COMETS. 



113 



observations had been made, twenty-four in number, for the purpose of comparing their 

 elements a work of immense labour and difficulty, of which the present Astronomer 

 Royal has remarked, that in all probability he was the only person then in existence 

 who could have performed it. He found the elements of two comets to coincide with 

 tolerable exactness with those of the comet of 1682, as follows : 



The general elements here are pretty closely analogous. With reference to the periodic 

 time, there is, from 



Aug. 24. 1531, to Oct. 16. 1607, - - 76 years 53 days, 



Oct. 16. 1607, to Sept. 4. 1682, - - 75 years wanting 42 days, 



a difference of about fifteen months. This Halley conjectured might arise from the dis 

 turbing action of the planets, a correct idea, and one of great sagacity, as the theory of 

 planetary disturbance was but then in its infancy. Upon these data, therefore, he ven 

 tured the conclusion that the three appearances were returns of the same comet, which 

 would reappear after the lapse of a similar interval. His words are : " Nothing seems 

 to contradict this my opinion, besides the inequality of the periodic revolutions, which 

 inequality is not so great neither, as that it may not be owing to physical causes ; for 

 the motion of Saturn is so disturbed by the rest of the planets, especially Jupiter, that 

 the periodic time of that planet is uncertain for some whole days together. How much 

 more, therefore, will a comet be subject to such like errors, which rises almost four times 

 higher than Saturn, and whose velocity, though increased but a very little, would be 

 sufficient to change its orbit from an elliptical to a parabolical one ? This, moreover, 

 confirms me in my opinion of its being the same comet, that in the year 1456, in the summer 

 time, was seen passing retrograde, between the earth and the sun, much after the same 

 manner ; which, though nobody made observations upon it, yet, from its period and the 

 manner of its transit, I cannot think different from those I have just now mentioned. 

 Hence I dare venture to foretell that it will return again in the year 1758." Subse 

 quently his tone grew more decided. Historical i-ecords supplied some further links to 

 the chain of cometary appearances after nearly the same interval. Thus : 



Years of comets - - 1155 1230 1305 1380 1456 1531 1607 1682 

 Intervals .... 75 75 75 75 75 75 75 



The astronomer was confirmed by this evidence in the accuracy of his prediction, and 

 called upon all posterity to remember that an Englishman had announced it. The pro 

 phecy was no random guess. It was not founded merely upon the coincidence of the 

 dates, but a result obtained, in the case of the last three comets, from the close agreement 

 of the elements of their orbits. 



Great curiosity was excited as the year 1758 approached, to ascertain whether the 

 prediction would be verified. No doubt existed upon the subject in the scientific world, 

 but some apprehension was felt lest circumstances should be unfavourable to a perception 

 of the phenomenon. " "We cannot doubt," observed Lalande, in 1757, " that it will 

 return ; and even if astronomers should not see it, they will not be the less persuaded of 



