Mr Milne's Prize Essay on Cornets. 347 



" II. This process of consolidation will evidently be the more power- 

 ful, the more that the Comet is subjected to the sun's calorific action ; 

 a condition which depends upon two circumstances ; one, the perihelion 

 distance of the Comet, the otlier, the time in which it completes its re- 

 volution. It follows from this consideration, that we may be able even 

 to estimate the degree of solidity wliich Comets have attained, simply 

 by taking into account these two circum8tanc€s ; and a reference to ob- 

 servation will at once shew whether or not the theory be correct. But 

 before attempting to apply this test, one remark must be made, which 

 shews that the application of it may not in all cases be conclusive. If 

 all Comets during their successive revolutions round the sun, were to 

 remain totally exempt from the possibility of receiving any accession of 

 foreign matter, tending to enlarge tlieir bulk, then we might expect that 

 the consideration of their perihelion distance and their period of revolu- 

 tion should always conespond with the amount of their solidity, or, in 

 other words, the actual size of their nucleus. But if we suppose with 

 Herschel, La Place, and other eminent astronomers, that there exist 

 multitudes of nebulae throughout space in every different stage of matu- 

 lity, from those whose formation has just commenced, to those whose 

 condensation by the attraction of the particles has already so far ad- 

 vanced, as will soon render them capable of gravitating towards the sun, 

 we must reckon it not impossible that Comets, in the extensive range of 

 their orbits, may occasionally meet with some of these nebulse, and thus 

 cany with them a new supply of unpenhelioned matter in their next 

 approach to the centre of the system. In this manner, the loss of sub- 

 stance to which, as we have above remarked, comets are exposed, bv vo- 

 latilization, may possibly be restored ; while, in process of time, they may 

 acquire a magnitude and solidity considerably surpassing what could have 

 arisen from the primitive quantity of their nebulous matter. Ceitainly 

 we are not at liberty to suppose, that this fortuitous junction of a comet 

 with nei>ulae takes place frequently ; but, in estimating the consolidation 

 of different Comets, in order to find whether the result corresponds with 

 what the frequency and nearness of their approach to the sun would 

 lead us to expect, we ought to recollect that the test is not infallible, 

 from the possibility of an accession of nebulous matter having occurred 

 in the manner we have now described. 



" Hei'schels tiieory, with respect to the agency of the solar beat, in 

 promoting the consolidation of comets, necessarily implies, that the en- 

 relope and tail gradually become less extensive, and that the nucleus, 

 upon whose surface the nebulous matter consolidates, gradually in- 

 creases in magnitude. In these respects, therefore, some difference 

 ought to be indicated by the physical appearance of those comets whose 



