In addition to her great craters, the moon seems to be thickly covered with little ones, many of them being as small as can be seen at all through a telescope. Whether these are all volcano-craters remains to be discovered. It is not supposed that any of them are now active. From time to time, signs of faint changes on the moon’s surface have been noticed, which it was thought might be owing to volcanic outbursts. Such an outburst as the worst eruptions of Mount Vesuvius would be invisible at this distance. But the said changes may be quite as well accounted for by the startling fortnightly variations of climate which the moon has to endure. The general belief now inclines to the idea that the moon-volcanoes are extinct, though no doubt there was in the past great volcanic activity there. A description has been given earlier of the rain of meteorites constantly falling to our earth, and only prevented by the atmosphere from becoming serious. But the moon has no such protecting atmosphere, and the amount of cannonading which she has to endure must be by no means small. Perhaps in past times, when her slowly-cooling crust was yet soft, these celestial missiles showering upon her may have occasionally made deep round holes in her surface. This is another guess, which time may prove to be true. Guesses at possible explanations of mysteries do no harm, so long as we do not accept them for truth without ample reason. [Illustration: LUNAR ERUPTION--BRISK ACTION.] The origin of the lunar craters must be referred to some ancient epoch in the moon’s history. How ancient that epoch is we have no means of knowing; but in all probability the antiquity of the lunar craters is enormously great. At the time when the moon was sufficiently heated to have these vast volcanic eruptions, of which the mighty craters are the survivals, the earth must have been very much hotter than it is at present. It is not, indeed, at all unreasonable to believe that when the moon was hot enough for its volcanoes to be active, the earth was so hot that life was impossible on its surface. This supposition would point to an antiquity for the moon’s craters far too great to be estimated by the centuries and the thousands of years which are adequate for the lapse of time as recognized by the history of human events. It seems not unlikely that millions of years may have elapsed since the mighty craters of Plato or of Copernicus consolidated into their present form. [Illustration: LUNAR ERUPTION--FEEBLE ACTION.]