The
seas of the torrid zone would be seen to be arranged in long parallel
layers, corresponding to the zone of the equatorial and tropical calms.
For an observer placed upon the moon the study of our geography would
not be so simple an undertaking as one might at first imagine.
There is nothing of this sort in Mars. In every climate and under every
zone its atmosphere is nearly perpetually clear and sufficiently
transparent to permit one to recognize at any moment whatever the
contours of the seas and continents, and, more than that, even the minor
configurations. Not indeed that vapors of a certain degree of opacity
are lacking, but they offer very little impediment to the study of the
topography of the planet. Here and there we see appear from time to time
a few whitish spots, changing their position and their form, rarely
extending over a very wide area. They frequent by preference a few
regions, such as the islands of the Mare Australe, and on the continents
the regions designated on the map with the names of Elysium and Tempe.
Their brilliancy generally diminishes and disappears at the 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.
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