Page:In the high heavens.djvu/138

 smallest world on which the continued existence of water would be possible. It would perhaps be going rather too far to say that a world almost the size of Mars must therefore be the smallest on which life could possibly be supported, but it is plain that our argument tends to support such a proposition.

The discussion we have just given will prepare us to believe that a planet with the size and mass of Mars may be expected to be encompassed with an atmosphere. Our telescopic observations completely bear this out. It is perfectly certain that there is a certain shell of gaseous material investing Mars. This is shown in various ways. We note the gradual obscuration of objects on the planet as they approach the edge of the disc, where they are necessarily viewed through a greatly increased thickness