Certain other difficulties have also to be met; as, for instance, the relative coldness of the climate of Mars. At its distance it gets considerably less than half as much light and heat as we receive. In addition to this, the rarity of its atmosphere would naturally be expected to decrease the effective temperature at the planet’s surface, since an atmosphere acts somewhat like the glass cover of a hot-house in retaining the solar heat which has penetrated it. It has been calculated that, unless there are mitigating circumstances of which we know nothing, the average temperature at the surface of Mars must be far below the freezing-point of water. To this it is replied that the possible mitigating circumstances spoken of evidently exist in fact, because we can _see_ that the watery vapor condenses into snow around the poles in winter, but melts again when summer comes. The mitigating agent may be supposed to exist in the atmosphere where the presence of certain gases would completely alter the temperature gradients. It might also be objected that it is inconceivable that the Martian engineers, however great may be their physical powers, and however gigantic the mechanical energies under their control, could force water in large quantities from the poles to the equator. This is an achievement that measures up to the cosmical standard. It is admitted by the champions of the theory that the difficulty is a formidable one; but they call attention to the singular fact that on Mars there can be found no chains of mountains, and it is even doubtful if ranges of hills exist there. The entire surface of the planet appears to be almost “as smooth as a billiard ball,” and even the broad regions which were once supposed to be seas apparently lie at practically the same level as the other parts, since the “canals” in many cases run uninterruptedly across them. Lowell’s idea is that these sombre areas may be expanses of vegetation covering ground of a more or less marshy character, for while the largest of them appear to be permanent, there are some which vary coincidently with the variations of the canals. As to the kind of machinery employed to force the water from the poles, it has been conjectured that it may have taken the form of a gigantic system of pumps and conduits; and since the Martians are assumed to be so far in advance of us in their mastery of scientific principles, the hypothesis will at least not be harmed by supposing that they have learned to harness forces of nature whose very existence in a manageable form is yet unrecognized on the earth. If we wish to let the imagination loose, we may conjecture that they have conquered the secret of those intra-atomic forces whose resistless energy is beginning to become evident to us, but the possibility of whose utilization remains a dream, the fulfillment of which nobody dares to predict.