m in an atmosphere so
comparatively rare as that of Mars. And these clouds, in some instances
at least, appear, like the cirrus streaks and dapples in our own air, to
float at a great elevation. Mr. Douglass, one of Mr. Lowell's associates
in the observations of 1894 at Flagstaff, Arizona, observed what he
believed to be a cloud over the unilluminated part of Mars's disk,
which, by micrometric measurement and estimate, was drifting at an
elevation of about fifteen miles above the surface of the planet. This
was seen on two successive days, November 25th and November 26th, and it
underwent curious fluctuations in visibility, besides moving in a
northerly direction at the rate of some thirteen miles an hour. But,
upon the whole, as Mr. Lowell remarks, the atmosphere of Mars is
remarkably free of clouds.

The reader will remember that Mars gets a little less than half as much
heat from the sun as the earth gets. This fact also has been used as an
argument against the habitability of the planet. In truth, those who
think that life in the solar system is confined to the earth alone
insist upon an almost exact reproduction of terrestrial conditions as a
_sine qua non_ to the habitability of any other planet. Venus, they
think, is too hot, and Mars too cold, as if life were rather a happy
accident than the result of the operation of general laws applicable
under a wide variety of conditions. All that we are really justified in
asserting is that Venus may be too hot and Mars too cold for _us_. Of
course, if we adopt the opinion held by some that the temperature on
Mars is constantly so low that water would remain perpetually frozen, it
does throw the question of the kind of life that could be maintained
there into the realm of pure conjecture.

The argument in favor of an extremely low temperature on Mars is based
on the law of the diminution of radiant energy inversely as the square
of the distance, together with the assumption that no qualifying
circumstances, or no modification of that law, can enter into the
problem. According to this view, it could be shown that the temperature
on Mars never rises above -200 deg. F. But it is a view that seems to be
directly opposed to the evidence of the telescope, for all who have
studied Mars under favorable conditions of observation have been
impressed by the rapid and extensive changes that the appearance of its
surface undergoes coincidently with the variation of the planet's
seaso