S48 Obfervailons on 



even to change the comet's path, from an elliplls, tea 

 parabola or hyperbola, fo that it fhall never return. 



The fecond method, which has been adopted, for af- 

 certaining the periodical time of a comet ; and, on which 

 coniiderable rehance has been placed, is, to compare the 

 elements which are commonly computed, with ihofe of 

 all the other comets, on which calculations have been 

 made. If feveral of them are found to have the fame 

 elements ; it is concluded they muft be one and the 

 fame comet. For it is fcarcely credible that in the end- 

 lefs variety which is found to prevail, in the direftions 

 and rate of their motions, there fhould be any two, 

 which fhould have precifely the fame perihelion diftance, 

 the fame longitude of their nodes, the fame place of peri- 

 helion, and the fame angle of inclmation to the echptic : 

 and, in addition to all this, that they fhould appear at 

 intervals of time fo regular, as to correfpond with the 

 fucceiiive returns of the fame comet. On this ground, 

 it has been fuppofed, that the comet of 1759 has a pe- 

 riod of about "(5 years, becaufe one with limilar elements 

 was feen in the years 1456, 1531, 1607, and 1682. But 

 no fuch correfpondence is found, in the cafe of the late 

 comet. From a comparifon of its elements with others 

 whofe motions have been heretofore obferved and cal- 

 culated ; it will be feen that this is one, which is not in- 

 cluded in the lift. In perihelion diftance, it nearly 

 agrees with one, which appeared in 1718; but differs 

 from it, more than 40 degrees, in the inclination of the 

 orbit. In the longitude of the node, it is v^ithin lefs 

 than a degree of that in 1 759 ; but varies materially 

 from it, in the longitude and diftance of the perihelion. 

 In the inclination of the orbit, it differs but little from 

 one in 1097 and another in 176S ; but has no agreement 

 with them, in the other elements. So that this method 

 of determining the periodical time fails, in its applica- 

 tion to the prefent cafe. And even, if the elements of 

 this comet, had been found to agree, with thofe of any 

 preceding one ; it v/ould have ferved rather to fhew, 

 in what interval of time it has returned, than to give us 



