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.

It is interesting to consider what the effect of such

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conditions would be if they arose at the surface of our own planet.  We assume a horizontal force in the crust adequate to set up tensile stresses of the order, say, of fifteen tons to the square foot and these stresses to be repeated every few hours; our world being also subject to the dynamic effects we recognise in and beneath its crust.

It is easy to see that the areas over which the satellite exerted its gravitational stresses must become the foci —­foci of linear form—­of tectonic developments or crust movements.  The relief of stresses, from whatever cause arising, in and beneath the crust must surely take place in these regions of disturbance and along these linear areas.  Here must become concentrated the folding movements, which are under existing conditions brought into the geosynclines, along with their attendant volcanic phenomena.  In the case of Mars such a concentration of tectonic events would not, owing to the absence of extensive subaerial denudation and great oceans, be complicated by the existence of such synclinal accumulations as have controlled terrestrial surface development.  With the passage of time the linear features would probably develop; the energetic substratum continually asserting its influence along such lines of weakness.  It is in the highest degree probable that radioactivity plays no less a part in Martian history than in terrestrial.  The fact of radioactive heating allows us to assume the thin surface crust and continued sub-crustal energy throughout the entire period of the planet’s history.

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How far willl these effects resemble the double canals of Mars?  In this figure and in the calculations I have given you I have supposed the satellite engaged in marking the planet’s surface with two lines separated by about the interval separating the wider double canals of Mars—­that is about 220 miles apart.  What the distance between the lines will be, as already stated, will depend upon the height of the satellite above the surface when it comes upon a part of the crust in a condition to be affected by the stresses it sets up in it.  If the satellite does its work at a point lower down above the surface the canal produced will be narrower.  The stresses, too, will then be much greater.  I must also observe that once the crust has yielded to the pulling stress, there is great probability that in future revolutions of the satellite a central fracture will result.  For then all the pulling force adds itself to the lifting force and tends to crush the crust inwards on the central line beneath the satellite.  It is thus quite possible that the passage of a satellite may give rise to triple lines.  There is reason to believe that the canals on Mars are in some cases triple.