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the smallness of their mass and the eccentricity of their 

 orbital paths. It is in these ever present points of dissimi- 

 larity that I apprehend we shall find the cause of those 

 visible, those varying, and those incidental differences from 

 the planets, with which the term comet has become insepar- 

 ably associated. It has not been observed that the smallest 

 comets are most remarkable for their phenomena or their 

 aspects. On the contrary the larger bodies of the class have 

 always presented the most striking appearances, whence I 

 infer that though these appearances are beheld only in con- 

 nection with bodies of comparatively trivial mass, yet that 

 insignificance of mass is not the primary element in the 

 formation of the phenomena under consideration. The 

 eccentricity of their orbits however having been a noticeable 

 feature in connection with all the most remarkable comets, 

 it is in this particular and the circumstances which accom- 

 pany it, that I think the clue will be found to a solution of 

 the enigma of their aspects. The most obvious difference 

 from the planets which we might expect in the case of a 

 comet on account of the smallness of its mass would be the 

 feeble coercion of the elastic power of its gaseous parts and 

 the consequent voluminous development of its atmosphere, 

 whilst the eccentricity of its orbit would undoubtedly give 

 rise to enormous changes in temperature of the particles 

 composing it. It is in this extension of atmosphere and 

 in the suddenness and violence of these thermal changes 

 that I think it possible to find an explanation of almost 

 every one of those appearances which are peculiar to comets 

 as the ordinary and every day phenomena of their meteor- 

 ology. 



Suppose for instance we have a planetary body composed 



