Mars as influencing Temperature._

Coming now to the special feature of Mars and its probable temperature,
we find that most writers have arrived at a very different conclusion
from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority
who 'recently, by the application of Stefan's law,' has found the mean
temperature of this planet to be-35 deg. F. Again, Professor J.H. Poynting,
in his lecture on 'Radiation in the Solar System,' delivered before the
British Association at Cambridge in 1904, gave an estimate of the mean
temperature of the planets, arrived at from measurements of the sun's
emissive power and the application of Stefan's law to the distances of
the several planets, and he thus finds the earth to have a mean
temperature of 17 deg. C. (=62-1/2 deg. F.) and Mars one of-38 deg. C. (=-36-1/2 deg.
F.), a wonderfully close approximation to the mean temperature of the
earth as determined by direct measurement, and therefore, presumably, an
equally near approximation to that of Mars as dependent on distance from
the sun, and '_on the supposition that it is earth-like in all its
conditions._'

But we know that it is far from being earth-like in the very conditions
which we have found to be those which determine the extremely different
temperatures of the earth, and moon; and, as regards each of these, we
shall find that, so far as it differs from the earth, it approximates to
the less favourable conditions that prevail in the moon. The first of
these conditions which we have found to be essential in regulating the
absorption and radiation of heat, and thus raising the mean temperature
of a planet, is a compact surface well covered with vegetation, two
conditions arising from, and absolutely dependent on, an ample amount of
water. But Mr. Lowell himself assures us, as a fact of which he has no
doubt, that there are no permanent bodies of water, great or small, upon
Mars; that rain, and consequently rivers, are totally wanting; that its
sky is almost constantly clear, and that what appear to be clouds are
not formed of water-vapour but of dust. He dwells, emphatically, on the
terrible desert conditions of the greater part of the surface of the
planet.

That being the case now, we have no right to assume that it has ever
been otherwise; and, taking full account of the fact, neither denied nor
disputed by Mr. Lowell, that the force of gravity on Mars is not
sufficient to retain water-vapour in its atmosph