satisfactory explanation of the manner in which a body, situated as near
the sun as Mercury is, could have its axial rotation gradually reduced
by the tidal attraction of the sun until it coincided in period with its
orbital revolution.

Accepting the accuracy of Schiaparelli's discovery, which was
corroborated in every particular in 1896 by Percival Lowell in a special
series of observations on Mercury made with his 24-inch telescope at
Flagstaff, Arizona, and which has also been corroborated by others, we
see at once how important is its bearing on the habitability of the
planet. It adds another difficulty to that offered by the remarkable
changes of distance from the sun, and consequent variations of heat,
which we have already discussed. In order to bring the situation home to
our experience, let us, for a moment, imagine the earth fallen into
Mercury's dilemma. There would then be no succession of day and night,
such as we at present enjoy, and upon which not alone our comfort but
perhaps our very existence depends, but, instead, one side of our
globe--it might be the Asiatic or the American half--would be
continually in the sunlight, and the other side would lie buried in
endless night. And this condition, so suggestive of the play of pure
imagination, this plight of being a two-faced world, like the god
Janus, one face light and the other face dark, must be the actual state
of things on Mercury.

There is one interesting qualification. In the case just imagined for
the earth, supposing it to retain the present inclination of its axis
while parting with its differential rotation, there would be an
interchange of day and night once a year in the polar regions. On
Mercury, whose axis appears to be perpendicular, a similar phenomenon,
affecting not the polar regions but the eastern and western sides of the