the Comet of 1 811. 547 



or the hyperbola. If it Is ever to return, in a regular 

 orbit, it inuft revolve in an ellipfis. If its motion is in 

 either of the two other figures, after having once paffed 

 the perihelion, it will continually recede from the fun, 

 and will return no more ; unlefs its direftion fnould be 

 changed, by the attraction of fome other body. In each 

 of thefe cafes, however, if a portion of the path of a com- 

 et could be obtained by obfervation, with perfect exad- 

 nefs ; from this might be deduced, the remaining parts 

 of the orbit, on the fuppofition, that its figure fhould 

 remain unaltered. But here two infuperable difficul- 

 ties occur, one from theerrours of obfervation, the oth- 

 er from the liability of the comet, to be diverted from 

 its courfe, by the attraction of the planets, and perhaps 

 of the fixed flars. The ground of thefe difficulties, lies 

 not in the procefses for calculating the orbit 5 but in ta- 

 king the obfervations, and in the change of the orbit it- 

 felf. Errours too minute to be avoided, even by the 

 niceft inftruments hitherto in ufe, might be fufficient, in 

 certain cafes, efpecially where the orbit is very eccen- 

 tric, to make a variation of many years, in the periodi- 

 cal time. The moft able computers, have accordingly 

 differed whole centuries, in the periods, which they have 

 refpeclively affigned to the fame comet. The only ca- 

 fes to which calculation can be applied, with any hope 

 of fuccefs, are thofe in which the time of revolution is 

 very fhort. The comet of 1770, has been fuppofed to 

 be one of this clafs. Lexell, Pingre, and Burckhart all 

 agree in giving it a period of about five years and an half. 

 There is reafon to believe, that this is tlie orbit, which 

 really correfponds with that part of its motion, which 

 was obferved. But notwithftanding this, the comet 

 has never been feen fmce ; though it ought to have re- 

 turned fix or eight times, in the intervening forty years. 

 If its orbit was truly affigned, it feems it muil have 

 been fmce altered, by the attraction of the planets or 

 of fome other body, whofe influence may be fufficient, 

 not only to vary materially the time of revolution ; but 



