256 Joty—On the Origin of the Canals of Mars. 
matter of conjecture. For months, probably ; possibly, for many years. This 
point will need further consideration later on. 
If it be asked what record we may expect of such repeated stress in the crust, 
we can only answer that there is great probability that repeated disturbances of 
the crust will lead to eruption and upheaval if an energetic substratum underlies 
the surface crust. There is some reason to trace in certain cases terrestrial earth- 
quake disturbances to the tidal action of our own satellite. The more local and 
concentrated tidal effects of small satellites close to a planet’s surface should 
produce like seismic disturbances, but of more local activity. The path of the 
satellite will, in determining lines of weakness, probably be traced with upheaved 
districts similar to those which mark out many terrestrial coast lines which have 
themselves been referred to lunar attraction. Whether this view is in keeping 
with our present knowledge of the surface features of Mars will be considered 
further on. 
We must not overlook what goes on immediately beneath the satellite’s path of 
advance over the surface of its primary. We may assign to the vertical forces 
the function of elevating the crust in such a degree as, considered along with the 
instability of so thin a layer under compression, will allow us to disregard the 
elastic resistance to compression as seriously modifying the establishment of 
tensile stress in the regions without, But is it not probable that this very elevation 
will result ultimately in similar disturbances to those we may suppose will mark 
out the outer lines of stress? We have to consider more especially what will 
happen when once the tensile stresses have done their work. When this happens, 
the horizontal forces inwards will add themselves in part to the lifting forces, and 
assist towards producing central upheaval. 
The difficulties attending the estimate of disruptive stresses to which the 
planet’s crust is subjected when the relative motion of the satellite over the surface 
of the planet is very great, are so formidable that I reluctantly leave the problem 
unsolved. In the case of Phobos falling into Mars, when the satellite has attained 
the distance of 63 miles from the surface of Mars, his velocity will be somewhat 
over two miles per second. Taking into account the rotational velocity of Mars 
at its present value, the relative velocity will be very nearly two miles per second. 
Impulses will travel in the crust with the velocity of sound, and may therefore be 
assumed to travel outwards at the rate of three miles per second from points 
along the path of advance of Phobos. The addition of these waves will determine 
the magnitude and position of the lines of stress accompanying the satellite’s 
motion. This high relative velocity is no extreme case. Some of the lines of 
Mars were, according to this theory, formed at even higher relative velocities. A 
particular case is that when the relative velocity is that of sound in the material 
composing the crust of Mars. In this case the effects will continually add up, the 
