92 CELESTIAL PHENOMENA. 





possess as many as one hundred and fifty calculated paths of 

 comets ; but we have notices more or less precise of the 

 appearance of six or seven hundred of such bodies, and of 

 their passage through known constellations. Whilst the 

 classic nations of the West, the Greeks and the Bomans, do, 

 indeed, sometimes mention the place in the heavens where a 

 comet was first seen, but never afford us information respect- 

 ing its apparent path, the literature of the Chinese (who 

 observed nature diligently, and carefully recorded everything), 

 supplies us with circumstantial notices of the comets seen 

 by them, and of the constellations which each passed 

 through. These notices extend back to more than five 

 centuries before the Christian era, and many of them are 

 still found useful in astronomy ( 42 ). Of all planetary bodies, 

 comets, though their mean mass is probably much less than 

 the five-thousandth part of that of the earth, are those which 

 occupy the greatest space, their wide-spreading tails often 

 extending over many millions of miles. The cone of 

 light-reflecting gaseous matter which radiates from them has 

 been found in some instances, as in 1680 and 1811, to equal in 

 length the distance of the earth from the sun, or that of a line 

 including the orbits of the two planets, Yenus and Mercury. 

 It is even probable that the emanations from the comets of 

 1811 and 1823 mixed with our atmosphere. 



Comets show such diversities of form, diversities belong- 

 ing rather to the individual than to the class, that the 

 description given of one of these " wandering light-clouds" 

 (as they were called by Xenophanes, and Theon of Alex- 

 andria, contemporaries of Pappus), can only be applied with 

 much caution to another. The fainter telescopic comets are 

 f<or the most part without tails, and resemble Herscher* 



