THE MOON. 497 



diameter of from 100 to 120 geographical miles. 46 On the 

 contrary, the real annular mountains scarcely exceed 8 or 

 12 miles in diameter. Conon in the Apennines is 8; and 

 a crater which belongs to the shining region of Aris- 

 tarchus^ is said to present a breadth of only 25,576 feet, 

 exactly the half of the diameter of the crater of Rucu- 

 Pichincha, in the table-land of Quito, measured trigono- 

 metrically by myself. 



Since we have in this place adhered to comparisons with 

 well-known terrestrial phenomena and relations of magnitude, 

 it is necessary to remark that the greater part of the plain? 

 and annular mountains of the Moon, are to be considered in the 

 first place as craters of elevation, without continuous pheno- 

 mena of eruption in the sense of the hypothesis of Leopold 

 von Buch. What, according to the European standard, we 

 call great upon the Earth the elevation crater of Rocca 

 Monsina, Palma, Teneriffe, and Santorin becomes insignifi- 

 cant when compared with Ptolemy, Hipparchus, and many 

 others of the Moon. Palma has only 24,297 feet diameter; 

 Santorin, according to Captain Graves, new measurement, 

 33,148 feet; Teneriffe, at the utmost, 53,298 feet: conse- 

 quently, only only one-eighth or one-sixth of the two craters 

 of elevation of the Moon just mentioned. The small crater 

 of the Peak of Teneriffe and Vesuvius (from 319 to 426 feet 

 in diameter) could scarcely be seen by the aid of telescopes. 

 The by far greater number of the annular mountains have no 

 central mountain; and where there is one, it is described as 

 being dome-formed or level (HeveUtu, Macrobius], not as 

 an erupted cone ivith an opening.* 1 The active volcanos, 



** Beer and M'adler, p. 126. PtolemsDus is 96 miles in 

 diameter ; Alphous and Hipparchus 76 miles. 



47 Arzachel and Hercules are supposed to be exceptions: 

 the former to have a crater upon ils summit, the secoad a 



