COMETS. 547 



maintains that " all comets which are without a solid nucleus 

 (on account of their extremely small density), have no solar 

 heat, only the temperature of cosmical space." 1 * 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 

 simultaneously produce radiant light and radiant heat by 

 vibrations of the ether (waves of different lengths). The 

 darkening of the Moon by a comet, stated to have taken 

 place in the year 1454, which the Jesuit Pontanus, the first 

 translator of the Byzantine author, George Phranza, believed 

 that he had discovered in a monkish manuscript, has long 

 been mentioned in many astronomical works. This state- 

 ment 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 

 expressly, 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 tlie Moan quite in the ordinary manner, according 

 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 Constantinople (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, pp. 196-202.) 



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

 and the perturbation which it suffers from them without influ- 

 encing their periods of revolution (Cosmos, vol. i. p. 96), have 



" Cosmos, vol. iii. pp. 42 and 45. 



VOL. IV. S 



