Clii NOTES. 



est part of the crater-margin) by about thirty feet, and can be seen from Naples, 

 on the moon, on the contrary, many of the central mounts, as measured by 

 Madler and by Schmidt, are fully 1000 toises (about 6400 feet) lower than the 

 mean height of the encircling ridge, and even 100 toises below what may 

 be supposed to be the mean level of the general surface of that part of the 

 moon. (Madler, in Schumacher's Jahrbuch for 1841, S. 272 and 274; and 

 Julius Schmidt's " Der Mond," 1856, S. 62.) The lunar central mounts, or 

 it may perhaps rather be said " central masses of mountains," have usually 

 several summits: as in Theophilus, Petavius, and Bulliald. There are six 

 central mounts in Copernicus, while Alphons alone shows one regular central 

 peak with a sharp-pointed summit. The above relations remind us of the 

 Astroni in the Phlegrsean Fields, to whose dome-shaped central masses Von 

 Buch justly attributed great importance. " These masses (like the central 

 masses in the Ring Mountains in the moon) did not burst forth; no permanent 

 connection with the interior, no volcano was formed; but rather, as it were, a 

 model on a small scale of those great trachytic unopened domes, as Puy de Dome 

 and Chimborazo, which are so variously distributed over the earth's surface." 

 (PoggendorfFs Annalen, Bd. xxxvii. 1836, S. 183.) The encircling margin of 

 the Astroni has everywhere the form of a closed ellipse, never rising higher than 

 130 toises (831 feet) above the level of the sea. The summits of the central 

 domes are 658 feet lower than the highest part of the south-western crater-wall. 

 The domes form two parallel ridges clothed with thick bushes. (Julius Schmidt, 

 " Eruption des Vesuvs," S. 147; and the same author's " Der Mond," S. 70 and 

 103.) One of the most remarkable objects on the moon's surface is the Ring 

 Mountain Petavius, in which the whole interior crater-floor has expanded in a 

 convex form, and yet is crowned by a central~mount. The convexity, resembling 

 a blister-like swelling, is here a permanent form. In our terrestrial volcanoes 

 it is only temporarily that the crater-floor is sometimes so inflated by the force 

 of the vapours beneath as to rise almost to the height of the crater-margin ; but 

 as the vapours break through and burst forth, the inflated floor sinks down 

 again. The greatest diameters of terrestrial craters are those of the Caldeira de 

 Fogo, according to Charles Deville 4100 toises (4'3 geogr. miles); and the Cal- 

 deira of Palma, according to Leopold von Buch, 3100 toises; whereas, on the 

 moon, Theophilus is 50,000 toises, and Tycho 45,000 toises (or respectively 52 

 and 45'2 geogr. miles). Parasitical secondary craters, which have broken forth 

 on a marginal wall of a great crater, are very frequent on the moon. The crater- 

 floor in these parasites is usually empty, as on the great rent-asunder margin 

 of Maurolycus; it is more rare to see a small central mount, perhaps a cone of 

 eruption, as in Longomontanus. In a fine sketch of the crater-system of Etna, 



