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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.
This effect is at its maximum along two particular parallel lines
which are tangents to the circle of maximum horizontal stress and
which run parallel with the path of the satellite. The distance
separating these lines depend upon the elevation of the satellite
above the planet's surface. Such lines mark out the theoretical
axes of the "double canals" which future crustal movements will
more fully develop.