832 
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 belooked forif theper- 
turbations weresensible 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 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. 
(146.) That the discovery of Neptune took place at the 
ig eae time when it did was no accident. The conjunction 
very took Of Uranus and Neptune, when alone the perturbation 
place when of the elliptic elements is perceptible, is a rare phe- 
it did. 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 
ASTRONOMY.—MASKELYNE—DELAMBRE. 
(Diss. VI. 
Walker of the United States for many useful inves- 
tigations connected with the orbits of Uranus and 
Neptune ; but it is to be wished that the theory were 
completely re-examined, and also the problem of the 
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,' 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 
Mr Adams could, after such a memorabte triumph, 
abandon the pursuits of physical astronomy. The 
first has continued his researches on the orbits of 
comets and on the perturbations of the solar system ; 
and having been recently appointed to sueceed 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? 
CHAPTER III. 
ASTRONOMY. 
§ 1 MASKELYNE—DeELAMBRE.—Progress of Practical Astronomy from 1770 till eee the 
Lunar Theory deduced from Observation—The Density and Figure of the Globe. Cavendish ; 
Baily. Trigonometrical Surveys. 
(149.) 
As examples of those astronomers who most contri- whether of the motions of the heavenly bodies or of 
Practical buted in the period of which we now principally speak the figure and density of our own planet, I have se- 
Astronomy 
1770-1810, (1770-1810) to the progress of exact observation, lected Maskelyne and Delambre. Their characters 
1 According to the calculations of M. Peirson (Camb. Trans. vol. ix.). 
2 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 Connaissance 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 Sch her’s Nachricht 
(Ergiinzungsheft) (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. 
Principal 
inequality 
of Uranus 
and Nep- 
tune. 
(148.) 
Other 
works of 
MM. Adams 
and Lever- 
rier. 
