36 M. O. A. Rosenberger on the Ele7ne7its 



des Sc. de Paris, An 1 760, pp. 426, &c. They were all made 

 with a ^^-feet Newtonian telescope, which macrnified sixty-six 

 times. To this was adapted a micrometer with silken wires, 

 which could be turned so that one of the wires could be 

 easily brought into the parallel of the comet. Another move- 

 able wire parallel to the former was made use of to measure 

 the difference between the declinations of the comet and the 

 compared stars; a third, perpendicular to the others, served 

 as a transit icire. The field of the telescope embraced fifty- 

 three minutes. Messier observed from March 31st till April 

 13th, at the College of Louis le Grand, by a minute-watch, 

 which was set every evening to the true time of the observa- 

 tory. From April 14th till May 2nd, the observations were 

 continued at the house of the bookseller Desprez, on account of 

 a more unobstructed horizon. The clock used here was regu- 

 lated, by means of a minute-watch, with that of the observatory ; 

 so that the time stated may be wrong by some minutes. All his 

 other observations were made at theObservatoire de la Marine. 

 An observation of the 21st of May has been omitted, owing to 

 the position of the compared star being unknown. Moreover 

 all estimations included by Messier in his table of the ob- 

 served positions have been excluded. 



2. ManlJi {Mem. de V Acad. An 1759, p. 279.) observed 

 with a 3-feet telescope, which was mounted equalorially and 

 provided with a rhomboidal micrometer. It magnified only 

 eight times, and its field embraced 2"^ 41'. The instrument 

 showed stars of the 6th and 7th magnitude with the greatest 

 distinctness. He states the positions of the compared stars, 

 the differences of right ascension and declination, and the 

 places of the comet. But these statements are not always in 

 accordance; and owing to the uncertainty thereby produced, 

 several observations have been necessarily excluded. Where 

 the differences were small, I took the mean between the dif- 

 ferences of right ascension anil declination given by Maraldi 

 himself, and those resulting from the stated places of the com- 

 pared star and the comet. 



May 14th, a difference of right ascension is stated, evi- 

 dently belonging to the next day. 



F'or the 15th of May, we find two different times of obser- 

 vations belonging to one observation; viz. 8'' 49' and 9"' 27' 15", 

 on which account I took the mean of both for the right ascen- 

 sion, which at this period alters very slowly ; but took no no- 

 tice of the declination, for which, moreover, the statements 

 of Maraldi differ by 20" from each other. 



Maraldi mentions the observation of May 16th as particu- 

 larly difficult and uncertain, on which account I gave to the 



declination 



