SECT. XXXVI. LEXEL'S COMET. 341 



ter, so that Jupiter became the primum mobile. As- 

 suming the orbit to be such as Lexel had determined in 

 1770, La Place found that the action of Jupiter, previ- 

 ous to the year 1770, had so completely changed the 

 form of it, that the comet which had been invisible to us 

 before 1770, was then brought into view, and that the 

 action of the same planet producing a contrary effect, 

 has subsequently to that year removed it from our sight, 

 since it was computed to be revolving in an orbit whose 

 perihelion was beyond the orbit of Ceres. However, 

 the action of Jupiter during the summer of 1840 must 

 have been so great, from his proximity to that singular 

 body, that he seems to have brought it back to its former 

 path, as he had done in 1767, for the elements of the 

 orbit of a comet which was discovered in November, 

 1843, by M. Faye, agree so nearly with those of the 

 orbit of Lexel's comet as to leave scarcely a doubt of 

 their identity. From the smallness of the eccentricity, 

 the orbit resembles those of the planets, but this comet 

 is liable to greater perturbations than any other body in 

 the system, because it comes very near the orbit of 

 Mars when in perihelion, and very near that of Jupiter 

 when in aphelion ; besides, it passes within a compara- 

 tively small distance of the orbits of the minor planets, 

 and as it will continue to cross the orbit of Jupiter at 

 each revolution till the two bodies meet, its periodic 

 time, now about seven years, will again be changed, but 

 in the mean time it ought to return to its perihelion in 

 the year 1851. This comet might have been seen from 

 the earth in 1776, had its light not been eclipsed by that 

 of the sun. It is quite possible that comets frequenting 

 our system may be turned away, or others brought to 

 the sun, by the attraction of planets revolving beyond 

 the orbit of Uranus, or by bodies still farther removed 

 from the solar influence. 



Other three comets, liable to less disturbance, return 

 to the sun at stated intervals. Halley computed the 

 elements of the orbit of a comet that appeared in the 

 year 1682, which agreed so nearly with those of the 

 comets of 1607 and 1531, that he concluded it to be the 

 same body returning to the sun at intervals of about 

 seventy-five years. He consequently predicted its re- 

 FF2 



