392 HALLEY'S COMET. [SECT, xxxvi. 



the sun's rays, it was between thirty and forty degrees in 

 length. 



According to the observations of M. Valz, of Nismes, the 

 nebulosity increased in magnitude as it approached the sun ; 

 but no other comet on record has exhibited such sudden and 

 unaccountable changes of aspect. The nucleus, clear and 

 well denned, like the disc of a planet, was observed on one 

 occasion to become obscure and enlarged in the course of a 

 few hours. But by far the most remarkable circumstance 

 was the sudden appearance of certain luminous brushes or 

 sectors, diverging from the centre of the nucleus through the 

 nebulosity. M. Struve describes the nucleus of the comet, 

 in the beginning of October, as elliptical, and like a burning 

 coal, out of which there issued, in a direction nearly opposite 

 to the tail, a divergent flame, varying in intensity, form, and 

 direction, appearing occasionally even double, and suggesting 

 the idea of luminous gas bursting from the nucleus. On one 

 occasion M. Arago saw three of these divergent flames on 

 the side opposite the tail, rising through the nebulosity, which 

 they greatly exceeded in brilliancy : after the comet had 

 passed its perihelion, it acquired another of these luminous 

 fans, which was observed by Sir John Herschel at the Cape 

 of Good Hope. Hevelius describes an appearance precisely 

 similar, which he had witnessed in this comet at its approach 

 to the sun in the year 1682, and something of the kind seems 

 to have been noticed in the comet of 1744. Possibly the 

 second tail of the comet of 1724, which was directed towards 

 the sun, may have been of this nature. 



The influence of the ethereal medium on the motions of 

 Halley's comet will be known after another revolution, and 

 future astronomers will learn, by the accuracy of its returns, 

 whether it has met with any unknown cause of disturbance 

 in its distant journey. Undiscovered planets, beyond the 

 visible boundary of our system, may change its path and the 

 period of its revolution, and thus may indirectly reveal to us 

 their existence, and even their physical nature and orbit. The 



