162 
SECOND REPORT- 1832. 
to he hyperbolical. None of these determinations, I suppose, 
deserve much credit, except where the comet has been long ob¬ 
served and very ably discussed. 
In 1818 and 1819, in examining the coincidence of the ob¬ 
served places of a comet discovered by Pons, with the places 
given by parabolic elements, Encke * found that the supposi¬ 
tion of an ellipse of very short period was absolutely necessary; 
and his first calculations gave 1310 days. It was soon remarked 
that its elements were similar to those of the first comet of 1805, 
in calculating which Bessel had remarked {Mon. Corr. vol. 14,) 
that a parabolic orbit would not represent the observations. 
The interest excited by the discovery that we had a real pe¬ 
riodic comet of short period, will best be gathered from the 
successive parts of Zach’s Correspondance , vol. 2. Gibers 
pointed out its identity with that of 1795, on which he had 
long before remarked ( Berlin Ephemeris 1814,) that different 
calculators had found very different elements. Encke, in the 
Berlin Ephemeris 1822, showed that the sums of the squares 
of errors of observation in the comet of 1795, were reduced to 
less than half by taking an ellipse of 1200 days instead of a pa¬ 
rabola. In the same volume, the perturbations for these periods 
were given. Gibers soon after pointed out that the same 
comet had been observed before ; and this discovery is very 
curious. In the Conn, des Temps 1819, are given two obser¬ 
vations of a comet in 1786; from these alone no orbit could be 
determined. But Gibers found from the approximate elements, 
that these were certainly observations of the new periodic comet. 
Thus a series of observations extending through 33 years, or 
10 revolutions of the comet, was established. After very short 
examination, Encke found ( Berlin Ephemeris 1823,) that the 
periodic time given by the late observations was shorter than 
that from the earlier, or that the comet was gradually approach¬ 
ing the sun ; which would seem to prove the existence of a re¬ 
sisting medium. He however predicted its place approximately 
for 1822, when, on account of its southern declination, it could 
not be seen in Europe ; happily the Observatory at Paramatta 
* In a French elementary work, it is stated that M. Arago first remarked the 
similarity of the elements of the comet of 1819, with that of 1805. But the 
discovery was certainly made by Encke in the manner stated in the text. 
That M. Arago may have conceived there was some similarity, (not much, as 
may be seen on examining a table of comets,) is quite possible ; but nothing 
followed from this conjecture. Every calculation respecting this comet (except 
one by Damoiseau, which was a duplicate of one of Encke’s,) has been made by 
the German astronomers. 
