surface of the planet are visible, under favourable atmospheric conditions, with a good achromatic of 4 inches aperture. In general, however, they require telescopes of greater power to reveal their outlines with distinctness.

No object in the heavens presents so beautiful an appearance as Saturn viewed with an instrument of adequate power. The golden disc, faintly striped with silver-tinted belts; the circling rings, with their various shades of brilliancy and colour; and the perfect symmetry of the system as it sweeps across the dark background of the field of view, combine to form a picture as charming as it is sublime and impressive.

power of 60 applied to a reflector of 10 feet focal length. They can be detected, however, with smaller instruments. I have repeatedly seen all five with perfect distinct

ness through an achromatic of 4 inches aperture, and 5 feet focal length.

Since the diameter of Saturn when in opposition is about 19"-2, a power of 50 presents him to the eye with an apparent diameter of 16',-rather greater than the mean apparent semi-diameter of the moon; and it might at first sight appear that when the planet has so great an apparent diameter, the rings must be conspicuously visible. Such is not the case, however; and, in fact, this method of estimating the appearance of the magnified disc of a planet is altogether deceptive. The disc really appears of the dimensions calculated; but, in the first place, the apparent size of the moon is over-estimated by the unaided eye; and, secondly, when we so view the moon, its disc is not perceptibly distorted by atmospheric undulations, whereas these undulations are all magnified fifty-fold when we use a telescopic power of fifty, and the disc of a planet so viewed is correspondingly distorted. If to these considerations be added the difference in the illumination of Saturn and of the moon, (see Chap. VII.), and the loss of light by reflection at the surfaces of the lenses and by absorption in passing through them, it will readily be seen that an observer who should found his expectations on such a calculation as that given above, would be altogether disappointed by the view he would actually obtain of the planet.

CHAPTER IV.

THE PERIODIC CHANGES IN THE APPEARANCE OF SATURN'S SYSTEM.

IN describing his orbit about the sun, Saturn retains the direction of his polar axis (or axis of rotation) unaltered, or very nearly so. As in the case of the earth, there are small motions of this axis, owing to which, in the course of many thousands of years, the poles of the Saturnian heavens travel with an undulatory movement round two opposite small circles of the celestial sphere;* but so far as a single revolution about the sun is concerned, we may consider the polar axis of Saturn as retaining its direction absolutely unchanged. This axis is inclined at an angle of 63° 10' 32" to the plane of Saturn's orbit-in other words, the plane of Saturn's equator is inclined at an angle of 26° 49′ 28′′ to the plane in which Saturn moves. I propose, in this chapter, to consider the effect of this inclination in producing changes of appearance in Saturn's rings and disc, and in modifying the apparent orbits of his satellites; leaving to a future chapter the consideration of the variations- due to the same cause-in the Saturnian seasons, and in the appearance of the rings to the Saturnians.

* The complete revolution of Saturn's vernal equinox occupies upwards of 412,080 years, the annual precession of his equinoxes being 3"-145. The right ascension of the north pole of the Saturnian ecliptic is 18h. 23m. 317s.; and the declination 67° 22′ 20′′N. The poles of the Saturnian heavens revolve in two small circles, having this point and the opposite point on the celestial sphere for their respective poles, the angular radius of each small circle being 26° 49′ 28′′.

At present no conspicuous star lies near either pole of the Saturnian heavens. The right ascension of the northern pole is 2h. 23m. 1·7s., the declination 82° 52′ 36′′; this pole, therefore, lies near the northern foot of Cepheus, and nearly 6° from the polar star; the nearest visible star is 2 Ursæ minoris of the fifth magnitude-about 3° from Saturn's north pole. Saturn's south pole lies in the constellation Octans, and less than 1° from the star 8 Octantis of the fifth magnitude. As with the earth, the conspicuous stars, a Draconis and a Lyræ (the brilliant Vega) are possible north-polar stars for Saturn, and ‹ Argûs is a possible south-polar star; but many centuries must elapse before any of these stars will occupy such positions.