molecules escape every second from the box into a vacuum, how soon will the whole number of molecules in the box have disappeared? One might at first thought suppose perhaps in a day, or a year, or perhaps even as long as a century. The time required is easily computed, and we shall find that it will take just about 50,000,000 years before the last molecule in that cubic inch has got away. In point of fact, our atmosphere is probably disappearing at the present time at the rate of many millions of millions of molecules every second.
In the case of the Moon, most of the molecules seem to have already got away, but even if the density of the lunar atmosphere is only one ten-thousandth of our own, there are still enough left there to produce a pressure of several tons upon every square mile of surface. The density of our atmosphere is halved for every three and a half miles that we ascend, therefore at an altitude of forty-six miles the density of our atmosphere will be just equal to that at the Moon's surface. The density of the Moon's atmosphere is halved for every twenty-one miles that we ascend, therefore at an altitude of fiftysix miles the atmospheres of the Earth and Moon will have the same density, and at greater altitudes the Moon's atmosphere will be the denser of the two. Meteors entering the Earth's atmosphere first become visible at an elevation of about eighty miles, while the swiftly moving ones, such as the Leonids, are destroyed before coming within forty miles of the Earth. Meteors would first become visible on the Moon at an altitude of 200 miles above the surface; it therefore appears that a concentrated meteor shower upon the Moon would be quite as brilliant an affair as it would be upon the Earth. Our highest auroras sometimes reach an altitude of 600 miles. Similarly upon the Moon they would reach a height of 3,300 miles. We thus see that the Moon's atmosphere, assuming it to have a density no greater than that we have assigned to it, is even then a factor in selenography by no means negligible.
The next question that arises is, of what gases is the lunar atmosphere composed? The chief gases of our air—oxygen and nitrogen—would escape from the Moon's atmosphere about as readily as hydrogen does from that of the Earth. Carbonic acid would be retained with somewhat greater facility, but in general it is likely that any gas that was not constantly renewed from the Moon's interior would have practically disappeared from its surface long ago. Let us now see what gases are at the present time being given off from the Earth's interior. We find that there are only two that escape in large quantities—carbonic acid and water-vapour. The former would remain for some time
