856 
Its pertur- His successors have distinguished themselves chiefly 
bations 
calculated 
by the praiseworthy minuteness.and extent of their 
by various Calculations, which are amongst the most laborious, 
astrono- 
mers. 
(265.) 
Its return 
in 1835. 
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 caleu- 
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 1885, this Comet was watched 
until May 1836, and in the course of its Jong 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 peculiarheen reabsorbed into the nucleus, which presented 
features. 
then a uniform circular outline. The evolution of 
the highly-expansible luminous matter of the tail 
took 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 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss, VI. 
(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’! 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 
with a table of the dates of its probable perihelion 
passages, according to M. Laugier and Mr Hind.” 
The earlier appearances are deduced from the Chinese 
Annals :— 
Date. Interval. Date. Interval. 
B.C. 11°80 Years. Years. 
77°87 77-45 
A.D. 66:07 A.D. 989-70 
7517 ‘ 76:55 
141°24 1066°25 
77°02 79-05 
218-26 1146°30 
76:99 78:22 
295-25 1223-52 
. 7859 78°29 
37384 1301°81 
77°66 77-04 
451°50 1878-85 
79°34 77:59 
530°84 1456744 
77-96 75-21 
60880 1531:65 (0. s.) 
76-00 76-14 
684-80 1607°82 (N.8.) 
75°64 74:88 
760-44 1682:70 
76°82 6:49 
837-26 175919 
7499 76°68 
91225 1835-87 
"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. Brie: 
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, 
1 See Sir John Merschel’s Outlines of Astronomy, and Results of Observations at the Cape of Good Hope. 
2 Taken from Mr Hind’s small work on Comets. 
(266.) 
Table of its 
returns, 
(267.) 
Comet of 
Encke, 
