362 ORBITS OF COMETS. SECT. XXXV. 



that the two bodies were supposed to be identical ; by the subse- 

 quent computation of M. le Verrier, it appears, however, that 

 they are not the same, that they were both brought to our system 

 by Jupiter's attraction, and that they have been in it more than a 

 century, and have frequently come near the earth without having 

 been seen. From the smallness of the excentricity of LexePs 

 comet, 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 comparatively 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 have returned 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. There is still so much doubt 

 with regard to Lexel's comet that during the present year, 1858, 

 M. le Verrier has constructed a table of all the orbits in which 

 the comet may have moved after leaving Jupiter in 1770, which 

 will enable astronomers to recognise the comet even should the 

 elements of its orbit be much altered. He thinks it possible 

 that its path may have become hyperbolic, but that it is more 

 likely an augmentation of its periodic time may have taken place. 

 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 Neptune, or by bodies still 

 farther removed from the solar influence. 



Other cornets, 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 con- 

 cluded it to be the same body returning to the sun at intervals 

 of about seventy-five years. He consequently predicted its 

 reappearance in the year 1758, or in the beginning of 1759. 

 Science was not sufficiently advanced in the time of Halley to 

 enable him to determine the perturbations this comet might 

 experience ; but Clairaut computed that, in consequence of the 

 attraction of Jupiter and Saturn, its periodic time would be so 

 much shorter than during its revolution between 1607 and 1682, 



