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LUNAR CELESTIAL PHENOMENA.

and nitrogen (in the main), not a chemical compoundof these gases, we see that there is no absolute neces-sity for the proportion in which these gases appear inour atmosphere. In the atmosphere of another bodythey might be differently proportioned. Moreover,carbonic acid gas, which forms a comparatively smallpart of the terrestrial atmosphere, might form a muchlarger proportion of the atmosphere of another planet.It is also conceivable that other and denser gasesmight be present in other atmospheres.

But even when all such considerations as these havebeen taken into account, it remains certain that unlesswe assume the existence on the moon of gases un-known on earth, a lunar atmosphere would have aspecific gravity, under like conditions of pressure,differing in no marked degree from that of our earth’satmosphere. It would be a somewhat bold assumptionto take for the average specific gravity of the lunaratmosphere that of carbonic acid gas, which, as weknow, is almost exactly half as great again as that ofair. But even if we supposed the lunar atmospherecomposed of a gas as heavy as chlorine (which has aspecific gravity nearly 24 times as great as that of air),or like phosgene gas, which is nearly '3f times asheavy as air, the argument which follows would notbe seriously affected.

Our air is sufficient in quantity to form a layerabout 54 miles in depth over the whole surface of theearth, and as dense throughout as air at the sea-level.'This air, according to the laws of gaseous pressure,