Next in the series of the planets is our own earth. As the details of
this planet are to occupy us during nearly all the remainder of this
work, we shall for the present pass it by.

Beyond the earth we pass first to the planet Mars, a sphere which has
already revealed to us much concerning its peculiarities of form and
physical state, and which is likely in the future to give more
information than we shall obtain from any other of our companions in
space, except perhaps the moon. Mars is not only nearer to us than any
other planet, but it is so placed that it receives the light of the
sun under favourable conditions for our vision. Moreover, its sky
appears to be generally almost cloudless, so that when in its orbital
course the sphere is nearest our earth it is under favourable
conditions for telescopic observation. At such times there is revealed
to the astronomer a surface which is covered with an amazing number of
shadings and markings which as yet have been incompletely interpreted.
The faint nature of these indications has led to very contradictory
statements as to their form; no two maps which have been drawn agree
except in their generalities. There is reason to believe that Mars has
an atmosphere; this is shown by the fact that in the appropriate
season the region about either pole is covered by a white coating,
presumably snow. This covering extends rather less far toward the
planet's equator than does the snow sheet on our continents. Taking
into account the colour of the coating, and the fact that it
disappears when the summer season comes to the hemisphere in which it
was formed, we are, in fact, forced to believe that the deposit is
frozen water, though it has been suggested that it may be frozen
carbonic acid. Taken in connection with what we have shortly to note
concerning the apparent seas of this sphere, the presumption is