PHYSICAL CONSTITUTION OF COMETS. 



satisfactory examples in the more rare instances of those which 

 have distinct nuclei. 



In the ahsence of a more decisive test of the occultation of a 

 star by the nucleus, it has been maintained that the existence of 

 a solid nucleus may be fairly inferred from the great splendour 

 which has attended the appearance of some comets. A mere mass 

 of vapour could not, it is contended, reflect such brilliant light. 

 The following are the examples adduced by Arago : 



" In the year 43 before Christ, a comet appeared which was 

 said to be visible to the naked eye by daylight. It was the comet 

 which the Romans considered to be the soul of Caesar transferred 

 to the heavens after his assassination. 



" In the year 1402 two remarkable comets were recorded. The 

 first was so brilliant that the light of the sun at noon, at the end 

 of March, did not prevent its nucleus, or even its tail, from being 

 seen. The second appeared in the month of June, and was visible 

 also for a considerable time before sunset. 



" In the year 1532, the people of Milan were alarmed by the 

 appearance of a star which was visible in the broad daylight. At 

 that time Yenus was not in a position to be visible, and conse- 

 quently it is inferred that this star must have been a comet. 



"The comet of 1577 was discovered on the 13th of November 

 by Tycho Brahe, from his observatory on the isle of Huene, in the 

 Sound, before sunset. 



" On the 1st of February, 1744, Chizeaux observed a comet 

 more brilliant than the brightest star in the heavens, which soon 

 became equal in splendour to Jupiter, and in the beginning of 

 March it was visible in the presence of the sun. By selecting a 

 proper position for observation, on the 1st of March it was seen at 

 one o'clock in the afternoon without a telescope." 



Such is the amount of evidence which observation has supplied 

 respecting the existence of a solid nucleus. The most that can 

 be said of it is, that it presents a plausible argument, giving some 

 probability, but no positive certainty, that comets have visited 

 our system which have solid nuclei, but, meanwhile, this can 

 only be maintained with respect to a few : most of those which 

 have been seen, and all to which very accurate observations have 

 been directed, have afforded evidence of being mere masses of 

 semi-transparent matter. 



82. Although by far the great majority of comets are not 

 attended by tails, yet that appendage, in the popular mind, is 

 more inseparable from the idea of a comet than any other attribute 

 of these bodies. This proceeds from its singular and striking 

 appearance, and from the fact that most comets visible to the 

 naked eye have had tails. In the year 1531, on the occasion of 



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