34 



ASTRONOMY. MASKELYNE DELAMBRE. 



[Diss. VI. 



at that time, it would be unreasonable to expect a 

 correct determination of the form and ellipticity of the 

 orbit of Neptune, such as might be looked forif theper- 

 turbations were sensible through the entire orbits; and 

 in fact, by varying the position of the perihelion and 

 the amount of excentricity, we may, for an assumed 

 mean distance, obtain any value whatever for the in- 

 terval of the two planets at a particular time. We 

 have seen the origin of the false assumption of mean 

 distance on the part both of M. Leverrier and Mr 

 Adams, and we find that the mathematical solution 

 corrects to a great extent the error of that assump- 

 tion by giving a correspondingly incorrect position of 

 the perihelion, and also an exaggerated measure of 

 the excentricity, by which two circumstances the 

 planets of both mathematicians would have had, near 

 the time of conjunction, a distance from the sun of 

 only 32 or 33 radii of the earth's orbit, the true dis- 

 tance being about T *y part less. This still remaining 

 error was palliated, and evidently might for a time 

 have been completely masked, by assuming a mass of 

 Neptune proportionally too great, as indeed the table 

 we have given shows was the case. 



That the discovery of Neptune took place at the 

 time when it did was no accident. The conjunction 

 of Uranus and Neptune, when alone the perturbation 

 place when of the elliptic elements is perceptible, is a rare phe- 

 nomenon, occurring but once in about 172 years. 

 The last conjunction previous to 1822 was in 1649 : 

 we have seen that the attention of astronomers was 

 importunately called to the subject by the irregu- 

 larities in the motions of Uranus at the first con- 

 junction succeeding its discovery. 

 (147.) We are indebted to Professor Peirce and Mr 



(146.) 

 and why 

 the disco- 

 very took 



Walker of the United States for many useful inves- Principal 

 tigations connected with the orbits of Uranus and inequality 

 Neptune ; but it is to be wished that tbe theory were nd J^ 

 completely re-examined, and also the problem of the tune, 

 inverse method of perturbations which has now be- 

 come a systematic portion of physical astronomy. 

 The near commensurability of the two periods pre- 

 sents a peculiar case of perturbation similar to the 

 long inequality of Jupiter and Saturn ; but as its 

 period is no less than 4047 years, 1 although the 

 coefficient of the inequality is considerable, it will 

 not perceptibly alter the motions of either planet 

 except in a long course of years. 



It will not be supposed that either M. Leverrier or (148.) 

 Mr Adams could, after such a memorable triumph, Other 

 abandon the pursuits of physical astronomy. The ^ k ! j? f 

 first has continued his researches on the orbits of an( j Lever 

 comets and on the perturbations of the solar system ; rier. 

 and having been recently appointed to succeed M. 

 Arago in the direction of the Paris Observatory, we 

 cannot doubt that he will infuse new life into its 

 management. Mr Adams has made the discovery of 

 some important oversights in the details of the lunar 

 theory. One of these, with reference to Laplace's 

 Theory of the Secular Acceleration of the Moon's 

 mean Motion, has been referred to in a note to Art. 

 (62). Though Mr Adams occupies no public post, and 

 though he has declined the honours of a title, his con- 

 tinued residence at Cambridge must influence very 

 beneficially the studies of the place, where some of 

 his many friends have founded in memory of his 

 achievements a perpetual prize for the advancement 

 of physical astronomy, which is denominated the 

 Adams Prize. 2 



CHAPTER III. 



(149.) 

 Practical 



ASTRONOMY. 



1 MASKELYNE DELAMBRE. Progress of Practical Astronomy from 1770 till 1810 Of the 

 Lunar Theory deduced from Observation The Density and Figure of the Globe. Cavendish ; 

 Baily. Trigonometrical Surveys. 



As examples of those astronomers who most contri- 

 buted in the period of which we now principally speak 

 (1770-1810) to the progress of exact observation, 



whether of the motions of the heavenly bodies or of 

 the figure and density of our own planet, I have se- 

 lected Maskelyne and Delambre. Their characters 



1 According to the calculations of M. Peirson (Camb. Trans, vol. ix.). 



9 It has been our business to condense the history of the discovery of Neptune within a compass proportioned to the general 

 scheme of this Dissertation. The reader who desires farther details will consult M. Leverrier's Memoir on the Perturbations of 

 the Herschel planet (Uranus) in the Connaissan.ee des Temps for 1849 (also published separately), and in the Comptes Rendus for 

 1846 ; Mr Adams' papers in the Nautical Almanac for 1851, and the Astronomical Society's Memoirs, vol. xvi. ; Mr Airy's singularly 

 curious and impartial Historical Account in the 7th vol. of the Astronomical Society's Monthly Notices ; M. von Lindenau's History 

 in Schumacher's Nachrichten (Erganzungsheft) (favourable to M. Leverrier) ; Grant's History of Physical Astronomy (advocating 

 principally the claims of Mr Adams). Sir J. Herschel's Outlines of Astronomy contain a very valuable description of the scientific 

 part of the question. 



