The numerous volcanoes are particularly characteristic of the Moon. They vary in magnitude from a diameter of over 200 km. (125 miles), for example the colossal Clavius with its companion craters, down to dimensions just visible with the aid of a telescope. The largest exceed our biggest many times in width and differ essentially from them, inasmuch as their bottoms are flat, occasionally provided with smaller volcanic cones--see the crater Longomontanus to the right of Tycho on Fig. 26--and surrounded by a high (inwardly often very steep, outwardly more sloping) wall as on Clavius, Longomontanus, and Tycho. The largest, as for example Clavius, may be compared to a province such as Bohemia, surrounded as it is on all sides by mountains. The elevated ring, as well as the interior of Clavius, is adorned with numerous large and small craters. The smallest of these resemble hemispherical excavations in the crust of the Moon, or they may be small volcanic cones which break through the walls. Sometimes they are strung out like pearls along rents in the ground. All these volcanoes have undoubtedly given passage from the interior to the surface of the Moon for enormous volumes of gases previously enclosed in the lunar magma. Nor is it less certain that these gases have consisted largely of water vapour. If this had been condensed to water, oceans and rivers would have been formed, and on the bottom of the seas would have been deposited sediments carried down from the mountains. Such, however, is not the case. The so-called “seas” on the Moon are indeed on a lower level than their surroundings, but their surface is even (see Fig. 27 with Mare Serenitatis below and Mare Tranquillitatis above to the left; see also Fig. 29 with Mare Imbrium below; it is bounded on the right by the “Carpathians”). The “seas” consist of volcanic rocks, and are not at all covered with loose sediments which if present ought to reflect light better than the volcanic vitreous rocks. But the lunar “seas” are much darker than the environments. This shows that seas proper, or bodies of water, have probably never existed on the Moon. Even before the surface had changed from its molten condition the water vapour had departed from the atmosphere, and the new quantities which the volcanoes emitted from the depths below disappeared so rapidly that lakes were never formed. The history of other atmospheric gases on the Moon was no doubt similar. All evidence, therefore, points to the conclusion that life never inhabited its rough surface. Fig. 27 shows that the “sea-bottoms” are not free from volcanoes. They also abound in folds, corresponding to mountain-chains on the Earth. These folds indicate old breaks in the crust while it was yet very thin. To the right, in Mare Serenitatis, appear a few white spots which W. H. Pickering ascribed to snow. The largest is the much discussed “crater” (?) Linné. Mare Serenitatis is surrounded by a ring of volcanoes.