NUMBER OP COMETS. 



the universe which extend to enormous distances beyond its 

 limits, and after passing among the planets and approaching more 

 or less near to the sun, they again disappear, issuing to distances 

 not less remote. 



The number of those which have been actually seen, and whose 

 appearances have been recorded, amounts to many hundreds. 

 But when the chances against these bodies being visible during 

 the intervals, often very brief, of their passage through the solar 

 system, the vast numbers of them which can only be seen by the 

 aid of telescopes, the frequency of their position being such that 

 they are only above the horizon of observers during the day, or 

 that they can only be within the range of vision in latitudes 

 where no observers are found, are severally considered, it will be 

 evident that the number of comets actually seen must form a very 

 small fraction of the total number which have visited our 

 system. 



Reasoning upon the common principles of the doctrine of pro- 

 babilities, Arago has shown that the number of comets which 

 have passed through the system cannot be less than three and 

 a half millions, but that it is possible that they may amount to 

 twice that number. Even with the limited information respecting 

 these bodies, which was attainable by Kepler, that astronomer 

 declared that " there are more comets in space than fishes in the 

 ocean." 



Of the many hundreds whose appearances have been recorded, 

 dating from the earliest historical notices of these bodies, about 

 two hundred have been observed during the short intervals of 

 their appearance, with sufficient precision to enable astronomers to 

 calculate the paths or orbits in which they moved. These calcu- 

 lations have led to a result of the highest importance, inasmuch as 

 they have established demonstratively the fact that these comets 

 are masses of ponderable matter. The forms of their orbits prove 

 this. It has been shown by Newton that if a body move in a 

 certain form of curve, called by geometers a conic section, having 

 a point called its focus at the centre of the sun, it must be subject 

 to the attraction of the sun's gravitation, and it must reciprocally 

 attract the sun. Is^ow these comets have been ascertained by 

 observation to move in these very curves, the sun being in their 

 common focus. Hence they and the sun mutually attract each 

 other, according to the universal law of gravitation. They are, 

 therefore, masses of ponderable matter. 



But these masses are not only attracted by the sun but by the 

 planets, primary and secondary, near to which they pass, and they 

 are ascertained to deviate considerably, by reason of such attrac- 

 tions, from the paths they would follow if subject only to the sun's 



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