THE planet Mars has for a long time signalized itself to observers by the remarkable traits of its constitution. In consequence of its relative nearness, the telescope has been able to furnish us with a number of data respecting its physical geography and its meteorology; and it has been a very rich source of results concerning the philosophy of the solar system and the physical universe in general.
It is well known that Mars displays some bright spots, and others dark, of which we have every reason to consider the former to be continents, the latter seas. Toward the poles appear large white zones, varying in size at different times, which are caps of ice, susceptible of occasional breakings-up like our icebergs. In the thin and transparent atmosphere we can distinguish clouds, currents, and sometimes whirlwinds quite like the cyclones that rage among us.
Besides these intimate analogies with the earth, the study of Mars reveals especial features, some of which are most satisfactorily explained by considerations of comparative geology. With the tenuity of the atmosphere is associated a much smaller extension of the seas, and the relative repartition of land and water is very different from what prevails on the earth. Astronomers observe, as one of the most remarkable peculiarities of the surface of this planetary neighbor of ours, a large number of long and narrow passages and seas like bottle-necks. In our globe the oceans are of three times the surface of the continents; and Europe, Asia, and Africa form together a single island, while another island is formed by the union of the two Americas. But, on Mars, an almost complete equality exists between the surfaces occupied by the continents and by the seas. Further, they are mingled with one another in such a complicated manner that a traveler might visit nearly all the quarters of the planet, either by land or by boat, without having to leave the element on which he began his journey.
This much assumed, it should be recollected that Mars is older in the planetary series than the earth; that is, having been individualized at a more ancient period, and having a smaller volume, it has reached a more advanced stage in the sidereal evolution. Hence the planet represents now, in its great lines and independent of its individual characteristics, a condition which the earth will ultimately attain. One of the effects of the secular cooling of the earth is to determine the progressive absorption of the waters of the ocean by the successively consolidated rocky masses. Hence a striking comparison might be made between the present Martial seas and the terrestrial oceans after we shall have supposed they have been in a more or less great part absorbed. The results of innumerable soundings have permitted the