226 Royal Astronomical Society. 



of view. Now, it is precisely on comets or nebulae, whose condensa- 

 tion towards the centre is feeble and which (like the comets of Encke 

 and Biela,) have no nuclei, that this action is most powerful, and its 

 effect most extensive. I would not, however, be understood by any 

 means to deny the reality of a more rapid law of variation than that 

 which results from the foregoing consideration. If it subsist, of course 

 we must look out for another explanation. At all events, the con- 

 clusion we have arrived at, regarded as a geometrical result, is not 

 devoid of a certain degree of interest j and the cause I have assigned, 

 having the character of a vera causa, cannot be excluded from some 

 share in the production of the observed effect, even should it not be 

 found sufficient to account for the whole. 



" There is, however, another way in which the apparent dimen- 

 sions of a comet may be conceived to vary with its proximity to the 

 sun, while its real volume may remain unaltered, or even undergo a 

 contrary change. The nebulous portion of the comet, or that which 

 reflects the sun's rays, is not improbably of the nature of a fog, i. e. 

 a collection of discrete particles of a vaporizable fluid floating in a 

 transparent medium. Now, as these molecules, during the comet's 

 approach to the sun, absorb its rays and become heated, a portion 

 of them will be constantly passing from the liquid and visible to the 

 gaseous and invisible state. As this change must commence from 

 without and be propagated inwards, the effect will be a diminution 

 of the comet's visible bulk. On the other hand, in its recess from 

 the sun, it will part with the heat thus acquired by radiation, which, 

 in conformity with the general analogy of radiant caloric, will take 

 place chiefly from the unevaporated or nebulous mass within, whose 

 dimensions will therefore begin and continue to increase by the pre- 

 cipitation immediately above it of fresh nebulous matter, just as we 

 see fogs in cold still nights forming on the surface of the ground, and 

 gradually extending upwards as the heat near the surface is dissi- 

 pated. The comet will thus appear to enlarge rapidly in its visible 

 dimensions, while the real volume is in fact slowly shrinking by the 

 general abstraction of heat from the mass. 



"This process might goon in the entire absence of any solid or fluid 

 nucleus ; but supposing such a nucleus to exist, and to have ac- 

 quired a considerable increase of temperature in the vicinity of the 

 sun, evaporation from its surface would afford a constant and co- 

 pious supply of vapour, which, rising into its atmosphere and con- 

 densing at its exterior parts, would tend yet more to dilate the visible 

 limits of the nebula. Some such process would naturally enough 

 account for the appearances which have been noticed in the heads of 

 certain comets, where the stratum, void of nebula, has been observed 

 interposed, as it were, between the denser portion of the head or nu- 

 cleus and the coma*. It is analogous to the meteorological pheno- 

 menon of a definite vapour plane, so commonly observed ; and, in 

 certain cases, may admit of two or more alternations of nebula and 

 clear atmosphere. 



* See my father's observations on the comet of 181 1. 



