104 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



the primaries, we find that the satellites had their rotation annulled 

 because they were too near their planets ; whereas the planets, except- 

 ing Mercury, have preserved the independence of their rotation 

 because they were far too distant from the Sun. 



Another interesting question is raised by the existence of an atmos- 

 phere around Mercury. That gaseous envelop, like the one surround- 

 ing Mars, is absolutely invisible ; but the frequent presence of cloudy 

 veils over the markings of the superficies constitutes a solid proof of 

 its existence. The clouds of the planet were discovered by the acute- 

 ness of Schiaparelli, who found that they often appear as white 

 streaks on the limb, and that they also veil the dark areas, being 

 curiously more frequent on the evening than on the morning phase. 

 The Italian astronomer compared these veils to the clouds of the 

 Earth — a natural, though at present impossible, assumption, as will 

 soon be seen. Yet, excepting this comparison, my independent re- 

 sults have entirely confirmed the statements of Schiaparelli on this 

 question of the atmospheric veils of Mercury. The limb of the 

 planet often showed to me temporary, irregular, whitish arcs of cloud, 

 stretching sometimes over 3,000 miles in length. Toward the central 

 regions, these veils tended to become invisible, their presence being 

 indirectly revealed by the temporary pallor, or invisibility, of the 

 subjacent markings of the surface. It was rarely that a spot be- 

 longing to the superficies would preserve its intensity unimpaired for 

 many successive days; and the veils in question presented all the 

 degrees of condensation, froiii the greatest rarefaction up to an opac- 

 ity which completely obliterated dark areas of the soil measuring 

 more than 2,000 miles across. The changes were sometimes so rapid, 

 that a spot of the length just mentioned, visible with its real intensity 

 through a clear Mercurian air one day, would be utterly invisible 

 24*" later, and conversely. Very often the dark areas had their in- 

 tensity locally diminished for weeks, with an alternate succession 

 of various degrees of pallor, extinction, and final return to their nor- 

 mal appearance. In the course of my inquiry, the hooked dark spot 

 to the right of my chart, named " Solitudo Criophori ", was much 

 more often rendered invisible by local veils than ajiy other marking. 



The clouds of Mercury are much more frequent and more obliterat- 

 ing than those of Mars, whose nature is, however, quite a different 

 one. 



It is certain that these veils cannot be composed of droplets of 

 water or of particles of ice, like our own clouds. The enormous heat 

 radiated from the Sun renders the existence of water in the liquid 

 state on the sun-lit hemisphere of Mercury impossible, while the 

 deductions of Johnstone Stoney from the kinetic theory of gases 

 make the presence even of aqueous vapour extremely doubtful in 



