wards of 2000 to one. Let any one figure to himself the condition of our globe, were the sun to be septupled, to say nothing of the greater ratio! or were it diminished to a seventh, or to a 300th of its actual power! Again, the intensity of gravity, or its efficacy in counteracting muscular power and repressing animal activity on Jupiter is nearly three times that on the Earth, on Mars not more than one third, on the Moon one sixth, and on the four smaller planets probably not more than one twentieth; giving a scale of which the extremes are in the proportion of sixty to one. Lastly, the density of Saturn hardly exceeds one eighth of the mean density of the earth, so that it must consist of materials not much heavier than cork. Now, under the various combinations of elements so important to life as these, what immense diversity must we not admit in the conditions of that great problem, the maintenance of animal and intellectual existence and happiness, which seems, so far as we can judge by what we see around us in our own planet, and by the way in which every corner of it is crowded with living beings, to form an unceasing and worthy object for the exercise of the Benevolence and Wisdom which presides over all! (436.) Quitting, however, the region of mere speculation, we will now show what information the telescope affords us of the actual condition of the several planets within its reach. Of Mercury we can see little more than that it is round, and exhibits phases. It is too small, and too much lost in the constant neighbourhood of the Sun, to allow us to make out more of its nature. The real diameter of Mercury is about 3200 miles : its apparent diameter varies from 5" to 12". Nor does Venus offer any remarkable peculiarities: although its real diameter is 7800 miles, and although it occasionally attains the considerable apparent diameter of 61", which is larger than that of any other planet, it is yet the most difficult of them all to define with telescopes. The intense lustre of its illuminated part dazzles the sight, and exaggerates every imperfection of the telescope; yet we see clearly that its surface is not mottled over with permanent spots like the moon; we perceive in it neither mountains nor shadows, but a uniform brightness, in which sometimes we may, indeed, fancy obscurer portions, but can seldom or never rest fully satisfied of the fact. It is from some observations of this kind that both Venus and Mercury have been concluded to revolve on their axes in about the same time as the Earth. The most natural conclusion, from the very rare appearance and want of permanence in the spots, is, that we do not see, as in the Moon, the real surface of these planets, but only their atmospheres, much loaded with clouds, and which may serve to mitigate the otherwise intense glare of their sunshine. (437.) The case is very different with Mars. In this planet we discern, with perfect distinctness, the outlines of what may be continents and seas. (See Plate I. fig. 1., which represents Mars in its gibbous state, as seen on the 16th of August, 1830, in the 20-feet reflector at Slough.) Of these, the former are distinguished by that ruddy colour which characterizes the light of this planet (which always appears red and fiery), and indicates, no doubt, an ochrey tinge in the general soil, like what the red sandstone districts on the Earth may possibly offer to the inhabitants of Mars, only more decided. Contrasted with this (by a general law in optics), the seas, as we may call them, appear greenish.* These spots, however, are not always to be seen equally distinct, though, when seen, they offer always the same appearance. This may arise from the planet not being entirely destitute of atmosphere and clouds†; and what adds greatly to the probability of this is the appearance of brilliant white spots at its poles, one of which appears in our figure, which have been conjectured with a great deal of probability to be snow; as they disap * I have noticed the phænomena described in the text on many occasions, but never more distinct than on the occasion when the drawing was made from which the figure in Plate J. is engraved. Author. + It has been surmised to have a very extensive atmosphere, but on no sufficient or even plausible grounds. pear when they have been long exposed to the sun, and are greatest when just emerging from the long night of their polar winter. By watching the spots during a whole night, and on successive nights, it is found that Mars has a rotation on an axis inclined about 30° 18′ to the ecliptic, and in a period of 24h 39m 21s in the same direction as the earth's, or from west to east. The greatest and least apparent diameters of Mars are 4" and 18", and its real diameter about 4100 miles. (438.) We come now to a much more magnificent planet, Jupiter, the largest of them all, being in diameter no less than 87,000 miles, and in bulk exceeding that of the Earth nearly 1300 times. It is, moreover, dignified by the attendance of four moons, satellites, or secondary planets, as they are called, which constantly accompany and revolve about it, as the moon does round the earth, and in the same direction, forming with their principal, or primary, a beautiful miniature system, entirely analogous to that greater. one of which their central body is itself a member, obeying the same laws, and exemplifying, in the most striking and instructive manner, the prevalence of the gravitating power as the ruling principle of their motions of these, however, we shall speak more at large in the next chapter. (439.) The disc of Jupiter is always observed to be crossed in one certain direction by dark bands or belts, presenting the appearance, in Plate I. fig. 2., which represents this planet as seen on the 23d of September, 1832, in the 20-feet reflector at Slough. These belts are, however, by no means alike at all times; they vary in breadth and in situation on the disc (though never in their general direction). They have even been seen broken up, and distributed over the whole face of the planet; but this phænomenon is extremely rare. Branches running out from them, and subdivisions, as represented in the figure, as well as evident dark spots, like strings of clouds, are by no means uncommon; and from these, attentively watched, it is concluded that this planet revolves in the surprisingly short period of 9h 55m 50s (sid. time), on an axis perpendicular to the direction of the belts. Now, it is very remarkable, and forms a most satisfactory comment on the reasoning by which the spheroidal figure of the earth has been deduced from its diurnal rotation, that the outline of Jupiter's disc is evidently not circular, but elliptic, being considerably flattened in the direction of its axis of. rotation. This appearance is no optical illusion, but is authenticated by micrometrical measures, which assign 107 to 100 for the proportion of the equatorial and polar diameters. And to confirm, in the strongest manner, the truth of those principles on which our former conclusions have been founded, and fully to authorize their extension to this remote system, it appears, on calculation, that this is really the degree of oblateness which corresponds, on those principles, to the dimensions of Jupiter, and to the time of his rotation. (440.) The parallelism of the belts to the equator of Jupiter, their occasional variations, and the appearances of spots seen upon them, render it extremely probable that they subsist in the atmosphere of the planet, forming tracts of comparatively clear sky, determined by currents analogous to our trade-winds, but of a much more steady and decided character, as might indeed be expected from the immense velocity of its rotation. That it is the comparatively darker body of the planet which appears in the belts is evident from this, that they do not come up in all their strength to the edge of the disc, but fade away gradually before they reach it. (See Plate I. fig. 2.) The apparent diameter of Jupiter varies from 30" to 46". (441.) A still more wonderful, and, as it may be termed, elaborately artificial mechanism, is displayed in Saturn, the next in order of remoteness to Jupiter, to which it is not much inferior in magnitude, being about 79,000 miles in diameter, nearly 1000 times exceeding the earth in bulk, and subtending an apparent angular diameter at the earth, of about 16". This stupendous globe, besides being attended by no less than seven satellites, or moons, is surrounded with two broad, flat, extremely thin rings, concentric with the planet and with each other; both lying in one plane, and separated by a very narrow interval from each other throughout their whole circumference, as they are from the planet by a much wider. The dimensions of this extraordinary appendage are as follows *:— Exterior diameter of exterior ring Exterior diameter of interior ring Miles. = 176418. = 151690. Equatorial diameter of the body Interval between the planet and interior ring Thickness of the rings not exceeding The figure (fig. 3. Plate I.) represents Saturn surrounded by its rings, and having its body striped with dark belts, somewhat similar, but broader and less strongly marked than those of Jupiter, and owing, doubtless, to a similar cause. That the ring is a solid opake substance is shown by its throwing its shadow on the body of the planet, on the side nearest the sun, and on the other side receiving that of the body, as shown in the figure. From the parallelism of the belts with the plane of the ring, it may be conjectured that the axis of rotation of the planet is perpendicular to that plane ; and this conjecture is confirmed by the occasional appearance of extensive dusky spots on its surface, which, when watched, like the spots on Mars or Jupiter, indicate a rotation in 10h 29m 17s about an axis so situated. (442.) The axis of rotation, like that of the earth, preserves its parallelism to itself during the motion of the planet in its orbit; and the same is also the case with the ring, whose plane is constantly inclined at the * These dimensions are calculated from Prof. Struve's micrometric measures, Mem. Art. Soc. iii. 301. with the exception of the thickness of the ring, which is concluded from my own observations, during its gradual extinction now in progress. The interval of the rings here stated is possibly somewhat too small. |