5,50 COSMOS. 



densed there, can scarcely be looked upon as pressing upon 

 a vesicular vapoury envelope. ** Although in the streamers of 

 the comets the outlines of the reflecting portion of the 

 vapoury envelope is generally very indefinite, the circum- 

 stance that in some individuals (for example, Halley's Comet 

 at the 2nd of January, 1836, at the Cape of Good Hope), a 

 sharpness of outline has been observed on the anterior para- 

 bolic part of the body, such as our masses of clouds seldom 

 present, is so much the more striking and instructive as to the 

 molecular condition of these bodies. The celebrated observer 

 at the Cape compared the unusual appearance, testifying to 

 the intensity of the mutual attraction of the particles, with 

 that of an alabaster vessel strongly illuminated in the in- 

 terior. 84 



Since the appearance of the astronomical part of my 

 Delineation of Nature, the cometary world has presented a 

 phenomenon whose mere possibility could scarcely have been 

 suspected beforehand. Biela's Comet, an interior one of 



18 Valz, Essai sur la determination de la densite de V ether 

 dans V espace planetaire, 1830, p. 2 ; and Cosmos, vol. i. p. 91. 

 The so-carefully observing and always unprejudiced Hevelius 

 had also directed attention to the increase in the size of the 

 cometary nuclei, with increased distance from the Sun. (Pingre, 

 CometoqrapJiie, torn. ii. p. 193.) The determinations of the 

 diameter of Encke's Comet in the perihelion is very difficult, 

 if accuracy is desired. The comet is a nebulous mass, in 

 which the centre, or one point of it, is the brightest, even 

 prominently bright. From this point, which however presents 

 no appearance of a disc, and cannot be called a comet-head, 

 the light decreases very rapidly all around, and at the same 

 time the vapour elongates towards one disc, so that this 

 elongation appears as a tail. The measurements, therefore, 

 refer to this mass of vapour, whose circumference, without 

 having really definite boundaries, decreases in perihelion. 



84 Sir John Herschel, Results of Astronomical Observation* 

 at the Cape of Good Hope, 1847,' 366, pi. *v. 



