I may add that the inadmissible span of 180 degrees and spans
very near this value, which are not well admissible, are so far
as I can find, absent. The curves are not great circles.
You will require of me that I should explain the centres of
radiation so conspicuous here and there on Lowell's map. The
meeting of more than two lines at the oases is a phenomenon
possibly of the same nature and also requiring explanation.
In the first place the curves to which I have but briefly
referred actually give rise in most cases to nodal, or crossing
points; sometimes on the equator, sometimes off the equator;
through which the path of the satellite returns again and again.
These nodal points will not, however, afford a general
explanation of the many-branched radiants.
It is probable that we should refer such an appearance
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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.