646 Mr Alilne's Prize Essay mi Cornets. 



now proposed. There is no individual perhaps in the annals of astrono- 

 my who has contributed more to our knowledge of the heavens than Sir 

 William Hei-schel, both by extending the limits of our vision into the 

 most distant parts of the universe, and by investigating the laws which 

 govern the more complicated phenomena of nature. But of all his con- 

 tributions to the science, none are so important in themselves, or so well 

 calculated to disclose to us the secret and marvellous operations going 

 on in the workshop of Nature, as the discoveries which he has made 

 concerning nebulae. These nebulas, it is supposed, are formed by the 

 partial condensation of matter, probably the etherial medium itself dif- 

 fused throughout the universe ; and that their number must be prodi- 

 gious, is sufficiently proved by the fact, that Herschel, by his ow« ef- 

 forts alone, discovered 2000 of them. Some of the nebulae are found 

 to have so strong a resemblance to many comets, which, on account of 

 their distance from the sun, can just be discerned from the earth, that 

 they are not unfrequently confounded * ; and it is only by a nearer ap- 

 proach, or by an intimate acquaintance with all the nebulae in the same 

 quarter of the heavens, that astronomers are able to distinguish them. 

 Now, it is the opinion of Herschel, and his opinion is strongly sup- 

 ported by the authority of La Place f, that Comets are originally minute 

 nebulae, which, by the continual approximation of their particles, have 

 at length acquired such a degree of density, as to be capable of being 

 attracted by the sun, and of describing an orbit of their own. As the 

 nebulous mass approaches the sun, one result, as we have seen, is the 

 expansion of its parts, and their prolongation into what has been termed 

 the Tail : But, another result, according to Herschel, and one no less 

 important, is a gradual consolidation of the nebulous matter by the agency 

 of the solar heat, " It is admitted on all hands," says he, " that the act of 

 " shining denotes a decomposition, in which at least light is given out; 

 but that many other elastic volatile substances escape at the same time, 

 especially in so high a degree of rarefaction, is far from improbable. 

 Since light then, certainly, and very likely other subtile fluids also, 

 escape in great abundance during a considerable time before and after a 

 comet's nearest approach to the sun, I look," says Herschel, " upon a 

 perihelion passage in some degree as an act of consolidation J." 



• " By the gradual increase of the distance of our Comet," says Herschel, 

 speaking of the Comet of 1807, " we have seen that it assumed the semblance 

 of a nebula ; and it is certain, that had I met with it in one of my sweeps of 

 the zones of the heavens, as it appeared on either of the days between the 

 6th December and the 21st February, it would have been put down in the 

 list I have given of nebulje." 



t Connoissance des Teriis, 1816. % Phil. Trans. 1812-14. 



