Page:In the high heavens.djvu/325

 the meteorites which now arrive here left the moon some few days previously. If, however, the moon's volcanoes are all extinct, it is then excessively unlikely that any of the meteorites that are now falling could have been derived from this source. The lunar origin of the meteorites must therefore he rejected, if it can be shown that the lunar volcanoes are all now extinct.

At the time when it was believed that the moon might be the source of meteorites, there was thought to be good reason for the supposition that some of the lunar volcanoes retained their igneous energy. Modern research has, however, demonstrated that the lunar volcanoes are absolutely silent and ineffective. No doubt some slight indications of change have been detected, in certain details, on the lunar surface, but I do not think that, even if we admit every case of change which has been alleged by recent observers, it could be contended that any one of the lunar volcanoes now possesses the necessary activity. We are, therefore, forced to discard the lunar theory of meteorites altogether, for the simple reason that if the moon ever did project meteoritic masses, they must have tumbled down on the earth ages ago, at the time when the lunar volcanoes were still active. We do not, therefore, look for any lunar explanation of the meteorites which fall down here in these modern days, when the volcanoes in our satellite have become extinct. Let us, therefore, go further a-field, and search for the possible origins of these bodies in the volcanoes of other worlds.

It will be convenient, at this point, to lay down the principle by which we shall be guided in determining the critical velocity which would be necessary to project bodies away from any particular globe. It can be