498 



COSMOS. 



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

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

 xJianchini (August 16, 1725), and Short (April 22, 1751), 

 observed, 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-coloured night side of the Moon. 4 ' 



lateral crater. These points, important in a geognostic point 

 of view, deserve fresh investigation with more perfect instru- 

 ments. (Schroter, Selenotopographische Fragments, Th. ii. 

 tub. 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.) 



* Op. cit. p. 151. Arago, in the Annuaire for 1842, 

 p. 526. (Compare also Immanuel Kant, Schriften der Phy- 

 sischen Geographic, 1839, pp. 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, Hevelius, and Cleomedes), are illusions of a similar 

 nature to the supposed volcanic eruptions perceptible to us 

 upon the Moon. (See Schroter, Selenotopographische Frag- 

 mente, Th. i. pp. 412-523 ; Th. ii. pp. 268-272.) The 

 question, what is the smallest object whose height can be 

 measured with the instruments which are at present at our 

 command? is in general difficult to answer. According to 

 the report of Dr. Robinson upon the beautiful reflecting tele- 

 scope of Lord Rosse, extents of 220 feet (80 to 90 yards) 

 are discerned with the greatest distinctness. Madler calcu- 

 lates, that in his observations shadows of 3* were capable 

 of being measured, a length which, under certain presupposi- 

 tions as to the position of a mountain, and the altitude of the 

 Sun, would indicate a mountain elevation of 120 feet. How- 

 ever, he points out, at the same time, that the shadows must 

 nave a certain degree of breadth, in order to be visible and 

 measurable. The shadow of the great pyramid of Cheops, 



