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 we 

 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, believed 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 mist 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 1 2th of May, 1 454. (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 was 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 

 1st of July, 1770, to a distance of 363 times the Earth's ra- 

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



