rs were,
then, fully acquired to science in or before the year 1862. The first
was that of the seasonal fluctuations of the polar spots; the second,
that of the general permanence of certain dark gray or greenish patches,
perceived with the telescope as standing out from the deep yellow ground
of the disc. That these varieties of tint correspond to the real
diversities of a terraqueous globe, the "ripe cornfield"[976] sections
representing land, the dusky spots and streaks, oceans and straits, has
long been the prevalent opinion. Sir J. Herschel in 1830 led the way in
ascribing the redness of the planet's light to an inherent peculiarity
of soil.[977] Previously it had been assimilated to our sunset glows
rather than to our red sandstone formations--set down, that is, to an
atmospheric stoppage of blue rays. But the extensive Martian atmosphere,
implicitly believed in on the strength of some erroneous observations by
Cassini and Roemer in the seventeenth century, vanished before the sharp
occultation of a small star in Leo, witnessed by Sir James South in
1822;[978] and Dawes's observation in 1865,[979] that the ruddy tinge is
deepest near the central parts of the disc, certified its
non-atmospheric origin. The absolute whiteness of the polar snow-caps
was alleged in support of the same inference by Sir William Huggins in
1867.[980]

All recent operations tend to show that the atmosphere of Mars is much
thinner than our own. This was to have been expected _a priori_, since
the same proportionate mass of air would on his smaller globe form a
relatively sparse covering.[981] Besides, gravity there possesses less
than four-tenths its force here, so that this sparser covering would
weigh less, and be less condensed, than if it enveloped the earth.
Atmospheric pressure would accordingly be of about two and a quarter,
instead of fifteen terrestrial pounds per square inch. This corresponds
with what the telescope shows us. It is extremely doubtful whether any
features of the earth's actual surface could be distinguished by a
planetary spectator, however well provided with optical assistance.
Professor Langley's inquiries[982] led him to conclude that fully twice
as much light is absorbed by our air as had previously been
supposed--say 40 per cent. of vertical rays in a clear sky. Of the sixty
reaching the earth, less than a quarter would be reflected even from
white sandstone; and this quarter would again pay heavy toll in esc