

PHYSICAL CONSTITUTION OF COMETS. 



491 



On the 1st of April, 1796, Dr. Olbers, at Bremen, saw a star of the sixth or 

 seventh magnitude, and although it was covered by a comet, he found that its 

 light was not perceptibly diminished. The observer in this case did not feel 

 sure that the nucleus was between the eye and the star. 



MESSIER, when observing a comet in 1774, saw a small telescopic star be- 

 side it, and having looked at it again after the lapse of some hours, he ob- 

 served a second star near the first. He explained this by the supposition that 

 at the moment of his first observation the nucleus of the comet concealed the 

 second star. 



WARTMANN states that on the night of the 28th November, 1828, a star of 

 the 8th magnitude was completely eclipsed by Encke's comet. Here again, 

 however, it is objected that Wartmann's telescope was too feeble to be trusted 

 in such an observation. 



In the absence 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 splendor which has attended the appearance of 

 some comets. A mere mass of vapor 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 bv 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 Venus was not in 

 a position to be visible, and consequently it is inferred that this star must have 

 been a comet. 



The comet of 1577 was discovered on the 13th of November by Tycho Bra- 

 che, 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 splendor 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 within the nebulosity of comets. The most 

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

 ability, but no positive certainty, that comets have visited our system which 

 have solid nuclei, but, meanwhile, this can only be maintained with respect to 

 few; most of those which have been seen, and all to whictf very accurate ob- 

 servations have been directed, have afforded evidence of being mere masses of 

 semi-transparent vapor. 



THE TAIL. 



Although by far the great majority of comets are not attended by tails, yel 

 that appendage, in the popular mind, is more inseparable from the idea of a 

 comet than any other attribute of these bodies. This circumstance probably 

 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 occa- 

 sion of one of the visits of Halley's comet to the solar system, Pierre Apian 

 observed that the comet generally presented its tail in the direction from the 



