ere, we must conclude
that the surface of that planet, like that of the moon, has been moulded
by some form of volcanic action modified probably by wind, but not by
water. Adding to this, that the force of gravity on Mars is nearer that
of the moon than to that of the earth, and we may r reasonably conclude
that its surface is formed of volcanic matter in a light and porous
condition, and therefore highly favourable for the rapid loss of surface
heat by radiation. The surface-conditions of Mars are therefore,
presumably, much more like those of the moon than like those of the
earth.

The next condition favourable to the storing up of heat--a covering of
vegetation--is almost certainly absent from Mars except, possibly, over
limited areas and for short periods. In this feature also the surface of
Mars approximates much nearer to lunar than to earth-conditions. The
third condition--a dense, vapour-laden atmosphere--is also wanting in
Mars. For although it possesses an atmosphere it is estimated by Mr.
Lowell (in his latest article) to have a pressure equivalent to only
2-1/2 inches of mercury with us, giving it a density of only one-twelfth
part that of ours; while aqueous vapour, the chief accumulator of heat,
cannot permanently exist in it, and, notwithstanding repeated
spectroscopic observations for the purpose of detecting it, has never
been proved to exist.

I submit that I have now shown from the statements--and largely as the
result of the long-continued observations--of Mr. Lowell himself, that,
so far as the physical conditions of Mars are known to differ from those
of the earth, the differences are all _unfavourable_ to the conservation
and _favourable_ to the dissipation of the scanty heat it receives from
the sun--that they point unmistakeably towards the temperature
conditions of the moon rather than to those of the earth, and that the
cumulative effect of these adverse conditions, acting upon a
heat-supply, reduced by solar distance to less than one-half of ours,
_must_ result in a mean temperature (as well as in the extremes) nearer
to that of our satellite than to that of our own earth.

_Further Criticism of Mr. Lowell's Article._

We are now in a position to test some further conclusions of Mr.
Lowell's _Phil. Mag._ article by comparison with actual phenomena. We
have seen, in the outline I have given of this article, that he
endeavours to show how the small amount of solar heat received by Mars
is