COMETS. 193 



of Encke's Comet, or they mix with the old cosmical matter 

 which has not aggregated into spheres, or condensed into the 

 rings, and which appears to us as the zodiacal light. We 

 see, as it were, before our eyes, material particles disappear, 

 and can scarcely conjecture where they again collect. How- 

 ever probable may be the condensation, in the neighborhood 

 of the central body of our system, of a gaseous fluid filling 

 space, still, in the case of the comets, whose nuclei, accord- 

 ing toValz, diminish in the perihelion, this fluid, condensed 

 there, can scarcely be looked upon as pressing upon a vesicu- 

 lar vapory envelope.* Although in the streamers of the 

 comets the outlines of the reflecting portion of the vapory 

 envelope is generally very indefinite, the circumstance that, 

 in some individuals (for example, Halley's Comet at the 2d 

 of January, 1836, at the Cape of Good Hope), a sharpness 

 of outline has been observed on the anterior parabolic 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 in- 

 tensity of the mutual attraction of the particles, with that of 

 an alabaster vessel strongly illuminated in the interior.! 



Since the appearance of the astronomical part of my De- 

 lineation 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 

 short period, 6| years in its revolution, has separated into 

 two comets of similar figure though unequal dimensions, both 

 having a head and tail. So long as they could be observed, 

 they did not unite again, and proceeded on their course sep- 

 arately, almost parallel with each other. Hind had, on the 



* Valz, Esstxi sur la Determination de la Densite de V Ether dans I'espace 

 PlanUaire, 1830, p. 2; and Cosmos, vol. i., p. 106. The so-carefully 

 observing and always unprejudiced Hevelius had also directed atten- 

 tion to the increase in the size of the cometary nuclei, with increased 

 distance from the Sun. (Pingre, Comilographie, 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 center, or one point of it, is the brightest, even prominently 

 bright. From this point, which, however, presents no appearance of a 

 disk, and can not be called a comet-head, the light decreases very rapid- 

 ly all around, and at the same time the vapor elongates toward one disk, 

 so that this elongation appears as a tail. The measurements, therefore, 

 refer to this mass of vapor, whose circumference, without having really 

 definite boundaries, decreases in perihelion. 



\ Sir John Herschel, Results of Astronomical Observations at the Cap*- 

 of Good Hope, 1847, $ 366, pi. XV. and xvi. 



Vol. IV— I 



