the force of gravity on Mars is only thirty-eight per cent of that on
the earth. A steam-shovel driven by a certain horse-power would be
nearly three times as effective there as here. A man of our stature on
Mars would find his effective strength increased in the same
proportion. But just because of the slight force of gravity there, a
Martian might attain to the traditional stature of Goliath without
finding his own weight an encumbrance to his activity, while at the
same time his huge muscles would come into unimpeded play, enabling
him single-handed to perform labors that would be impossible to a
whole gang of terrestrial workmen. The effective powers of huge
machines would be increased in the same way; and to all this must be
added the fact that the mean density of the materials of which Mars is
composed is much less than that of the constituents of the earth.
Combining all these considerations, it becomes much less difficult to
conceive that public works might be successfully undertaken on Mars
which would be hopelessly beyond the limits of human accomplishment.

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