Joty—On the Origin of the Canals of Mars. 251 
cases, that it has been asserted that some of these bodies actually come within 
Mars’ mean distance of the Sun, and move to distances beyond the orbit of 
Jupiter.* Is it not probable, then, that in the past both Mars and Jupiter may 
have drawn from their orbits, and picked up some of these minute wanderers ? 
This suggestion is not altogether new ; Mars’ present satellites have been ascribed 
to this source. 
The assumption of the slowing down of Mars’ axial velocity needs no justifica- 
tion here. Solar tidal influence must always act in this direction. If many small 
bodies, especially if rotating retrogradely to the planet’s axial motion, were at 
yarious times absorbed, this would be also a cause of diminished axial rotation— 
a diminution, of course unknown in amount, but probably small. 
Gravitational effect of a Small Satellite moving close to the surface of a Planet. 
A small solid satellite, moving close to the surface of its primary, will give 
rise to certain stresses in the crust of the latter, due to gravitational attraction. 
What the magnitude and distribution of these stresses will be when the satellite 
is moving relatively to the planet’s surface at a very high velocity is a problem 
not touched upon in this'Paper. The case of the satellite moving slowly, 
relatively to the surface of its primary, or at rest, relatively to this surface, is 
alone dealt with. ‘The importance of dealing with this question is, of course, to 
find if the tidal disturbances of minute satellites are likely to affect the crust in 
a manner calculated to give rise to such appearances as are presented by the double 
“canals” of Mars. 
Immediately beneath the satellite the gravitational pull of this body is all 
vertical in direction ; outwards, from this point, the force acting upon the surface 
material is a pull directed towards the satellite’s centre. Resolving this force at 
any point into a vertical and a horizontal component, the first goes to elevate the 
solid crust of the planet ina very feeble degree; the second tends to pull it in 
beneath the satellite. The latter adds up as we go outwards, one particle pulling 
upon the next one. The tensile stress thereby set up is, however, at the same 
time diminished owing to the increased area over which the total force is 
distributed. The integration outwards shows that there is, as was to be antici- 
pated, a circle of maximum stress, the circular base of a cone having the centre of 
the satellite at its apex, the semi-angle of the cone being closely 71°. It is 
assumed that the crust is insufficiently thin, and so far affected by the central 
lifting force as to be unstable under horizontal compression. 
* Proctor, doc cit., p. 560. 
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