Page:In the high heavens.djvu/139

 of Martian atmosphere. We also observe the clearness with which objects are exhibited at the centre of the disc of Mars, and though this may be in some measure due to the absence of distortion from the effects of foreshortening, it undoubtedly arises to some extent from the fact that objects in this position are viewed through a comparatively small thickness of the atmosphere enveloping the planet.

Clouds are also sometimes seen apparently floating in the upper region of Mars. This, of course, is only possible on the supposition that there must be an atmosphere which formed the vehicle by which clouds were borne along. It is, however, quite obvious that the extent of the Martian atmosphere must be quite insignificant when compared with that by which our earth is enveloped. It is a rare circumstance for any of the main topographical