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