106 COSMOS. 



explained by tlie position of their orbits and their distance from 

 the Sun. It would seem to indicate, in some individuals, the 

 existence of an inherent process of condensation, and an in- 

 creased or diminished capacity of reflecting borrowed light. 

 In the comet of 1618, and in that which has a period of three 

 years, it was observed first by Hevelius that the nucleus of 

 the comet diminished at its perihelion and enlarged at its 

 aphelion, a fact which, after remaining long unheeded, was 

 again noticed by the talented astronomer Valz at Nismes. 

 The regularity of the change of volume, according to the dif- 

 ferent degrees of distance from the Sun, appears very striking. 

 The physical explanation of the phenomenon can not, howev- 

 er, be sought in the condensed layers of cosmical vapor occur- 

 ring in the vicinity of the Sun, since it is difficult to imagine 

 the nebulous envelope of the nucleus of the comet to be vesic- 

 ular and impervious to the ether.* 



The dissimilar eccentricity of the orbits of comets has, in 

 recent times (1819), in the most brilliant manner enriched our 

 knowledge of" the solar system. Encke has discovered the ex- 

 istence of a comet of so short a period of revolution that it re- 

 mains entirely within the limits of our planetary system, at- 

 taining its aphelion between the orbits of the smaller planets 

 and that of Jupiter. Its eccentricity must be assumed at 0*845, 

 that of Juno (which has the greatest eccentricity of any of the 

 planets) being 0*255. Encke's comet has several times, al- 

 though with difficulty, been observed by the naked eye, as in 

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

 in 1822. Its period of revolution is about S^d years; but, 

 from a careful comparison of the epochs of its return to its 

 perihelion, the remarkable fact has been discovered that these 

 periods have diminished in the most regular manner between 

 the years 1786 and 1838, the diminution amounting, in the 

 course of 52 years, to about lyVl^h. days. The attempt to 

 brmg into unison the results of observation and calculation in 

 the investigation of all the planetary disturbances, with the 

 view of explaining this phenomenon, has led to the adoption 

 of the very probable hypothesis that there exists dispersed in 

 space a vaporous substance capable of acting as a resisting 

 medium. This matter diminishes the tangential force, and 

 with it the major axis of the comet's orbit. The value of the 

 constant of the resistance appears to be somewhat different 

 before and after the perihelion ; and this may, perhaps, be as- 



* Arago, in the Anniiaire, 1832, p. 217-220. Sir John Herschel, 

 Asiron., § 488. 



