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 features, such as the outlines of its so-called continents or coasts of its so-called seas, to be obscured by clouds to an extent which is appreciable except by very refined observations.

Quite otherwise would be the appearance which our globe would present to any observer who should view it say from Mars, or from some other external world at the same distance. The greater part of our globe would seem swathed with vast clouds through which only occasional peeps could be had at the actual configuration of its surface. I dare say a Martian astronomer who had an observatory with sufficiently good optical appliances, and who possessed sufficient patience, might in the course of time, and by availing himself of every opportunity, gradually limn out a chart of the earth which would in some degree represent that with which we are familiar in our atlases. It would, however, be a very tedious matter owing to the interruptions to the survey caused by the obscurities in our atmosphere. The distant astronomer would never be able to comprehend the whole of our earth's features in a bird's-eye glance as we are able to do those features on that hemisphere of Mars which happens to be turned towards us on a clear night.

As to what the composition of the atmosphere on Mars may be we can say but little. In so far as the sustenance of life is concerned, the main question, of course, turns on the presence or the absence of oxygen. It may be pertinent to this inquiry to remark here that a globe surrounded by air may at one epoch of its career have free oxygen as an ingredient in its atmosphere, while at other epochs free oxygen may be absent. This may arise from another cause besides the possible loss of the gas by