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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 onwards.  These stresses rapidly replace one another as the satellite travels along.  They are resisted by the inertia of the crust, and are taken up by its elasticity.  The nature of this succession of alternate compressions and rarefactions in the crust possess some resemblance to those arising in an earthquake shock.

If we consider the effects taking place laterally to the line of motion we see that there are no such changes in the nature of the forces in the crust.  At each passage of the satellite the horizontal tearing stress increases to a maximum, when it is exerted laterally, along the line passing through the horizontal projection of the satellite and at right angles to the line of motion, and again dies away.  It is always a tearing stress, renewed again and again.