Page:The Solar System - Six Lectures - Lowell.djvu/82



resembles the system in the light; the one joins on to the other. The points where the system in the light areas strike the dark are the points from which the canals in the dark regions set out. The two are thus but parts of one world-wide whole. Whatever purpose the one subserves is thus taken up and extended by the other.

Nor does the communication come to an end in the dark regions. From the southern portions of these, in the southern hemisphere, other canals run straight into the polar cap; in the northern hemisphere, similarly, canals penetrate to the most northern limit of the snow.

Lastly, the rifts which appear in the caps during the process of melting turn out to be where subsequently are seen canals. Now, as there are no mountains on Mars, differences of level cannot be a cause of melting; areas of vegetation could.

We may sum up our present knowledge of the surface conditions of the planet as follows:

(1) Change takes place upon the planet's surface; this proves the presence there of an atmosphere.

(2) The limb-light, the apparent evidence of a twilight, and the albedo, all point to a density for this atmosphere very much less than our own.