tions and precipitations, I do not know of any decisive arguments that can be opposed to the opinion. The spots are not gathered in large masses, but are disposed in areas and zones of small extent; are greatly ramified, and alternate with considerable uniformity with clear spaces. We may, therefore, conclude that no vast oceans or great continents exist on Mercury; but that land and sea interpenetrate one another and give rise to conditions very different from those which exist on the earth, but which may be more desirable.
Mercury is a world that differs from ours as much as Mars does. The sun lights it and warms it much more intensely than it does the earth, and in a very different way. If life exists in that world, it is doubtless under conditions so different from ours that we can hardly imagine them. The eternal presence of the sun, darting its rays almost vertically on some regions, and its perpetual absence in the opposite countries, would seem intolerable to us. And yet, if we reflect upon it, we shall remark that such a contrast would produce a more rapid, more powerful, and more regular atmospheric circulation than that which spreads the elements of life over the earth; and it possibly is brought about in this way that as complete and even perhaps more perfect equilibrium of temperature is produced on the whole planet than with us.
Mercury, by directing the same face toward the sun during its whole revolution, is peculiarly distinguished from the other planets, all of which the length of whose rotation has been determined, turn round their axes in a few hours. This mode of rotation, however, which would be unique among the planets, seems common enough among the satellites. All testimony is to the effect that our moon has always conformed to it. The first three satellites of Jupiter probably behave in the same way, and the observations of Auwers and Engelmann demonstrate that the fourth does so. Cassini verified the same fact for Japhet, the eighth satellite of Saturn. It may, therefore, be considered the rule among the satellites, while it is an exception among the planets.
The exception may probably be attributed to the proximity of Mercury to the sun, and perhaps also to the fact that it has no satellites; and depends, I think, on the way Mercury was formed when the solar system took its present shape. The peculiarity constitutes a new datum to be added to those which astronomers will have to take account of in studying solar and planetary cosmogony—Translated for the Popular Science Monthly from a French version by F. Terby in Ciel et Terre.