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 there are no large oceans. The only likeness to the conditions of our earth which we can perceive is in a general pointing of the somewhat triangular masses of what appears to be land toward one pole. As a whole, the conditions of the Martial lands and seas as regards their form, at least, is more like that of Europe than that of any other part of the earth's surface. Europe in the early Tertiary times had a configuration even more like that of Mars than it exhibits at present, for in that period the land was very much more divided than it now is.