I must briefly refer to what is one of the most interesting features of the Martian lines:  the manner in which they appear to come and go like visions.

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