e time cloudy the mean transmission is 35 per
cent." These statements seem incompatible with that quoted above.

The figure he uses in his calculations for the actual albedo of the
earth, 0.75, is also not only improbable, but almost self-contradictory,
because the albedo of cloud is 0.72, and that of the great cloud-covered
planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only
0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it
only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these
estimates, while the fact that both Venus and Jupiter are wholly
cloud-covered, while we are only half-covered, renders it almost
certain that our albedo is far less than Mr. Lowell makes it. It is
evident that mathematical calculations founded upon such uncertain data
cannot yield trustworthy results. But this is by no means the only case
in which the data employed in this paper are of uncertain value.
Everywhere we meet with figures of somewhat doubtful accuracy. Here we
have somebody's 'estimate' quoted, there another person's 'observation,'
and these are adopted without further remark and used in the various
calculations leading to the result above quoted. It requires a practised
mathematician, and one fully acquainted with the extensive literature of
this subject, to examine these various data, and track them through the
maze of formulae and figures so as to determine to what extent they
affect the final result.

There is however one curious oversight which I must refer to, as it is a
point to which I have given much attention. Not only does Mr. Lowell
assume, as in his book, that the 'snows' of Mars consist of frozen
water, and that therefore there _is_ water on its surface and
water-vapour in its atmosphere, not only does he ignore altogether Dr.
Johnstone Stoney's calculations with regard to it, which I have already
referred to, but he uses terms that imply that water-vapour is one of
the heavier components of our atmosphere. The passage is at p. 168 of
the _Philosophical Magazine._ After stating that, owing to the very
small barometric pressure in Mars, water would boil at 110 deg. F., he adds:
"The sublimation at lower temperatures would be correspondingly
increased. Consequently the amount of water-vapour in the Martian air
must on that score be relatively greater than our own." Then follows
this remarkable passage: "Carbon-dioxide, because of its greater
specific gravity,