atmosphere would then have contained almost exactly one-sixth as many molecules per square mile as that of the Earth. But since the force of gravity at the Moon's surface is, as we have seen, also one-sixth of that at the surface of the Earth, the density of the lunar atmosphere must have been only one thirty-sixth part of that of the Earth's. This would correspond to a pressure of .83 inches of mercury at the Earth's surface, and we should not under any circumstances expect to find a lunar atmosphere of greater density than this. But even this is 300 times more dense than what we actually find at the present time.
It is now known that the gases of which our atmosphere is composed are made up of little particles or molecules, each far too small to be seen even with the most powerful microscope, and each moving at ordinary temperatures with about the velocity of a cannon-ball. A few move many times faster than this, and a few many times slower. That they do not hurt us is due to their small size, but the result that we do feel is the well-known gaseous pressure. Now it may be shown mathematically that if a cannonball or any other body were fired directly upward from the Moon, with a velocity slightly exceeding one and a half miles a second, it would quit its surface never again to return to it. Those molecules, therefore, that are moving with the highest velocity, if they happen to be pointed in the right direction, will escape, and as time goes on, if the average velocity does not diminish, practically all of the molecules will ultimately get away.
It is also known that the molecules of the light gases like hydrogen and water vapour are moving much faster than the molecules of the heavy gases like carbonic acid and argon. The lighter gases would therefore escape first, and if the other gases were so heavy that their molecules never attained a speed of one and a half miles a second, they would never escape at all. It is probable, however, that all gases are escaping with more or less rapidity, not only from the Moon, but also from the Earth. In the case of the Earth it is a much slower process, because on account of our great mass as compared to that of the Moon a molecule to escape from us would have to attain a velocity of about seven miles a second. But even if we did lose a few million molecules every second it would not much matter.
Perhaps the best way to give an idea of the size of a molecule is to say that the number contained in a box holding a single cubic inch at the ordinary atmospheric pressure is represented by the figure sixteen followed by twenty-one ciphers. But this number is too large for us to understand. Let us, therefore, suppose that 10,000,000 of these
