is, therefore, merely by way of comparison, that I propose to examine, in this chapter, the adaptation of the physical conditions of the planet Saturn to the wants of beings constituted like the inhabitants of our earth: it is not necessary to establish the subsistence of conditions so adapted, in order to prove that the planet is the abode of living creatures.

If an inhabitant of our earth could be placed on the surface of another planet, it is probable that the first circumstance in his new condition that would attract his attention would be the change in his own weight. If he were removed to Jupiter, he would find his weight more than doubled, and would be unable to move without pain and difficulty. If, on the other hand, he were removed to Mars or Mercury, he would find his weight diminished by more than one half, and his activity and apparent muscular power correspondingly increased. If he were removed to Saturn, the change in his weight would vary with the latitude of the spot to which we suppose him to be conveyed. Owing to the compression of Saturn's globe, his vast size, and his rapid rotation on his axis, gravity varies with latitude in a much more marked manner than on our earth. If Saturn were not rotating, the weight of a terrestrial pound would be about 1.19 lbs. at Saturn's pole, and 1.17 lbs. at his equator. But the centrifugal force at Saturn's equator is about 0·164,* by which amount gravity is still further diminished. Thus, a man weighing 12 st. on earth, would weigh only a few ounces over 12 st. at Saturn's equator, but would weigh more than 14 st. at Saturn's pole. The difference of weight in the former case would hardly be appreciable: in the latter it would prove a heavy burden; but its effect would be somewhat diminished by perfect adjustment, since it would be distributed over the whole body. A Saturnian would find his weight increased in nearly the same degree, if he travelled from Saturn's equator to either pole: we shall presently see that this is not the only circumstance in which the physical conditions of Saturn's arctic and temperate zones present a marked contrast to those prevailing at and near his equator.

By this is meant that a mass weighing 1 lb. on the earth and revolving about a centre with the same velocity and at the same distance as points on Saturn's equator about Saturn's centre, would require, to retain it in its orbit, a force sufficient to counterpoise a weight of 0.164 lbs.

It has been considered probable that the appearance of Saturn's surface differs greatly from that of our earth. And this for two reasons:-In the first place, it is urged that his density being so small, he must be composed of materials very much lighter than, and therefore very different from, those composing our earth; and, in the second place, that fluids upon his surface must either be of less density than the planet, and therefore very different from our oceans, or if of greater density, must all be collected in one hemisphere.* Saturn's globe may, however, be hollow, and the mean density of the materials of this hollow globe not very different from the mean density of the materials composing our earth.† And, again, it has not been established by rigid mathematical inquiry, that oceans upon a planet of Saturn's figure, would necessarily be collected wholly, or almost wholly, in one hemisphere, if their density exceeded that of the planet. On the contrary, it appears probable that fluid masses on the surface of such a planet would tend to form two vast polar oceans, since gravity is so much greater

* Whewell's 'Astronomy and General Physics:' (Bridgewater Treatise.)

+ Whether the earth is solid throughout or merely a spherical shell is a question on which the world of science is divided. The increase of heat as we proceed inwards seems to indicate that at no very great depth the heat must be so intense that all known substances would be converted into fluids. On the other hand, Mr. Hopkins has shown that the precession of the earth's pole is not such as it would be if the earth were a shell of such comparatively small thickness containing a vast fluid mass. Arguments of some force have also been urged to show that the above-mentioned increase of heat is not to be considered as an indication of a fluid nucleus; and it is certain that man has penetrated the earth's crust to a distance absolutely insignificant compared with the dimensions of the earth's globe. Yet it seems clear that the balance of probability is largely in favour of a continual increase of heat inwards, in even a greater ratio than that observed near the surface. And it appears not improbable that at a depth of a thousand miles the heat should be so intense that all known substances would at ordinary pressures be converted into vapour. But the pressure exerted by a vapourous nucleus on the surface of the fluid shell next to it, and by transmission on the solid shell, must be so immense that the interior parts of the solid and liquid shells must owe their solidity and fluidity respectively to the intensity of such pressure, and not to the insufficiency of the heat in those parts to change respectively solidity into fluidity, and fluidity into gaseity, at ordinary pressures. Thus the thickness of either shell may be far greater than would appear from any calculation founded on ordinary pressures. It is also conceivable that the immensity of the pressure exerted by the gaseous nucleus would be sufficient to modify the motions of the fluid shell, and that by combining the effects of such modification with the increased thickness of the fluid crust deduced from the consideration mentioned above, the prec ion of the earth's pole might be accounted for as exactly as on the supposition of the solidity of the whole mass of the earth's globe.

at Saturn's poles than at his equator. But even if it were proved that the former arrangement must inevitably subsist, what, after all, is such an arrangement but an almost exact counterpart of what is observed on our own earth? * It is true that tidal waves could not sweep round such an ocean, as round the oceans that surround the earth, but an ocean whose tides are ruled by eight satellites, and restrained by the attractions of a stupendous ring, may require arrangements altogether different, in this respect, from those prevailing on our earth. The appearance of Saturn, however, is not favourable to the supposition that the ocean masses on his surface are confined to a single hemisphere; for the bright bands on

* This arrangement on the earth is modified by the tendency towards the poles that might be expected from the earth's form; so that while the southern hemisphere (or more exactly the hemisphere of which New Zealand forms the central region) is evidently that towards which the main body of the water is attracted, the northern polar regions are also occupied by a vast ocean, connected with the southern ocean by Behring's Straits and the Atlantic. From this arrangement and the conformation of the land, it is obvious that at present the centre of gravity of our globe lies nearer the southern than the northern pole. M. Adhemar has suggested that this displacement of the centre of gravity from the centre of figure is due to the vast masses of ice collected at the southern pole, and that as the duration of the antarctic summer is now continually increasing, those masses will diminish, and the frozen masses at the arctic pole increase, until the centre of gravity is nearer the northern than the southern pole, when the great southern ocean will rush northward. He conceives, in fact, that a vast flood takes place twice in every revolution of the vernal equinox (that is, twice in 25,868 years), the ocean masses rushing alternately from pole to pole; and he imagines that the successive states of submersion and emergence undoubtedly passed through by every part of the earth's surface may be better explained in this way than by the supposition of alternate elevations and depressions from internal causes. The close observer of nature will not readily accept the idea of such cataclysmal floods, destroying all living creatures on the face of the earth at each eruption. It is not altogether improbable, however, that the ocean masses may oscillate from pole to pole in a more gradual manner, and that during such oscillations inundations might take place, insignificant when compared with the universal floods imagined by Adhèmar, but sufficient to constitute tremendous local catastrophes, and to leave lasting traces of their effects. The results of such oscillations would differ in no respect from those of elevations and subsidences of continents. It may be remarked that while elevations and depressions of large tracts of the earth's surface have undoubtedly taken place, it appears improbable that whole continents should be so raised or depressed; and the expression sometimes met with in works on geology, that a whole hemisphere may be elevated by internal forces while the opposite hemisphere is depressed, is simply an absurdity. Such changes are inconsistent with the simplest law of mechanics, that 'action and reaction are opposite and equal.' Forces tending to elevate one hemisphere must bear upon and therefore tend to elevate the opposite hemisphere,--must tend, in fact, to lengthen that diameter of the earth along which their resultant acts.

Saturn's disc, which are probably vast belts of clouds drawn from oceans upon his surface, are found equally in the northern and southern hemispheres, and extend completely round Saturn's globe.

The climatic conditions on the surface of Saturn undoubtedly differ in the most striking manner from those which prevail on the earth. We may consider three points on which these conditions depend; namely:--the distance of Saturn from the sun; the inclination of his axis to the plane of his orbit; and the respective lengths of the Saturnian day and year.

We have seen that Saturn's mean distance from the sun is more than 9 times as great as the mean distance of the earth. Thus the diameter of the sun's disc appears less to the Saturnians than to us in the proportion of 2 to 19; while the apparent surface of the solar disc, which varies as the square of the apparent semidiameter, appears diminished to about th part of the apparent surface of the disc visible to us. The quantity of light and heat received on any part of Saturn's surface is therefore only th part of the quantity received on a part of the earth's surface of equal extent, and equally inclined to the solar rays. In fact, notwithstanding the immensity of Saturn's globe, the whole of the light and heat received upon it, when Saturn is at his mean distance from the sun, is considerably less than the light and heat similarly received on the earth. It does not necessarily follow, however, that the climate of Saturn is so bleak and frigid as that of the earth would be under a corresponding diminution of the solar heat; for, independently of the consideration that the climate of any planet may be greatly affected by internal heat, there can be no doubt that the amount and density of the atmosphere that surrounds a planet has a most important influence on the climatic conditions that prevail upon its surface.* That Saturn has a very

extensive, and therefore (at his surface) a very dense atmosphere, seems probable from the appearance presented by his disc in powerful telescopes, as well as from his vast absolute dimensions. Such an atmosphere can, of course, have no effect in increasing the

* If the atmosphere of our earth were suddenly subjected to such a change that heat radiating from the earth passed through the air as freely as the sun's direct heat, the earth would no longer be habitable by such races as now exist upon its surface.

amount of heat received upon any part of Saturn's surface, or rather, tends somewhat to diminish that amount; but by preventing radiation, it may serve to maintain a mean temperature as high as the mean temperature of our globe, or even considerably higher.* The amount of light received would not be increased by such an arrangement, except by the comparatively small amount refracted towards the planet by the atmosphere, and the consequent lengthening of the Saturnian twilights. That the surface of Saturn is illuminated with considerable brilliancy, however, may be inferred from the brightness of his disc. Although it is less splendid than the discs of planets nearer the sun,† there is no approach to the sombreness and gloom that one would expect from a diminution of the solar light to so small a fraction of that received upon our earth. It has been calculated, however, that under such a diminution the sun would still supply 560 times as much light as the moon at full-a calculation confirmed by the small loss of light in partial eclipses of the sun. There is therefore little reason for supposing that the quantity of light received by Saturn would be insufficient even for such forms of life as are found upon our earth; still less reason is there for supposing that no forms of life whatever could subsist on Saturn's surface.‡

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*Mr. Hopkins has calculated that if the atmosphere of our earth were increased in height by about 40,000 feet, the earth would be maintained at its present temperature, if exposed only to the radiation of space, in the total absence of the sun. See Nichol's Cyclopædia, Appendix, Atmospheres of Planets.' This result, however, can hardly be considered as satisfactorily established.

† Reference is not here made to absolute splendour, which depends on the magnitude of a planet and its distance from the earth, but to the intrinsic brilliancy of the disc, which is independent of those relations. The faintness of Saturn's light compared with that of the moon was very observable at the reappearance of Saturn on the moon's bright limb after the occultation of May 8th, 1859.

It is probable that our own earth once received much less light than at present. This is indicated by the size of the eye-orbits in many extinct species of animals, and by the development attained by creatures of the bat kind, which now form an insignificant class of the earth's inhabitants. Thus, Hugh Miller, speaking of the remains of animals of the secondary division, says, ' enormous jaws, bristling with pointed teeth, gape horrid in the stone, under staring eye-sockets a full foot in diameter;' and again, 'here we see a winged dragon,' the Pterodactylus Crassirostris, that, armed with sharp teeth and strong claws, had careered through the air on leathern wings like a bat.'-' Testimony of the Rocks,' Lecture III. The pterodactyles of the greensand exhibit not uncommonly a spread of wing of eight or nine yards! See also Note B, Appendix I.

M