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 square miles; that is to say,
they cover an area, in round numbers, seven times that of the snow-cap.
Only one-seventh of a foot of water, accordingly, could possibly be
made available for their fertilisation, supposing them to get the entire
advantage of the spring freshet. Upon a stint of less than two inches of
water these fertile lands are expected to flourish and bear abundant
crops; and since they completely enclose the polar area they are