in the ordinary fossiliferous strata. Their dwelling-place may have been in one or more of the numerous localities of our continents not yet explored, or in those immense regions of the earth which are now covered by the great oceans of the globe; and till these oceans have quitted their beds, or some great convulsions have upheaved and laid bare the strata above which the races in question may have lived and died, we are not entitled to maintain it as a demonstrated truth, that the ancient earth was under the sole dominion of the brutes that perish.

But without waiting for the result of catastrophes like these, the future of geology, even if restricted to existing continents and islands, may be pregnant with startling discoveries, and the remains of intellectual races may be found even beneath the primitive Azoic formations of the earth. The astronomers of the present day have penetrated far into the celestial depths, compared with those of the preceding age, descrying in the remotest space glorious creations, and establishing mighty laws. Like them, may not geologists descend deeper into the abyss beneath, and discover in caverns yet

unexplored the upheaved cemeteries of primordial times. The earth has yet to surrender its strongholds of gigantic secrets, and startling revelations are yet to be read on sepulchres of stone. It is not from that distant bourne where the last ray of starlight trembles on the telescopic eye that man is to receive the great secret of the world's birth, or of his future destiny. It is from the deep yaults to which primæval life has been consigned that the history of the dawn of life is to be composed. Geologists have read that chronology backwards, and are decyphering downwards its pale and perishing alphabet. They have reached the embryos of vegetable existence, the probable terminus of the formation which has buried them. But who can tell what sleeps beyond! Another creation may lie beneath-more glorious creatures may be entombed there. The mortal coils of beings more lovely, more pure, more divine than man, may yet read to us the unexpected lesson that we have not been the first, and may not be the last of the intellectual race.

In order to compare the condition of the earth with that of the moon and the other

planets of the Solar system, we must know something of its atmosphere, of its action in refracting, reflecting, and polarizing light, and of the phenomena which it will exhibit to other planets in its various states, as modified by the aqueous vapour which it contains, whether it exists in minute vesicles, or in masses of clouds. The light reflected by the atmosphere, when in its purest state, is a rich blue, becoming paler and paler as the aqueous vapour is increased. When the light of the sun reaches the eye, after having been transmitted through great lengths of the earth's atmosphere, it is bright red, passing into orange and yellow when the length of its path is diminished. Considering, then, the diversity of climate in any one hemisphere of the globe, it is hardly possible that the earth, as seen from any given point in space, could appear free from clouds. When the sky is blue over large portions of the tropical regions, and smaller portions of the temperate and arctic zones, it is elsewhere covered with fleecy clouds, or throwing down its superabundant vapours in rain, or hail, or snow. The banks of fleecy clouds will reflect a brilliant light to the distant

eye, while the pure air will exhibit the colour of the land, or of the ocean, mixed with its own native tint of blue; and, in certain positions of the sun, the red beams into which his pure rays have been changed by absorption, will display themselves in certain parts of the terrestrial disc. When the Earth, therefore, is reduced by distance to the apparent size of Mars and Jupiter, it will exhibit a tint composed of all those which we have described.

When the blue light of the sky, and the reflected light of the clouds, are examined by an observer on the surface of the earth, it is found to be polarized, like the light which is reflected from the surfaces of transparent bodies; and, therefore, a greater or less portion of the light which reaches the eye of an observer, placed on another planet, must be polarized, and exhibit all the properties of that species of light. We thus obtain a certain test of the existence of water in the other planets of the system, and we are enabled to ascertain the truth of certain speculations respecting their condition, which affect the question of a plurality of worlds.

1 See Johnston's Physical Atlas.

CHAPTER IV.

ANALOGY BETWEEN THE EARTH AND THE OTHER

PLANETS.

WITH the information contained in the preceding chapter, respecting the structure of the earth and its atmosphere, we are now in a condition to compare it as an inhabited world with the other planets of our system, and to ascertain, from the analogies which exist between them, to what extent it is probable that they are either inhabited, or in a state of preparation, as the earth once was, for the reception of inhabitants.

In making this comparison, the first point which demands our attention is the position which the earth occupies in the Solar system. In reference to the number of the planets, which is nine, reckoning the asteroids as one, Jupiter is the fifth, or middle planet, and is otherwise