342 HALLEY'S COMET. SECT. XXXVI. 



appearance 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, 

 and Uranus, in the other; yet, notwithstanding these 

 sources of uncertainty, and our ignorance of all the pos- 

 sible causes of derangement from unknown bodies on 

 the confines of our system, or in the regions beyond it, 

 the comet has appeared exactly at the time, and not far 

 from the place, assigned to it by astronomers ; and its 

 actual arrival at its perihelion a little before noon on the 

 16th of November, 1835, only differed from the com- 

 puted time by a veiy few days. 



The fulfilment of this astronomical prediction is truly 

 wonderful if it be considered that the comet is seen only 

 for a few weeks, during its passage through our system, 

 and that it wanders from the sun for seventy-five years 

 to twice the distance of Uranus. This enormous orbit 

 is four times longer than it is broad ; its length is about 

 3420 millions of miles, or about thirty-six times the mean 

 distance of the earth from the sun. At its perihelion 

 the comet comes within nearly fifty-seven millions of 

 miles of the sun, and at its aphelion it is sixty times 

 more distant. On account of this extensive range it 

 must experience 3600 times more light and heat, when 

 nearest to the sun than in the most remote point of its 



