We now have another elaborate estimate of the comparative amounts of
heat actually received by Mars and the Earth, dependent on their very
different amounts of atmosphere, and this estimate depends almost wholly
on the comparative _albedoes_, that of Mars, as observed by astronomers
being 0.27, while ours has been estimated in a totally different way as
being 0.75, whence he concludes that nearly three-fourths of the
sun-heat that Mars receives reaches the surface and determines its
temperature, while we get only one-fourth of our total amount. Then by
applying Stefan's law, that the radiation is as the 4th power of the
surface temperature, he reaches the final result that the actual heating
power at the surface of Mars is considerably _more_ than on the Earth,
and would produce a mean temperature of 72 deg. F., if it were not for the
greater conservative or blanketing power of our denser and more
water-laden atmosphere. The difference produced by this latter fact he
minimises by dwelling on the probability of a greater proportion of
carbonic-acid gas and water-vapour in the Martian atmosphere, and thus
brings down the mean temperature of Mars to 48 deg. F., which is almost
exactly the same as that of the southern half of England. He has also,
as the result of observations, reduced the probable density of the
atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of
that of the Earth.

_Critical Remarks on Mr. Lowell's Paper._

The last part of this paper, indicated under pars. 4 and 5, is the most
elaborate, occupying eight pages, and it contains much that seems
uncertain, if not erroneous. In particular, it seems very unlikely that
under a clear sky over the whole earth we should only receive at the
sea-level 0.23 of the solar rays which the earth intercepts (p. 167).
These data largely depend on observations made in California and other
parts of the southern United States, where the lower atmosphere is
exceptionally dust-laden. Till we have similar observations made in the
tropical forest-regions, which cover so large an area, and where the
atmosphere is purified by frequent rains, and also on the prairies and
the great oceans, we cannot trust these very local observations for
general conclusions affecting the whole earth. Later, in the same
article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per
cent. of the heat reaching them: a clear sky at sea-level 60 per cent.
As the sky is half th