546 SCIENCE PROGRESS 



they do to each other, but also that, in the distant regions of 

 space to which it travels, a small change of velocity would have 

 an exaggerated effect on the period. We now know that the 

 effect of these planets can be even greater than that necessary to 

 explain Halley's fifteen months, and that the total range is five 

 years, from seventy-four years to seventy-nine. H alley then 

 found that a similar comet had appeared in 1456, which he had 

 little doubt was the same object. So confident did he now feel, 

 that he ventured to predict the return of the comet in 1758; 

 subsequently he made this a year later, having found that 

 Jupiter's action would delay it. He added that, " If the comet 

 returns in accordance with my prediction, posterity will not 

 forget that this first prediction was made by an Englishman." 

 When the time approached, the French mathematicians 

 Clairaut and Lalande made calculations to fix the date of the 

 return as accurately as possible. They received material 

 assistance from a lady, Madame Lepaute, in this huge under- 

 taking, which involved a study of the comet's motion for two 

 revolutions — viz. from 1607 to 1682 — in order to find the rate at 

 which the comet was moving in 1682, and from that year to 1759 

 in order to find how much this rate was altered in the interval. 

 They just finished their work in time, announcing in November, 

 1758, that the comet would pass its perihelion (or nearest point 

 to the sun) on April 13 following. It was actually found by 

 Palitsch on Christmas Day, and passed perihelion just a month 

 earlier than the time predicted, this being a small discordance 

 when it is considered that the masses of the planets were still 

 imperfectly known, and that Uranus and Neptune had not yet 

 been discovered. 



It is a curious illustration of the dictum that " A prophet 

 has no honour in his own country," that, while the French 

 astronomers thus triumphantly vindicated Halley's prediction, 

 in England the Gentleman' s Magazine expressed grave doubt as 

 to whether the comet would reappear at all, and referred to 

 Halley somewhat contemptuously. However, after the comet 

 was seen, it changed its tone, and published a poem, from which 

 these lines are taken : 



Hah ! there it flames, the long-expected star, 

 And darts its awful glories from afar ! 

 Punctual at length the traveller appears, 

 From its long journey of near fourscore years. 



