ocean of air. It is the atmosphere which, to a large extent, mitigates the fierceness with which the sun's rays would beat down on the globe if it were devoid of such protection. Again, at night, the atmospheric covering serves to screen us from the cold that would otherwise be the consequence of unrestricted radiation from the earth into space. It is, therefore, obvious that the absence of a copious atmosphere, though perhaps not so absolutely incompatible with life of some kind, must still necessitate types of life of a wholly different character from those with which we are familiar. In attempting, therefore, to form an estimate of the probability of life on another world, it is of essential importance to consider whether it possesses an atmosphere.
We may here lay down a canon which appears to be pretty general among the celestial bodies accessible to our observations. It may be thus stated. The larger the body the more abundant the atmosphere by which that body is surrounded. Of course this rule has to be understood with certain qualifications, and perhaps some exceptions to it might be suggested, but as a broad general fact it will hardly be questioned. Thus, to take at once the largest body of our system and one of the smallest—the sun and the moon—they both provide striking exemplifications of the principle in question. It is well known that the sun is enveloped by an atmosphere remarkable for the prodigious extent that it occupies. On the other hand the moon, which is by far the smallest of the bodies readily accessible to our observations, is, if not entirely devoid of gaseous investment, at all events only provided with the scantiest covering of this nature. But the chief interest that the principle we have laid
