HISTORY OF PHYSICAL ASTRONOMY.

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of the New Planet.” He proposed taking a sweep of the heavens in thedirection of the ecliptic 30° long, and 10° broad, the centre being theplace indicated by the theory as the locus of the planet. The paper con-tained the details of a plan for conducting the examination. In his letterhe says, “ I only add at present that in my opinion the importance ofthis inquiry exceeds that of any current work which is of such a natureas not to be totally lost by delay.” * Professor Cliallis declined Mr. Airy’soffer of an assistant, having himself formed the resolution (as he had pre-viously intimated to Mr. Adams j-) of searching for the planet on the oc-casion of the approaching opposition. Having received a paper from Mr.Adams, containing instructions relative to the theoretical place of theplanet, he commenced his observations for this purpose on the 29th July j.The plan contemplated was to sweep over the region of the zodiac selectedfor examination at least three times, completing each sweep before thecommencement of the following one. It was concluded that to accomplishthis object three hundred hours of observing would be required. Thediscovery of the planet would he effected by finding that one of the starsin the examined region had not the same position in each sweep. Mr.Challis continued to prosecute his search till the end of September.We shall find that his observations contained more than one place of theplanet, and that their subsequent comparison, which was included in theplan of operations, would have infallibly led to its discovery.

We now return to Mr. Adams. On the 2nd September, 1846, he trans-mitted a second paper to the Astronomer Royal, containing an account ofhis further researches on the Trans-Uranian planet. A comparison of hisoriginal results with the observations of Uranus had induced him tosuspect that the mean distance was somewhat too great. He therefore dimi-nished it to the extent of ^th, assuming it to be equal to 37.5, and thenrepeated his previous solution. He hoped that by this means the theorywould be brought to agree better with the observations of recent years,and that a smaller value would be obtained for the eccentricity, which ap-peared to him too large, as it resulted from his first hypothesis. In hiscommunication to the Astronomer Royal, he gave the elements of theplanet by the two hypotheses of the mean distance, and appended a list ofresidual errors of longitude formed by a comparison of his theory with agreat number of observations of the planet included between the years1712 and 1840. These errors were smaller by the second hypothesisthan by the first, and the eccentricity was also considerably diminished.Mr. Adams was of opinion that by continuing to diminish the mean dis-tance the theory might be rendered still more accordant with observation,and he was induced to conclude, from an examination of the residual errorsof recent years, that, by assigning to the mean distance a value equal to33.6, a very near approximation to the truth would be obtained. This

* Airy’s Hist. Statement, No. 16.

f Challis’ Report to the Syndicate of the University of Cambridge .

7 Ibid. It must be borne in mind that at this time Le Verrier had obtained an ap-proximate value of the position of the planet, but had not assigned determinate values tothe mass or the elements of the orbit. A knowledge of the mass was necessary for thepurpose of ascertaining the class of stars among which it might be expected that theplanet would be found. Mr. Adams, guided by his theoretical results, had mentioned toProfessor Challis that the planet would be equal to a star of the ninth magnitude. Thiscircumstance induced Mr. Challis to note the positions of all stars down to the tenth mag-nitude. The actual discovery of the planet has shewn that the range of search adoptedwas at once necessary and sufficient.