astronomy, the late Miss Agnes Clerke. In the _Edinburgh
Review_ (of October 1896) there is an article entitled 'New Views about
Mars,' exhibiting the writer's characteristic fulness of knowledge and
charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying
the surplus water across the equator, far into the opposite hemisphere,
for purposes of irrigation there (which we see he again states in the
present volume), Miss Clerke writes: "We can hardly imagine so shrewd a
people as the irrigators of Thule and Hellas[6] wasting labour, and the
life-giving fluid, after so unprofitable a fashion. There is every
reason to believe that the Martian snow-caps are quite flimsy
structures. Their material might be called snow _souffle_, since, owing
to the small power of gravity on Mars, snow is almost three times
lighter there than here. Consequently, its own weight can have very
little effect in rendering it compact. Nor, indeed, is there time for
much settling down. The calotte does not form until several months after
the winter solstice, and it begins to melt, as a rule, shortly after the
vernal equinox. (The interval between these two epochs in the southern
hemisphere of Mars is 176 days.) The snow lies on the ground, at the
outside, a couple of months. At times it melts while it is still fresh
fallen. Thus, at the opposition of 1881-82 the spreading of the northern
snows was delayed until seven weeks after the equinox: and they had,
accordingly, no sooner reached their maximum than they began to decline.
And Professor Pickering's photographs of April 9th and 10th, 1890,
proved that the southern calotte may assume its definitive proportions
in a single night.

[Footnote 6: Areas on Mars so named.]

"No attempt has yet been made to estimate the quantity of water
derivable from the melting of one of these formations; yet the
experiment is worth trying as a help towards defining ideas. Let us
grant that the average depth of snow in them, of the delicate Martian
kind, is twenty feet, equivalent at the most to one foot of water. The
maximum area covered, of 2,400,000 square miles, is nearly equal to that
of the United States, while the whole globe of Mars measures 55,500,000
square miles, of which one-third, on the present hypothesis, is under
cultivation, and in need of water. Nearly the whole of the dark areas,
as we know, are situated in the southern hemisphere, of which they
extend over, at the very least, 17,000,000