I was much interested in the account which your last number (vol. xviii. p. 197) contained of the presumably new lunar crater discovered by Dr. Klein in the Mare Vaporum. Is it really necessary to ascribe the formation of such a crater to present volcanic action? It seems to me that this singular phenomenon of the birth of a new crater may be more likely owing to such action having, in long-past ages, left (as in all probability it would leave) extensive caverns beneath the visible surface of our satellite. Such caverns might, in consequence of the gradual changes which the action of the sun's rays, alternating with intense cold, must produce on the lunar rocks, occasionally give way. A crater-like cavity would then be caused on the moon's surface by this subsidence, such as are not unfrequently seen in mining districts where old workings have fallen in. The fact that the new crater is elliptical, and not round, seems to add to the probability of its having been caused by some such “settling” process. If the crater were produced by active volcanic agency, it would surely be circular, or nearly so. I do not find this mode of quasi-crater formation suggested in Nasmyth's book, nor, so far as I can remember, in any other. Is it not, however, a possible cause of change on the surface of our satellite?