Cur. IIT., § 5.]  ASTRONOMY.—M. ENCKE—GAMBART. 857 
another part of the world at every revolution :— 
namely, in 1825, 1828, 1832, 1835, 1838, 1842, Its appa- 
1845, 1848, and 1852; so that the Comet has been ™“* 
observed at fourteen (not all consecutive) returns. 
to the orbit of Jupiter), and made the subject by 
him of a series of investigations altogether peculiar. 
On this account it bears the name of Encke, instead 
of that of its discoverer Pons, although M. Encke 
himself, with unaffected modesty, always describes it | The complete establishment of the existence and pe- , bah? 
as the Comet of Pons. riodicity of a comet, quite in the interior of the planet- eras 
Aue Newton gave, in the Principia, his celebrated so- ary system (its greatest distance from the Sun being tion 
orbits de- 
rived from 
observa- 
tion. 
lution of the problem of determining a Comet’s orbit 
assumed to be parabolic, from three geocentric places. 
This solution has been simplified and improved by 
Lagrange and Boscovich, and also by Olbers. La-~ 
place gave a method for an elliptic orbit, which may 
represent any number of observations. Gauss, in 
his Theoria Motéis Corporum Celestium, treated the 
subject with great skill and generality, I am unable 
to state who first attempted to discriminate an ellip- 
tic from a parabolic cometary orbit, or to determine 
the period in the former from observations at one ap- 
parition only. It is evident that such outstanding 
differences as are irreconcilable with a parabolic 
orbit, will be most perceptible in the case of comets 
whose orbits have a tolerably short major axis, or 
whose period is not very great, and will be mate- 
rially increased by watching a comet through a con- 
siderable part of its orbit, which the assiduous appli- 
cation of telescopes to every part of the heavens has 
of late years rendered much more frequent than for- 
merly. Amongst others, Bessel, who has signalized 
himself by a capital performance in this, as in every 
other department of Astronomy, applied rigorous me- 
thods to determine the orbits of the comets of 1807 and 
1815; the latter of which will very probably return 
to its perihelion in 1887. It is undeniable, however, 
that expert calculators have often been deceived in 
assigning orbits, even when believed to be of short 
period, founded upon a single apparition. 
four times the Earth’s distance, and its least distance 
but one-third of the Earth’s), was a discovery in itself 
highly interesting. But something yet remained be- 
hind, Professor Encke, in comparing the earlier with 
the later apparitions of the Comet, detected a gradual 
acceleration of its movement, which amounted be- 
tween 1786 and 1838, to 1-8 days, on a period of about 
1211 days; being about 2} hours per revolution. 
Whatever may be the cause of this, the fact is undis- 
puted, even by Bessel, who was indisposed to accept 
M.Encke’s explanation. This fact, it will be observed, 
is uniquein Astronomy. The major axis and periodie 
times of the planets and satellites have, as we have 
seen in the chapter on Physical Astronomy, no secu- 
lar variation. The moon’s apparent acceloration has 
been otherwise accounted for. 
and at an early period, attributed the acceleration of ; 
the Comet’s mean motion to the effect of a slightly aium. 
resisting medium, insensible in the case of the planets, 
partly owing to their incomparably greater density (for 
this Comet appears to be one of the most loosely ag- 
gregated bodies known, being transparent to its very 
centre); and also, to the circumstance, that the density 
of the ether or resisting medium is assumed to dimi- 
nish rapidly at a distance from the Sun. M. Encke 
supposes it to decrease in density with the square of 
the distance, and only to affect the Comet sensibly 
within 25 days preceding or following its perihelion. 
That the effect of resistance is to accelerate the return 
(269. M. Encke, however, was more fortunate in the case of the Comet is evident, by considering that the pro- 
M. Encke’s of the first comet of 1819. Using the methods of jectile force becoming gradually extinguished, the 
= Gauss, he showed that an Elliptic Orbit of about 3} Sun’s attraction must be more available to pull the 
comet of years must be admitted, and that the comet had pro- body inwards at each revolution, thus shortening the 
1819—pe- bably been already observed in 1786, by Méchain,in major axis of the ellipse, and diminishing the time. 
riod 3 1795 by Miss Herschel,! and in 1805 by Pons. He Tn order satisfactorily to arrive at any such con-_ (271.) 
_ investigated with great labour the effects of the planet- clusion, it was of course necessary to estimate with na 
ary perturbations on this body, which, in the case of great accuracy the perturbing effects of the planets Encke’s 
Jupiter, are occasionally very large, if that planet be on the Comet’s motion; and it is not a little curious comet ap- 
in the part of its orbit near the aphelion position of and satisfactory, that the movements of this insigni- Se tad 
the comet, when it approaches the orbit of Jupiter. ficant erratic body should have occasioned a mate- asses of 
The careful calculations of M. Encke for the next re- rial rectification of the masses of two of the Planets. Mercury 
turn in 1822 were verified by the observations of Sir M. Encke very early suspected that the received nd Jupi- 
Thomas Brisbane, at that time fortunately governor mass of Jupiter was too small, a fact clearly esta- Las 
of New South Wales, where he had, with character- _blished afterwards by Mr Airy ; and in 1838 M. Encke 
istic liberality, founded an Observatory. Since then, showed that the mass of Mercury (which, not having 
this body, insignificant in its physical appearance a satellite, was little more than guessed at previously) 
"(being to all appearance a small cloud of vapour had been assumed nearly three times too great by La- 
without a solid nucleus), has been detected in one or grange. The perihelion of the Comet approaches much 
Miss Caro- . 2 Caroline Lucretia Herschel, sister of Sir William and aunt of Sir John Herschel, deserves a passing notice, not only as the 
line Her- ‘imdependent discoverer of eight comets (of which five were first seen by her), but as the indefatigable and intelligent assistant of 
schel. Sir William Herschel during the busiest years of his life. For this service King George IIL., carrying out his judicious liberality 
to her brother, granted her a small pension. She died at Hanover 9th January 1848, aged 97. 
VOL, I, 
5a 
M. Encke at once, attributed 
to a resist~! 
