be ground for considering already the former value too high; both are very uncertain. If we accept that of Lowell, we find that each square metre of the surface of Mars supports a column of air, whose mass is only about one-fifth of the mass resting on each square metre of ocean surface on Earth. [Illustration: Fig. 14. The planet Venus, with sunlit atmosphere (to the left), as observed by Langley at the Venus passage December 6, 1882. ] The dense clouds which float above Venus have long ago led to the assumption that the atmosphere of that planet must be far deeper than that of the Earth. Its strong refractory power has also contributed to this belief. When Venus is close to the sun-disc the dark body stands forth surrounded by a ring of light (see Fig. 14). It is, however, recognized that this phenomenon requires no greater air density than that on Earth for its appearance. In this connection we should remember that the inside limit of the vapour shell which we in this manner observe, is the cloud-wall, not the ground. And these clouds, we have every reason to believe, float on account of the heat prevailing at a great height in the atmosphere, so high in fact that they form an impenetrable wall already where the cirrus clouds appear in our sky. If these suppositions are correct, the light-ring mentioned is caused by a quarter only of the air-masses on Venus, and its total air-covering must be far deeper than that of the Earth. The latter occupies probably in this respect as well as in reference to position in space a middle ground between Mars with its extremely thin and Venus with its comparatively dense atmosphere. If so, we might expect the atmosphere on Mercury to be denser yet, while we already have seen that it is almost wholly lacking on that planet. The explanation is that Mercury has lost its spin around its own axis and therefore always presents the same side to the Sun--just as the Moon and probably all other satellites turn one side only toward their respective central bodies--hence the opposite side becomes so cold that all gases are there condensed to fluids or solids except the two most volatile ones, hydrogen and helium, which on the other hand leave the planet on its hot side. If Venus, therefore, as held by several astronomers from Schiaparelli to Lowell, always turned one side only toward the Sun, this planet also would be without any perceptible air-covering. According to investigations by Bjelopolsky of Venus’s spectrum, which investigations, however, are in complete disagreement with corresponding measurements by Slipher, that planet has a period of rotation on its own axis of about 29 hours. This figure is very uncertain and a new determination is therefore highly desirable.