98 CELESTIAL PHENOMENA. 



their distance from the sun. In particular individuals, it 

 certainly indicates internal processes of condensation, and 

 increased or diminished capability of reflecting light. In 

 the comet of 1618, as in that which has a period 

 of revolution of three years, Hevelius saw the nucleus 

 lessen at the perihelion, and enlarge at the aphelion of the 

 comet : this remarkable phsenomenon, which had long re- 

 mained unheeded, has since been observed by the distin- 

 guished astronomer, Valz, at Nismes. The regularity of the 

 alteration of the volume according to the distance from the 

 sun, appeared exceedingly striking ; the physical explanation 

 of the phenomenon cannot well be sought in the greater 

 condensation of the cosmical ether in the vicinity of the 

 sun, for it is difficult to imagine the nebulous envelope of 

 the nucleus of the comet to be, like a vesicle, impervious to 

 the ether ( 52 ). 



The very dissimilar excentricities of the elliptical paths of 

 different comets, has led in modern times (1819) to a bril- 

 liant accession to our knowledge of the solar system. Encke 

 has discovered the existence of a comet having so short a 

 period of revolution, that it always remains within our plane- 

 tary system, and even reaches its aphelion, or greatest dis- 

 tance from the sun, between the orbits of Jupiter and of the 

 small planets. The excentricity of its orbit is 0'845 ; that 

 of Juno, which has the greatest excentricity amongst the 

 planets, being 0'255. This comet has been more than once 

 seen, though with difficulty, by the naked eye ; in Europe, 

 in 1819, and, according to Riimker, in New Holland, in 

 1 822. Its period of revolution is about 3'3 years ; but from 

 the most careful comparison of the epochs of its return to 

 its perihelion, the remarkable fact has been discovered 



