meridian hour of the place, and is re-enforced in the morning and evening with very marked variations.  It is possible that they may be layers of clouds because the upper portions of terrestrial clouds where they are illuminated by the sun appear white.  But various observations lead us to think that we are dealing rather with a thin veil of fog instead of a true nimbus cloud, carrying storms and rain.  Indeed, it may be merely a temporary condensation of vapor under the form of dew or hoar frost.

Accordingly, as far as we may be permitted to argue from the observed facts, the climate of Mars must resemble that of a clear day upon a high mountain.  By day a very strong solar radiation, hardly mitigated at all by mist or vapor; by night a copious radiation from the soil toward celestial space, and because of that a very marked refrigeration.  Hence a climate of extremes, and great changes of temperature from day to night, and from one season to another.  And as on the earth at altitudes of 5,000 and 6,000 meters (17,000 to 20,000 feet) the vapor of the atmosphere is condensed only into the solid form, producing those whitish masses of suspended crystals which we call cirrus clouds, so in the atmosphere of Mars it would be rarely possible (or would even be impossible) to find collections of cloud capable of producing rain of any consequence.  The variation of the temperature from one season to another would be notably increased by their long duration, and thus we can understand the great freezing and melting of the snow which is renewed in turn at the poles at each complete revolution of the planet around the sun.

As our chart demonstrates, in its general topography Mars does not present any analogy with the earth.  A third of its surface is occupied by the great Mare Australe, which is strewn with many islands, and the continents are cut up by gulfs, and ramifications of various forms.  To the general water system belongs an entire series of small internal seas, of which the Hadriacum and the Tyrrhenum communicate with it by wide mouths, whilst the Cimmerium, the Sirenum, and the Solis Lacus are connected with it only by means of narrow canals.  We shall notice in the first four a parallel arrangement, which certainly is not accidental, as also not without reason is the corresponding position of the peninsulas of Ausonia, Hesperia, and Atlantis.  The color of the seas of Mars is generally brown, mixed with gray, but not always of equal intensity in all places, nor is it the same in the same place at all times.  From an absolute black it may descend to a light-gray or to an ash color.  Such a diversity of colors may have its origin in various causes, and is not without analogy also upon the earth, where it is noted that the seas of the warm zone are usually much darker than those nearer the pole.  The water of the Baltic, for example, has a light, muddy color that is not observed in the Mediterranean.  And thus in the seas of Mars we see the color become darker when the sun approaches their zenith, and summer begins to rule in that region.