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as is shown at the Sinus Titanum to the perturbations of the
satellite's path due to the surface features on Mars. Observe
that the principal radiants are situated upon the boundary of the
dark regions or at the oases. Higher surface levels may be
involved in both cases. Some marked difference in topography must
characterise both these features. The latter may possibly
originate in the destruction of satellites. Or again, they may
arise in crustal disturbance of a volcanic nature, primarily
induced or localised by the crossing of two canals. Whatever the
origin of these features it is only necessary to assume that they
represent elevated features of some magnitude to explain the
multiplication of crossing lines. We must here recall what
observers say of the multiplicity of the canals. According to
Lowell, "What their number maybe lies quite beyond the
possibility of count at present; for the better our own air, the
more of them are visible."
Such innumerable canals are just what the present theory
requires. An in-falling satellite will, in the course of the last
60 or 80 years of its career, circulate some 100,000 times over
Mars' surface. Now what will determine the more conspicuous
development of a particular canal? The mass of the satellite; the
state of the surface crust; the proximity of the satellite; and
the amount of repetition over the same ground. The after effects
may be taken as proportional to the primary disturbance.
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It is probable that elevated surface features will influence two
of these conditions: the number of repetitions and the proximity
to the surface. A tract 100 miles in diameter and elevated 5,000
or 10,000 feet would seriously perturb the orbit of such a body as
Phobos. It is to be expected that not only would it be effective
in swaying the orbit of the satellite in the horizontal direction
but also would draw it down closer to the surface. It is even to
be considered if such a mass might not become nodal to the
satellite's orbit, so that this passed through or above this
point at various inclinations with its primary direction. If
acting to bring down the orbit then this will quicken the speed
and cause the satellite further on its path to attain a somewhat
higher elevation above the surface. The lines most conspicuous in
the telescope are, in short, those which have been favoured by a
combination of circumstances as reviewed above, among which
crustal features have, in some cases, played a part.
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 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.