to William Herschel that
we owe the first systematic study of these remarkable polar caps. This
illustrious astronomer was rewarded by a very interesting discovery. He
found that these arctic tracts on Mars vary both in extent and
distinctness with the seasons of the hemisphere on which they are
situated. They attain a maximum development from three to six months
after the winter solstice on that planet, and then diminish until they
are smallest about three to six months after the summer solstice. The
analogy with the behaviour of the masses of snow and ice which surround
our own poles is complete, and there has until lately been hardly any
doubt that the white polar spots of Mars are somewhat similarly
constituted.

As the period of revolution of Mars around the sun is so much longer
than our year, 687 days instead of 365, the seasons of the planet are,
of course, also much longer than the terrestrial seasons. In the
northern hemisphere of Mars the summer lasts for no fewer than 381 days,
and the winter must be 306 days. In both hemispheres the white polar cap
in the course of the long winter season increases until it reaches a
diameter of 45 deg. to 50 deg., while the long summer reduces it to a small area
only 4 deg. or 5 deg. in diameter. It is remarkable that one of these white
regions--that at the south pole--seems not to be concentric with the
pole, but is placed so much to one side that the south pole of Mars
appears to be quite free from ice or snow once a year.

Although many valuable observations of Mars were made in the course of
the nineteenth century, it is only since the very favourable opposition
of 1877 that the study of the surface of Mars has made that immense
progress which is one of the most remarkable features of modern
astronomy. Among the observers who produced valuable drawings of the
planet in 1877 we may mention Mr. Green, whose exquisite pictures were
published by the Royal Astronomical Society, and Professor Schiaparelli,
of Milan, who almost revolutionised our knowledge of this planet.
Schiaparelli had a refractor of only eight inches aperture at his
disposal, but he was doubtless much favoured by the purity of the
Italian sky, which enabled him to detect in the bright portions of the
surface of Mars a considerable number of long, narrow lines. To these he
gave the name of canals, inasmuch as they issued from the so-called
oceans, and could be traced across the reputed continents for
cons