156 COSMOS. 



we call great upon the Earth the elevation crater of Rocca 

 Monsina, Palma, Tenerifie, 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 one eighth or one sixth of the two craters of 

 elevation of the Moon just mentioned. The small crater of 

 the Peak of Tenerifie 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 (Hevelius, Maci'obius], not as 

 an erupted cone with an opening* The active volcanoes 

 which are stated to have been seen in the right side of the 

 Moon (May 4, 1783) ; the phenomena of light in Plato, which 

 Bianchini (August 16, 1725) and Short (April 22, 1751) ob- 

 served, are here mentioned only as of historical interest, since 

 the sources of deception have long been fathomed, and lie in 

 the more powerful reflection of the terrestrial light which 

 certain parts of the surface of our planet throw upon the ash- 

 colored night side of the Moon.f 



* Arzachel and Hercules are supposed to be exceptions : the former 

 to have a crater upon its summit, the second a lateral crater. These 

 points, important in a geognostic point of view, deserve fresh investi- 

 gation with more perfect instruments. (Schroter, Selenotopographi&cht 

 Fragments, th. ii., tab. 44 and 68, fig. 23.) Hitherto no signs have ever 

 been detected of lava streams collected in deep hollows. The radiated 

 lines which issue from Aristoteles in three directions are ranges of hills. 

 (Beer and Madler, p. 236.) 



t Op. cit., p. 151. Arago, in the Annuaire for 1842, p. 526. (Com- 

 pare also Immanuel Kant, Schriften der Pkysischen Geographic, 1839, 

 p. 393-402.) According to recent and more complete investigations, 

 the temporary changes said to have been observed upon the surface of 

 the Moon (the formation of new central mountains and craters in the 

 Mare Crisium, Heveliits, and Clcomedes), are illusions of a similar na- 

 ture to the supposed volcanic eruptions perceptible to us upon the Moon. 

 (See Schroter, SelenotopographischeFragmente, th. i., p. 412-523 ; th. ii., 

 p. 268-272.) The question, what is the smallest object whose height 

 can be measured with the instruments which are at present at our com- 

 mand ? is in general difficult to answer. According to the report of Dr. 

 Robinson upon the beautiful reflecting telescope of Lord Rosse, extents 

 of 220 feet (80 to 90 yards) are discerned with the greatest distinctness. 

 Madler calculates that, in his observations, shadows of 3" were capable 

 of being fteasnred ; a length which, under certain presuppositions as to 

 the position of a mountain, and the altitude of the Sun, would indicate 

 a mountain elevation of 120 feet. However, he points out. at the same 

 time, that the shadows must have a certain degree of breadth in order 

 to be visible and measurable. The shadow of the great pyramid of 



