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.

186

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.

I have spoken all along of the satellite moving slowly over the
surface of Mars. I have done so as I cannot at all pronounce so
readily on what will happen when the satellite's velocity over
the surface of Mars is very great. To account for all the lines
mapped by Lowell some of them must have been produced by
satellities moving relatively to the surface of Mars at
velocities so great

187

as three miles a second or even rather more. The stresses set up
are, in such cases, very difficult to estimate. It has not yet
been done. Parallel lines of greatest stress or impulse ought to
be formed as in the other case.

I now ask your attention to another kind of evidence that the
lines are due in some way to the motion of satellites passing
over the surface of Mars.

I may put the fresh evidence to which I refer, in this way: In
Lowell's map (P1. XXII, p. 192), and in a less degree in
Schiaparell