aping
back to space. Thus not more than perhaps ten or twelve out of the
original hundred sent by the sun would, under the most favourable
circumstances, and from the very centre of the earth's disc, reach the
eye of a Martian or lunar observer. The light by which he views our
world is, there is little doubt, light reflected from the various strata
of our atmosphere, cloud or mist-laden or serene, as the case may be,
with an occasional snow-mountain figuring as a permanent white spot.

This consideration at once shows us how much more tenuous the Martian
air must be, since it admits of topographical delineations of the
Martian globe. The clouds, too, that form in it seem in general to be
rather of the nature of ground-mists than of heavy cumulus.[983]
Occasionally, indeed, durable and extensive strata become visible.
During the latter half of October, 1894, for instance, a region as large
as Europe remained apparently cloud-covered. Yet most recent observers
are unable to detect the traces of aqueous absorption in the Martian
spectrum noted by Huggins in 1867[984] and by Vogel in 1873.[985]
Campbell vainly looked for them,[986] visually in 1894,
spectrographically in 1896; Keeler was equally unsuccessful;[987]
Jewell[988] holds that they could, with present appliances, only be
perceived if the atmosphere of Mars were much richer in water-vapour
than that of the earth. There can be little doubt, however, that its
supply is about the minimum adequate to the needs of a _living_, and
perhaps a life-nuturing planet.

The climate of Mars seems to be unexpectedly mild. Its _theoretical_
mean temperature, taking into account both distance from the sun and
albedo, is 34 deg. C. below freezing.[989] Yet its polar snows are both
less extensive and less permanent than those on the earth. The southern
white hood, noticed by Schiaparelli in 1877 to have survived the summer
only as a small lateral patch, melted completely in 1894. Moreover, Mr.
W. H. Pickering observed with astonishment the disappearance, in the
course of thirty-three days of June and July, 1892, of 1,600,000 square
miles of southern snow.[990] Curiously enough, the initial stage of
shrinkage in the white calotte was marked by its division into two
unequal parts, as if in obedience to the mysterious principle of
duplication governing so many Martian phenomena.[991] Changes of the
hues associated respectively with land and water accompanied in lower
latitudes, and we