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

195

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 sprin