COMETS. 191 



maintains that " all comets which are without a solid nu- 

 cleus (on account of their extremely small density) have no 

 solar heat, only the temperature of cosmical space. "^ If wo 

 take into consideration the numerous and striking analogies 

 of the phenomena which, according to Melloni and Forbes, 

 luminous and non-luminous sources of heat present, it ap- 

 pears difficult, in the present state of our physical reasoning, 

 not to assume that processes go on in the Sun itself which si- 

 multaneously produce radiant light and radiant heat by vi- 

 brations of the ether (waves of different lengths). The dark- 

 ening of the Moon by a comet, stated to have taken place in 

 the year 1454, which the Jesuit Pontanus, the first trans- 

 lator of the Byzantine author, George Phranza, beheved that 

 he had discovered in a monkish manuscript, has long been 

 mentioned in many astronomical works. This statement 

 of the passage of a comet between the Earth and Moon in 

 1454 is quite as erroneous as that asserted by Lichtenberg 

 of the Comet of 1770. The Chronicon of Phranza first ap- 

 peared complete at Vienna in 1796, and it is said there ex- 

 pressly, that in the year of the world 6962, while an eclipse 

 of the Moon took place, a comet like a Qiiist appeared and 

 came near to the Moon quite in the ordinary manner, ac- 

 cording to the order and circular orbits of the heavenly 

 luminaries. The year of the world ( = 1450) is incorrect, 

 as Phranza says distinctly the eclipse of the Moon and the 

 appearance of the comet were seen after the taking of Con- 

 stantinople (May the 19th, 1453), and an eclipse of the Moon 

 actually happened upon the 12th of May, 1454. (See Jacobs, 

 in Zach's Monatl. Corresp., bd. xxiii., 1811, p. 196-202.) 



The relation of Lexell's Comet to the satellites of Jupiter, 

 and the perturbation which it suffers from them without in 

 fluencing their periods of revolution [Cosmos, vol. i,, p. 110), 

 have been more accurately investigated by Leverrier. Mes- 

 sier discovered this remarkable comet as a feeble nebulous 

 spot in Sagittarius upon the 14th of June, 1770 ; but eight 

 days after, its nucleus shone as brightly as a star of the 2d 

 magnitude. Before the perihelion passage, no tail Avas vis- 

 ible ; afterward it developed itself by slight emanations 

 scarcely one degree in length. Lexell found for his comet 

 an elliptic orbit, and the period of rotation of 5* 585 years, 

 which Burckhardt confirmed in his excellent prize essay 

 According to Clausen, it had approached the Earth upon the 

 Ifit of July, 1770, to a distance of 363 times the Earth's ra- 

 * Cosmos, vol. iii., p. 36 and 37. 



