286 " PHILOSOPHICAL TRANSACTIONS. [aNNO 1766. 



The second rock struck by Moses is, he thinks, 43 feet long, l6 broad, 13 

 high ; it has two cracks, oblique ones ; in them are some mouths, like those of 

 Meribah : it is of a hard stone, not granite or marble. 



IX, A Discovery, with Observations, of Tivo new Comet.';, in the Marine Observa- 

 tory at Paris. By M, Messier, of Paris, F. R. A'. From the French, p, 57. 

 The first of these comets Mr. M. discovered the 8th of March, 1766, and 

 from the observations he made of it, till the 15th, Mr. Pingre computed the 

 elements of its orbit, as follows : 



Place of the ascending node ^ 8 4° lO' 50" 



Inclination of tlie orbit 40 50 20 



Place of the perihelium 4 23 15 25 



Logarithm of the perihelion dist 9.703570. 



The comet passed its perihelion the 17 th of February, at 8*' 50"", mean time, 

 at the meridian of Paris. 



The motion of the comet retrograde. 



The 2d comet Mr. M. discovered April 8, the same year 1766; which he 

 continued to observe in the evenings till April 12. And from his observations, 

 M. Pingre calculated the elements of this comet as follows: 



Place of the ascending node 1' 17° 22' 12" 



Inclination of the orbit 8 18 45 



Place of the perihelion 6 26 5 13 



Perihelion distance 0.636825 



Logarithm of the perihelion distance 9.804020 



It passed the perihelion April 17, 0^ 26"" 13' mean time, at the meridian of Paris. 

 The motion of the comet direct. By these elements M. Pingre judged that 

 this comet might be seen again in the morning, after getting clear of the sun's rays. 

 Mr. Pingre s remarks on the two comets of this year. — ^The elements of the 

 first comet I give as absolutely certain, those of the 2d I cannot be so sure of. 

 The interval was only 4 days between the first and the last observation. The 

 last two days, and especially the last, the twilight and the moon-light must have 

 produced some uncertainty in the observation. The ephemeris is founded on 

 the certainty of the elements. Supposing there were no more than 3 or 4 

 minutes error in the last two observations, this would not much alter the theory 

 from what I have given ; but an alteration in the elements would produce 2 or 

 3 degrees difference in the place of the perihelion, which might be sufficient 

 to render the reapparition of the comet uncertain in these high latitudes. Some 

 German observations sent to Mr. Messier, made in the beginning of April, 

 induce me to conclude, that the place of the perihelion ought really to be placed 

 a little more eastward ; but these observations were sent in so confuseii a manner. 



