CXXX11 NOTESc 



and in Hevelius and Cleomedes), are to be attributed to an illusion similar to 

 that which occasioned the belief in volcanic eruptions visible to us in the 

 Moon. See Schrb'ter, Seleuotopogr. Fragm. Th. i. S. 412 523 ; Th. ii. 

 S. 268272. The question, what are the smallest objects whose height and 

 other dimensions can be measured in the present state of our instrumental 

 means, is oue to which it is difficult to give a general answer. We find in 

 Dr. Robinson's account of J .rd Rosse's magnificent reflector, that in it an 

 extent of 80 to 90 yards can be recognised with great clearness. Madler 

 reckons that, in his observations, shadows of 3 secouds were still measurable, 

 which, under certain suppositions respecting the situatiou of the mountain 

 and the height of the Sun, would correspond to an elevation of 120 French 

 (128 English) feet ; but he notices at the same time that the shadow must 

 have a convenient breadth in order to be appreciable, or even visible. The 

 shadow of the Great Pyramid of Cheops would, according to the known 

 dimensions of that monument, be scarcely one-ninth of a second in breadth, 

 even at its widest part, and would therefore remain invisible (Madler, in 

 Schumacher's Jahrbuch for 18*1, S. 264). Arago reminds his readers, that, 

 with a magnifying power of 6000 (which, besides, could not be applied to 

 the Moon with a proportionally successful result), the mountains of the 

 Moon would appear of the size of Mont Blanc seen with the naked eye from 

 the Lake of Geneva. 



( 593 ) p. 367. The "rills" are not numerous; they are at most 100 or 120 

 miles long ; sometimes forked (Gassendi), rarely resembling veins (Tries- 

 uecker), always shining ; do not run across the mountains ; are peculiar to the 

 flatter districts ; do not become either broader or narrower, and have nothing 

 marked about their extremities. Beer und Madler, S. 131, 225, and 249. 



( 5W ) p. 368. See my description of the " Nocturnal Life of Animals in 

 the Primeval Forests," in the Ansichten der Natur (3te Ausg.), Bd. i. S. 334 j 

 Aspects of Nature in Different Lands and Different Climates, Vol. i. p. 270. 

 Laplace's speculations (I am unwilling to apply to them a different term) 

 on the manner in which he supposes perpetual moonlight might have been 

 ensured (Exposition du Systeme du Monde, 1824, p. 232), have been refuted 

 in a memoir by Liouville, " Sur un cas particulier du probleme des trois 

 corps." " Quelques partisans des causes finales," said Laplace, " ont imagine 

 que la lune a ete don nee a la terre pour 1'eclairer pendant les nuits ; dans ce 

 cas, la nature n'aurait point atteint le but qu'elle se serait propose, puisque 

 nons sommes souvent prives a la fois de la lumiere du soleil et de celle de la 

 lune. Pour y parvenir il cut suffi de mettre a 1'origine la lune en opposition 



