HALLBY'S COMET. 



of those planets whose masses were sufficiently great, in accele- 

 rating or retarding its motion while passing near them. 



56. This inquiry, which presented great mathematical diffi- 

 culties and involved enormous arithmetical Jabour, was undertaken 

 by Clairaut and Lalande : the former, a mathematician and 

 natural philosopher, who had already applied with great success 

 the principles of gravitation to the motions of the moon, undertook 

 the purely analytical part of the investigation, which consisted 

 in establishing certain general algebraical formulae, by which the 

 disturbing actions exerted by the planets on the comet were 

 expressed ; and Lalande, an eminent practical astronomer, under- 

 took the labour of the arithmetical computations, in which he 

 was assisted by a lady, Madame Lepaute, whose name has thus 

 become celebrated in the annals of science. 



"When it is considered that the period of Halley's comet is about 

 seventy-five years, and that for two successive periods, it was 

 necessary to calculate every portion of its course separately in 

 this way, some notion may be formed of the labour encountered 

 by Lalande and Madame Lepaute. " During six months," says 

 Lalande, " we calculated from morning till night, sometimes 

 even at meals ; the consequence of which was, that I contracted 

 an illness which changed my constitution for the remainder 

 of my life. The assistance rendered by Madame Lepaute was 

 such, that without her we never could have dared to undertake 

 this enormous labour, in which it was necessary to calculate the 

 distance of each of the two planets, Jupiter and Saturn, from 

 the comet, and their attraction upon that body, separately, for 

 every successive degree, and for 150 years." 



The name of Madame Lepaute does not appear in Clairaut' s 

 memoir; a suppression which Lalande attributes to the influence 

 exercised by another lady to whom Clairaut was attached. 

 Lalande, however, quotes letters of Clairaut, in which he speaks 

 in terms of high admiration of "la savante calculatrice." The 

 labours of this lady in the work of calculation (for she also 

 assisted Lalande in constructing his " Ephemerides ") at length 

 so weakened her sight that she was compelled to desist. She 

 died in 1788, while attending on her husband, who had become 

 insane. See the article on comets, by Prof, de Morgan, in the 

 " Companion to the British Almanac" for the year 1833. 



These elaborate calculations being completed, Clairaut presented 

 the result of their joint labours, in a memoir to the Academy of 

 Sciences of Paris, in which he predicted the next arrival of the 

 comet at perihelion, on the 18th of April, 1759 ; a date, however, 

 which, before the re-appearance of the comet, he found reason to 

 change to the llth of April; and assigned the path which the 



N 2 179 



