Evidence of change may indeed be discovered which can be otherwise explained. The moon is exposed to the action of heat other than that which pervaded her own frame at the time of her first formation. The sun's heat is poured upon the moon during the long lunar day of more than a fortnight, while during the long lunar night a cold prevails which must far exceed that of our bitterest arctic winters. We know from the heat-measurements made by the present Lord Rosse, that any part of the moon's surface at lunar mid-day is fully five hundred degrees Fahrenheit hotter than the same part two weeks later at lunar midnight. The alternate expansions and contractions resulting from these changes of temperature cannot but produce changes, however slowly, in the contour of the moon's surface. Professor Newcomb, indeed, considers that all such changes must long since have been completed. But I cannot see how they can be completed so long as the moon's surface is uneven, and at present there are regions where that surface is altogether rugged. Mighty peaks and walls exist which must one day be thrown down, so unstable is their form; deep ravines can be seen which must one day be the scene of tremendous landslips, so steep and precipitous are their sides. Changes such as these may still occur on so vast a scale that telescopists may hope from time to time to recognise them. But changes such as these are not volcanic; they attest no lunar vitality. They are antecedently so probable, indeed, while volcanic changes are antecedently so unlikely, that when any change is clearly recognised in the moon's surface, nothing but the most convincing evidence could be accepted as demonstrating that the change was of volcanic origin and not due to the continued expansion and contraction of the lunar crust. And now let us see how stands the evidence in the few cases which seem most to favour the idea that a real change has taken place.