90 Astronomical and Nautical Collections. 
No. Year. 
the observations, and hence all the computed orbits varied 
several minutes from the observations. Burckhardt reduced 
them with greater care, and hence obtained the last elements, 
which are more correct. Mém. Inst. VIL. p. 226. Mon. 
Corr. XVI. p. 515. wit. 
66. 1763. Discovered by Messier the 28th Sept., observed before the pe- 
- vihelium from the 30th Sept. to the 25th Oct., after the 
perihelium from the 12th to the 25th Nov. Pingré and Lexell 
could not represent the observations sufficiently well by any 
conic section. Pingré II. p. 106. Acta. Ac. Sc. Petr. 1780. 
Pt. ii. p. 324. Burckhardt has corrected the observations of 
Messier, which were distorted by some errors in the places of 
Flamstead’s stars, and he has employed the observations of 
St. Jaques de Silvabelle, which were made public more lately. 
Mon. corr. X. p. 507. Conn. des tems. XIII. p. 344. 
67. 1764, Discovered by Messier, and observed from 3d Jan. to 11th Feb. 
The third orbit is that which has been corrected from all the 
observations. Pingré IT, p. 74. 
68. 1766. Discovered by Messier the Sth March, and observed for eight 
days only. Pingreé If. p. 75. 
69. 1766. Observed by Messier at Paris only five days, from the Sth to 
the 12th April. La Nux at the Isle of Bourbon followed it 
from the 29th April to the 13th May. Pingré IL. p. 76. 
The imperfect observations of La Nux, Pingré could not 
satisfactorily combine with the Parisian observations, and 
Burckhardt has attempted to do this by means of an ellipsis. 
Conn. des tems. 1821. 
70. 1769. Discovered by Messier the 8th Aug., and observed before the 
perihelium to the 15th Sept. ; after it from the 24th Oct. to 
the Ist Dec. The principal observations are found Mem. 
Par. 1769, p. 49; 1770, p. 24; 1775, p. 392. Maskelyne 
Astr. Obs. I. On the various orbits, besides Pingré II. 
p. 83, see especially Euler Recherches et Calculs sur la 
comeéte, 1769; 4 Pet. 1770; the two rare works of Asclepi 
De cometarum motu exercitatio ; 4 Rom. 1770, and De come- 
tarum motu Addenda, Rom. 1770; besides Bessel’s excellent 
prize memoir in the Berl. Astr. Jahrb. 1811. That Bessel’s 
elliptic orbit is preferable to all others, scarcely requires to be 
observed. The nodes and perihelium are determined by Bessel 
as at rest with respect to the stars for the Ist Jan. 1769. 
71. 1770. This comet, celebrated for an orbit deviating so greatly from 
the parabolic form, was discovered the 14th June by Messier, 
and observed till the 24 Oct. The observations have been 
collected by Messier, Mém. Par. 1776, p. 597. The short 
period of this comet, little exceeding five years and a half, 
which appeared so paradoxical when it was first computed 
by Lexell, was fully confirmed by Burckhardt’s investigations 
in his valuable prize memoir. Mém. Inst. 1806, p.1. Therea- 
son why the comet has not re-appeared since 1770, has been 
very satisfactorily explained in Laplace's Mée. Cd. vol. [V. 
-72. 1770. Was observed only four times at Paris, between the J0th and 
20th of Jan. 1771. Mem, Par.1771,  Pingré II. p. 90. 
73. 1771, Discovered by Messier the Ist April, and observed by him 
until the 19th June ; but by St, Jaques de Silvabelle at Mar~ 
