Something going on in Mars determines the phenomenon. On a
particular night a certain line looks single. A few nights later
signs of doubling are perceived, and later still, when the seeing
is particularly good, not one but two lines are seen. Thus, as an
example, we may take the case of Phison and Euphrates. Faint
glimpses of the dual state were detected in the summer
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and autumn, but not till November did they appear as distinctly
double. Observe that by this time the Antarctic snows had melted,
and there was in addition, sufficient time for the moisture so
liberated to become diffused in the planet's atmosphere.
This increase in the definition and conspicuousness of certain
details on Mars' surface is further brought into connection with
the liberation of the polar snows and the diffusion of this water
through the atmosphere, by the fact that the definition appeared
progressively better from the south pole upwards as the snow
disappeared. Lowell thinks this points to vegetation springing up
under the influence of moisture; he considers, however, as we
have seen, that the canals convey the moisture. He has to assume
the construction of triple canals to explain the doubling of the
lines.
If we once admit the canals to be elevated ranges—not necessarily
of great height—the difficulty of accounting for increased
definition with increase of moisture vanishes. We need not
necessarily even suppose vegetation concerned. With respect to
this last possibility we may remark that the colour observations,
upon which the idea of vegetation is based, are likely to be
uncertain owing to possible fatigue effects where a dark object
is seen against a reddish background.
However this may be we have to consider what the effects of
moisture increasing in the atmosphere of Mars will be with regard
to the visibility of elevated ranges,
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We assume a serene and rare atmosphere: the nights intensely
cold, the days hot with the unveiled solar radiation. On the hill
tops the cold of night will be still more intense and so, also,
will the solar radiation by day. The result of this state of
things will be that the moisture will be precipitated mainly on
the mountains during the cold of night—in the form of frost—and
during the day this covering of frost will melt; and, just as we
see a heavy dew-fall darken the ground in summer, so the melting
ice will set off the elevated land against the arid plains below.
Our valleys are more moist than our mountains only because our
moisture is so abundant that it drains off the mountains into the
valleys. If moisture was scarce it would distil from the plains
to the colder elevations of the hills. On this view the
accentuation of a canal is the result of meteorological effects
such as would arise in the Martian climate; effects which must be
influenced by conditions of mountain elevation, atmospheric
currents, etc. We, thus, follow Lowell in ascribing the
accentuation of the canals to the circulation of water in Mars;
but we assume a simple and natural mode of conveyance and do not
postulate artificial structures of all but impossible magnitude.
That vegetation may take part in the darkening of the elevated
tracts is not improbable. Indeed we would expect that in the
Martian climate these tracts would be the only fertile parts of
the surface.
Clouds also there certainly are. More recent observations
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