4i8 COSMOS. 



apart from the principal mass. Both kinds of knowledge, 

 however, the morphology of the rocky piles and the oryc- 

 tognosy of their composition, are equally necessary to the 

 perfect understanding of volcanic action ; nay, the latter, 

 founded on crystallisation and chemical analysis, on account 

 of the connection with plutonic rocks (porphyritic quartz, 

 greenstone and serpentine) is of even greater geognostic im- 

 portance. The little we believe we know of what is called 

 the volcanicity of the Moon depends too, from the very na- 

 ture of the knowledge, on configuration alone 65 . 



65 The progressive perfection of our acquaintance with the formation 

 of the surface of the Moon as derived from numerous observers, from 

 Tobias Mayer down to Lohrmann, Miidler and Julius Schmidt, has 

 tended on the whole rather to diminish than to strengthen our belief 

 in great analogies between the volcanic structures of the earth and 

 those of the moon ; not so much on account of the conditions of di- 

 mension and the early recognised ranging of so many ring-shaped 

 mountains, as on account of the nature of the rills and of the system of 

 rays which cast no shadows (radiations of light) of more than 400 miles 

 in length and from 2 to 16 miles in breadth, as in Tycho, Copernicus, 

 Kepler and Aristarchus. It is remarkable, however, that Galileo, in his 

 letter to Father Christoph Grienberger, SMe montuosita della Luna, 

 should have thought of comparing annular mountains, whose diameter 

 he considered greater than they actually are, to the circumvallated 

 district of Bohemia, and that the ingenious Robert Hooke in his 

 " Micrography " attributes the type of circular formation almost uni- 

 versally prevalent on the moon to the reaction of the interior of its 

 body on the exterior (vol. ii, p. 701, and vol. iv, p 496). With respect 

 to the annular mountain ranges of the moon, I have been of late much 

 interested with the relation between the height of the central mountain 

 and that of the circumvallation or margins of the crater, as well as by 

 the existence of parasitic craters on the circumvallation itself. The 

 result of all the careful observations of Julius Schmidt, who is occupied 

 with the continuation and completion of Lohrmami's Topography of 

 the Moon, establishes "that no single central-mountain attains the 

 height of the wall of its crater, but that in all cases it probably even 

 lies together with its summit considerably below that surface of the 

 inoon from which the crater is erupted. While the cone of ashes in the 

 crater of Vesuvius which rose on the 22nd of October 1822, according 

 to Brioschi's trigonometrical measurement, exceeds in height the Punta 

 del Palo, the highest edge of the crater on the north (618 toises above 

 the sea), by about 30 feet, and was visible at Naples, many of the 

 central mo'intains of the moon, measured by Madler and the Olmiitz 

 Astronomer, lie fully 6400 feet lower than the mean margin of cir 

 cumvallation, nay, even 100 toises below what may be taken as 

 t/ie mean surface-level in that part of the moon to which they respec 

 tival belong {Madler, iu Schumacher s Jahrbuchfur 1841, pp. 272 aud 



