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
overwhelmingly to the effect that Mars has seasons not unlike our own.

The existence of snow on any sphere may safely be taken as evidence
that there is an atmosphere. In the case of Mars, this supposition is
borne out by the appearance of its surface. The ruddy light which it
sends back to us, and the appearance on the margin of the sphere,
which is somewhat dim, appears to indicate that its atmosphere is
dense. In fact, the existence of an atmosphere much denser than that
of our own earth appears to be demanded by the fact that the
temperatures are such as to permit the coming and going of snow. It is
well known that the temperature of any point on the earth, other
things being equal, is proportionate to the depth of atmosphere above
its surface. If Mars had no more air over its surface than has an
equal area of the earth, it would remain at a temperature so low that
such seasonal changes as we have observed could not take place. The
planet receives one third less heat than an equal area of the earth,
and its likeness to our own temperature, if such exists, is doubtless
brought about by the greater density of its atmosphere, that serves to
retain the heat which comes upon its surface. The manner in which this
is effected will be set forth in the study of the earth's atmosphere.

[Illustration: Fig. 4.--Mars, August 27, 1892 (Guiot), the