Mr Milne's Prize Essay on Cornets. 345 



PART V Views respecting the system in general. — 1. 



Theories respecting the Origin of Planets and Coniets ; 2. An Ob- 

 jection to La Place's theory removed ; 3. Olber's theory as to the 

 Extent of the Planetary System, erroneous ; 4. The existence of an 

 Etijereal Medium proved by Comets ; 5. Comets indicate the uni- 

 versality of Gravitation ; 6. Conclusion." 



The extract is, Comets in various stages of Maturity. 



" 1. From a careful examination of those Comets whose motions are 

 exactly known, on their successive returns to the perihelion, much va- 

 luable information of a different nature may be obtained : For, if they 

 happen to have undergone any change in their physical constitution, 

 during the period of their absence, that change will probably be indica- 

 ted by a corresponding variation in their appearance. Since the ef- 

 fect of the solar power is so great (whatever be the mode of its opera- 

 tion) in pushing away the nebulous matter of the Comet, into the form 

 of a tail, it has been supposed that some of this nebulous matter may 

 even be altogether detached from the attraction of the nucleus, so as to 

 cause a gradual diminution in the Comet's substance ; and this effect, it 

 is obvious, will be the more easily produced, if the gravitation of the ne- 

 bulous particles to the nucleus be weakened by a rotatory motion of the 

 Comet. Now, an attentive examination of those Comets, whose ap- 

 proaches to the sun at the perihelion are near in respect of distance, and 

 frequent in respect of time, may enable us to judge whether or not this 

 supposition be well founded. But this is a point to which the attention 

 of astronomers has been too recently directed, to be yet very satisfacto- 

 rily fixed. Numerous data are requisite, which a constant and careful 

 observation can alone supply, before any decisive result can be obtained. 

 But certainly the observations of astronomers, as far as they have been 

 made, with regard both to the diminished size of the nucleus of all co- 

 mets after a perihelion passage, and the inferior brilliancy of Halley's in 

 particular, at its last appearance *, seems to confirm what other consi- 

 derations abundantly suggest, that a partial abstraction of nebulous mat- 

 ter does take place at every approximation of a comet to the sun. 



" The question, therefore, very naturally occurs, whether a Comet, 

 after a long succession of revolutions, will not be liable to become alto- 

 gether annihilated by this dispersion of its nebulous matter ? HerscbeKs 

 opinion respecting the constitution and formation of comets, here de- 

 serves our attention, as it satisfactorily resolves the difficulty which is 



" Brande'8 Astron. ii 6& 



