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 white patch
is the supposed Polar Snow Cap.]

As is shown by the maps of Mars, the surface is occupied by shadings
which seem to indicate the existence of water and lands. Those
portions of the area which are taken to be land are very much divided
by what appear to be narrow seas. The general geographic conditions
differ much from those of our own sphere in that the parts of the
planet about the water level are not grouped in great continents, and