These things are not only amusing, but important. There can be no question that the force of gravity on the moon actually is as slight as it has just been described. So, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. It has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the sun until various perturbations have brought them down upon the earth. A body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity. Another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. According to the theory of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. The escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmosphere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.[16] [Footnote 16: The discovery of free hydrogen in the earth's atmosphere, by Professor Dewar, 1901, bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. Since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. In a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.]