n you go to the edge of it in order to make a beginning.
Here the stress from the satellite has got to the edge of the
crust. It is greatly concentrated just at the extremities of the
cracks. It will, unler such circumstances probably carry on the

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tear. If it does not do so this time, remember the satellite will
some hours later be coming over the same place again, and then
again for, at least, many hundreds of times. Then also we are not
limited to the assumption that the

{Fig. 12}

satellite is as small as Phobos. Suppose we consider the case of
a satellite approaching Mars which has a diameter double that of
Phobos; a diameter still much less than that of the larger class
of asteroids. Even at the distance

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of 65 miles the stress will now amount to as much as from 15 to
30 tons per square foot. It is almost certain that such a stress
repeated a comparatively few times over the same parts of the
planet's surface would so rend the crust as to set up lines along
which plutonic action would find a vent. That is, we might expect
along these lines all the phenomena of upheaval and volcanic
eruption which give rise to surface elevations.

The probable effect of a satellite of this dimension travelling
slowly relatively to the surface of Mars is, then, to leave a
very conspicuous memorial of his presence behind him. You see
from the diagram that this memorial will consist o: two parallel
lines of disturbance.

The linear character of the gravitational effects of the
satellite is due entirely to the motion of the satellite
relatively to the surface of the planet. If the satellite stood
still above the surface the gravitational stress in the crust
would, of course, be exerted radially outwards from the centre of
the satellite. It would attain at the central point beneath the
satellite its maximum vertical effect, and at some radial
distance measured outwards from this point, which distance we can
calculate, its maximum horizontal tearing effect. When the
satellite moves relatively to the planet's crust, the horizontal
tearing force acts differently according to whether it is
directed in the line of motion or at right angles to this line.

In the direction of motion we see that the satellite

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creates as it passes over the crust a wave of rarefaction or
tension as at D, followed by compression just beneath the
satellite and by a reversed direction of gravitational pull as
the satellite passes o