NOMOS. 149 



In order to understand the nature of the extra- 

 ordinary and rapid changes in form which are so 

 very distinctive of comets, it is necessary to call to 

 remembrance the amazing size and rarity 

 of cometary bodies. Comets, then, are ^ e * d 

 by far the most voluminous as well as the 

 lightest of all bodies with which we have 

 any acquaintance. The head of the great comet, of 

 1811 was considerably more than a million miles in 

 diameter, and the greatest length of the tail was no 

 less than 130,000,000 miles. Usually, however, the 

 diameter of the head is about 100,000 miles, and the 

 length of tail is far less than that specified. But the 

 principal peculiarity of comets is less in their enor- 

 mous volume than in their astonishing rarity. They 

 are little more than shadows of substance, for stars 

 of the smallest magnitude remain distinctly visible 

 even when covered by their densest portions. The 

 light which would not pass through the most filmy 

 cloud passes through them without any sensible 

 obscuration. " The most unsubstantial clouds which 

 float in the highest regions of the atmosphere, and 

 seem at sunset to be drenched in light, and to glow 

 throughout their whole depth as if in actual ignition, 

 without any shadow or dark side, must be looked 

 upon as dense and massive bodies when compared 

 with the filmy and all but spiritual texture of a 

 comet."* A similar conclusion arises also out of 

 the infinitely small influence which cometary bodies 

 are found to exert upon the movements of the more 

 orderly denizens of the solar system. The comets 



* Herschel's Outlines, p. 344. 

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