58 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



astrono- 

 mers. 



Its pertur- His successors have distinguished themselves chiefly 

 bations jj v fa Q praiseworthy minuteness and extent of their 

 calculations, which are amongst the most laborious, 

 if not indeed the most laborious, which occur in Phy- 

 sical Astronomy. The computers who calculated 

 the return of Halley's Comet in 1835 were four in 

 number, MM. Damoiseau, Pontecoulant, Lehmann, 

 and Rosenberger. Their memoirs are all considered 

 by competent judges to be excellent, but especially 

 that of Rosenberger, who calculated more fully than 

 the others the perturbations from 1682 to 1759, and 

 who has introduced a theoretical correction of some 

 importance. Some idea of the extent of these calcu- 

 lations may be formed from the fact, that in some 

 parts of the orbit the Elements were made to vary 

 for intervals of only two days. The Comet of Halley 

 was rediscovered at Rome on the 6th August 1835, 

 in the Jesuits' College. The error of Rosenber- 

 ger's Ephemeris was only seven minutes of arc, and 

 the perihelion passage took place on the 16th No- 

 vember (civil reckoning), five days after the predicted 

 time. Bessel states the remarkable fact, that the coin- 

 cidence of the comet's path with the results of previous 

 calculation is as close as the use of five-place loga- 

 rithms in computing the perturbations would permit. 

 All this is very creditable to the state of Astronomy ; 

 still it is infinitely less remarkable than Clairaut's 

 approximation, only a little less close, made 77 years 

 before. It is a matter of regret that neither in the 

 matter of this comet, nor in any other point of theory 

 connected with Cometary movements, have our coun- 

 trymen made any considerable advance since the time 

 of Halley. 



At its return in 1835, this Comet was watched 

 until May 1836, and in the course of its long visibi- 

 lity was made the subject of minute and admirable 

 telescopic researches, by Bessel in Europe, and by 

 Sir John Herschel at the Cape of Good Hope. Their 

 observations strictly confirmed a remark of M. Valz, 

 that the Tails of Comets, though evidently generated 

 under the influence of the. neighbourhood of the Sun, 

 yet commonly disappear at the period of closest ap- 

 proach. This, at least, was the case with Halley's 

 Comet. The tail began to grow on the 2d October, 

 two months after its discovery, and diminished after 

 the 15th (a month before perihelion). The comet re- 

 appeared in the end of January without any vestige of 

 a tail, and then dilated in absolute bulk with in- 

 credible quickness. But before its final disappear- 

 ance, on the 5th May, the tail is supposed to have 

 Its peculiar been reabsorbed into the nucleus, which presented 

 then a uniform circular outline. The evolution of 

 the highly-expansible luminous matter of the tail 

 todk place from the nucleus in luminous fan-like 

 jets, directed on the whole towards the Sun, and 

 issuing on the side of the Comet exposed to that 

 body. The jets had a vibratory motion. The tail 



(265.) 

 Its return 

 in 1835. 



features. 



(directed from the Sun) was formed by the abrupt 

 inflexion of these effluent jets, in a manner resem- 

 bling electrical repulsion; and Sir John Herschel has 

 not hesitated to ascribe the phenomenon to the com- 

 bination of a repulsive force as respects the Sun, 

 (" of an energy very far exceeding the gravitating force 

 towards that luminary") with " a peculiar and highly 

 energetic attraction to the nucleus, differing from, 

 and exceptional to, the ordinary power of gravita- 

 tion." 1 The opinion that polar forces resembling 

 magnetism or electricity are necessary to explain the 

 phenomena of Comets, has been for some time cur- 

 rent in Germany, and received, I believe, the coun- 

 tenance of Bessel. That there exist in the movements 

 of these bodies anomalies so great as to render the 

 sufficiency and completeness of the Theory of Gravity 

 suspected by such high authorities, makes the state- 

 ment of them very important, though I confess a 

 great reluctance to share in the conclusion. 



I shall terminate this notice of Halley's Comet ( 266 . 

 with a table of the dates of its probable perihelion re * ur e n 

 passages, according to M. Laugier and Mr Hind. 2 

 The earlier appearances are deduced from the Chinese 

 Annals : 



The differences in the above periods are attributed to 

 the effects of perturbation. The extreme distance of 

 the Comet's path from the Sun is 35 times the ra- 

 dius of the Earth's orbit, which is only one-sixth part 

 greater than the distance of Neptune, and therefore 

 nearly within the recognized limits of the planetary 

 system. 



Comet of Encke. Next in interest to the Comet 

 of Halley is that discovered by Pons of Marseilles 

 in November 1819, which was first suspected, then 

 proved by Professor Encke, to revolve in an elliptic 

 orbit of short period (considerably interior, in fact, 



( 26 7-) 



1 See Sir John Herschel's Outlines of Astronomy, and Results of Observations at the Cape of Good Hope. 



2 Taken from Mr Hind's small work on Comets. 



