390 HALLEY'S COMET. [SECT, 



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 return- 

 ing 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, that it 

 would pass its perihelion on the 18th of April, 1759. The 

 comet did arrive at that point of its orbit on the 12th 

 of March, which was thirty-seven days before the time 

 assigned. Clairaut subsequently reduced the error to 

 twenty-three days ; and La Place has since shown that it 

 would only have been thirteen days if the mass of Saturn 

 had been as well known as it is now. It appears, from this, 

 that the path of the comet was not quite known at that 

 period ; and, although many observations were then made, 

 they were far from attaining the accuracy of those of the 

 present day. Besides, since the year 1759, the 'orbit of the 

 comet has been altered by the attraction of Jupiter in one 

 direction, and that of the earth, Saturn, Uranus, and Nep- 

 tune, in the other; yet, notwithstanding these sources of 

 uncertainty, and our ignorance of all the possible causes of 

 derangement from unknown bodies on the confines of our 



