cap begins to shrink in size (in the Northern Hemisphere), and this goes
on so rapidly that early in the June of Mars it is reduced to its
minimum. About the same time changes of colour take place in the
adjacent darker portions of the surface, which become at first bluish,
and later a decided blue-green; but by far the larger portion, including
almost all the equatorial regions of the planet, remain always of a
reddish-ochre tint.[4]

[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's
photographs of Mars on April 9th showed the southern polar cap of
moderate dimensions, but with a large dim adjacent area. Twenty-four
hours later a corresponding plate showed this same area brilliantly
white; the result apparently of a great Martian snowfall. In 1882 the
same observer witnessed the steady disappearance of 1,600,000 square
miles of the southern snow-cap, an area nearly one-third of that
hemisphere of the planet.]

The rapid and comparatively early disappearance of the white covering
is, very reasonably, supposed to prove that it is of small thickness,
corresponding perhaps to about a foot or two of snow in north-temperate
America and Europe, and that by the increasing amount of sun-heat it is
converted, partly into liquid and partly into vapour. Coincident with
this disappearance and as a presumed result of the water (or other
liquid) producing inundations, the bluish-green tinge which appears on
the previously dark portion of the surface is supposed to be due to a
rapid growth of vegetation.

But the evidence on this point does not seem to be clear or harmonious,
for in the four coloured plates showing the planet's surface at
successive Martian dates from December 30th to February 21st, not only