may be so related to the thickness of the crust, that the heat developed can find ready escape by conduction to the surface, and by radiation thence into space. Nay, from the part which water is known to play in producing volcanic phenomena, it may well be that in every region where water does not find its way in large quantities to the parts in which the subterranean heat is great, no volcanic action results. Mallet, following other experienced vulcanologists, lays down the law, "Without water there can be no volcano;" so that the neighbourhood of large oceans, as well as special conditions of the crust, must be regarded as probably essential to the existence of such outlets as Vesuvius, Etna, Hecla, and the rest. So much premised, let us enquire whether it is antecedently likely that in the moon volcanic action may still be in progress, and afterwards consider the recent announcement of a lunar disturbance, which, if really volcanic, certainly indicates volcanic action far more intense than any which is at present taking place in our own earth. I have already, I may remark, considered the evidence respecting this new lunar crater which some suppose to have been formed during the last two years. But I am not here going over the same ground as in my former paper ("Contemporary Review" for August, 1878). Moreover, since that paper was written, new evidence has been obtained, and I am now able to speak with considerable confidence about points which were in some degree doubtful three months ago. Let us consider, in the first place, what is the moon's probable age, not in years, but in development. Here we have only probable evidence to guide us, evidence chiefly derived from the analogy of our own earth. At least, we have only such evidence when we are enquiring into the moon's age as a preliminary to the consideration of her actual aspect and its meaning. No doubt many features revealed by telescopic scrutiny are full of significance in this respect. No one who has ever looked at the moon, indeed, with a telescope of great power has failed to be struck by the appearance of deadness which her surface presents, or to be impressed (at a first view, in any case), with the idea that he is looking at a world whose period of life must be set in a very remote antiquity. But we must not take such considerations into account in discussing the _a priori_ probabilities that the moon is a very aged world. Thus we have only evidence from analogy to guide us in this part of our enquiry. I note the point at starting, because the indicative mood is so much more convenient than the conditional, that I may frequently in this part of my enquiry use the former where the actual nature of the evidence would only justify the latter. Let it be understood that the force of the reasoning here depends entirely on the weight we are disposed to allow to arguments from analogy.