COMETS. ] 



ASTRONOMY. 



961 



passage ; and 5th, the perihelion distance. The observa- 

 tions will also give the direction of motion. Dr. Halley 

 alone, acting according to the method of Sir Isaac New- 

 ton, calculated twenty-four orbits, by a comparison of 

 which he was led to iuferthe periodic nature of the body 

 which bears his name ; and its certainty has been con- 

 firmed by the well-observed and authentic observations 

 of successive returns. 



Fig. 108. 



on March ]2th, 1759, and the comet was seen for the first 

 time at Dresden, by Pazlituh, on Christmas Day, 1758. 

 This body being thus ranked in the planetary system, the 

 investigation of the period of its next arrival at the peri- 

 helion was made the subject of a prize in several Euro- 

 pean academies. The investigations of Damoisean and 

 Pentecoulant, on this question, gave only the difference 

 of one week in the predicted time of the arrival at the 



Fig. 10Q. Fig. 110. 



Modern astronomers have, with great zeal, applied the 

 methods of Olbers and Gauss for this purpose, not only 

 to those comets which have appeared in their own times, 

 but also to others, whose paths in the heavens have been 

 led by the ancient Chinese and European annalists. 

 M. Langier has thus calculated, with a strong degree of 

 probability, that the comets of 451 and 760, mentioned 

 by the Chinese annalists, were returns of Halley's comet. 

 Mr. Hind has also been led, by calculation, to consider 

 the comets nl 218 and 295 as appearances of the same 

 body. But no certainty can be attached to any elements 

 of this comet till the year 1378, the observations of which 

 FiR. 111. Fig. 



perihelion an agreement very accordant, when the great 

 intricacy of the problem is considered. The predicted 

 return was stated to be on November 13th, 1835 ; and 

 although the comet was attentively looked for during 

 several months previously, it was not seen till August, 

 when it was discovered at the Collegio Romano, of Rome. 

 From the best elements, the return to perihelion took 

 place on November 16th, the predicted time being only 

 three days in error. 



With regard to the physical appearances of these 

 bodies, it is generally admitted, at present, that Cumets 

 possess a very small degree of density. 

 Ill- Fig. 118. 



have been discussed by M. Langier, and of which the 

 resulting elements are given with the others in the fol- 

 lowing table : 



Lnnit. of 

 Perih. 



Loti i. of 

 Node. 



1378, Nov. 8-764. 



1456, June, 8-917. 



1531, Aug. 24-H88. 



1607, Oct. 20-7I6. 



1682, .Srpt. 14-795. 



1759, March, 12-552. 



1835, NOT. 15 939. 



..299" 31'.. .47" 17'. 

 .301" O'...4830'. 

 .301" 3y...49 25'. 

 .301" 38'.. 48 40'. 

 .301 56'.. .bV IT. 

 ..303" 10'.. .53 50'. 

 .304" 32'.. .55" 10'. 



Inclina- 

 tion. 



.17 56' . 

 .17 5fi' . 

 .17 56' . 

 .17 12' . 

 ,.17" 44' . 

 .17 37' . 

 .17 45' . 



Lontr. Perih. 

 Dist. 



. 9-75CO R. 

 . 9 7675 R. 

 . 9-7538 R. 

 . 9 71/JJ R 

 . 9 7656 R. 

 . 9-7068 R. 

 . 9-7683 R. 



The Rtriking similarity of the elements of the comets 

 of 1531, 1607, and 1682, immediately presented itself to 

 the mind of Dr. Halley, who was also led to infer that, 

 by the influence of the planet Jupiter, the period of its 

 next return would be considerably retarded. This pre- 

 diction was found to be accurate by the laborious calcu- 

 lations of Clairant, who, after minutely computing the 

 disturbing effect of Jupiter and Saturn, found that its 

 next return would be delayed by 618 days, as he con- 

 cluded that the comet would arrive at its perihelion on 

 April 13th, 1759. 



The actual return, from the best elements, took place 



VOL. I. 



The above diagram (Fig. 308) will show the ap- 

 pearance of Halley's comot, as observed by Sir J. Her- 

 Fig. 114. schel, at the Cape of 



of Good Hope, on Oc- 

 tober 28, 1835. The 

 bright spot situated 

 in the head of the 

 comet, is called the 

 nucleus, and the lumi- 

 nous appendage which 

 accompanies and sur- 

 rounds this nucleus, 

 is called the tail. On 

 October 29, Sir J.Her- 

 schel observed the 

 comet with a twenty- 

 foot telescope, and 

 noted its appearance, 

 as in Fig. 109. A 

 little later, on the same evening, th-3 appearance was in 

 Fig. 110. On January 25, 1830, it had an appearance as 

 in Fig. Ill ; in the intermediate time the comet was not 

 visible, in consequence of its proximity to the solar rays. 

 On January 20, 27, and 28, 1836, it presented the appear- 

 ances indicated in the Figures 112, 113, 114. 



60 



