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 he was elected, 2 Feb. 1836, his successor as Plumian professor of astronomy and experimental philosophy in the university, and became at the same time director of the Cambridge observatory, where he resided, and exercised a genial hospitality during twenty-five years. He resigned the latter post in 1861, but retained the Plumian professorship, and continued to live at Cambridge. He was re-elected to his fellowship in 1870. There, after some years of impaired health, he died, 3 Dec. 1882, at the age of nearly seventy-nine, and was buried with his wife at the Mill Road cemetery. A son and daughter survive him.

Courteous in manner, kindly in disposition, simple and unassuming in character, Challis was nevertheless thrown into a position of intellectual antagonism to many of his most distinguished contemporaries by the peculiarity of his scientific views. A striking proof of the amiability of his disposition is afforded by the fact that he never lost consideration for an opponent, or allowed disagreement to degenerate into hostility. For some slight acerbity in the mode of carrying on a controversy with Mr. Adams in 1854 on points connected with the lunar theory (Phil. Mag'. viii. 98), he, fifteen years later, publicly expressed regret, while acknowledging the justice of the criticism he had then repudiated (Introduction to Principles, p. xxiv).

His aim was a lofty one. It was nothing less than the co-ordination of all the known facts of science under one general theory of physical action. Certain hydrodynamical theorems, which he believed himself to have demonstrated, admitted, in his firm conviction, of application to the observed laws of light, heat, gravity, molecular attraction, and electricity. The conclusion pointed to was that the physical forces are mutually related, because all are modes of pressure of the same ethereal medium. The work in which these views were most fully embodied, and for the sake of concentrating all his faculties on which he resigned, at some pecuniary inconvenience, his position at the observatory, was published in 1869, with the title, ‘Notes on the Principles of Pure and Applied Calculation; and Applications of Mathematical Principles to Theories of the Physical Forces.’ It cannot be said, however, to have reached its aim. A generalisation akin to, though of far wider scope than Newton's, rendering all physical phenomena mathematically deducible from a few simple laws, if attainable, has yet to be attained.

Challis's name must always be mentioned in connection with the discovery of Neptune. To him, in September 1845, Adams communicated his first results, which he conceived the idea of testing on a favourable opportunity, by a search with the Northumberland equatoreal for the unknown body. Regular observatory work, however, was pressing; and it was not until Leverrier's strikingly concordant indications became known in England that Challis wrote, 18 July 1846, in answer to a suggestion from Airy, ‘I have determined on sweeping for the hypothetical planet.’ The plan adopted was a highly laborious one. Its preliminary was the construction of a map of all stars down to the eleventh magnitude contained in a zodiacal belt 30° long by 10° broad. The work was begun on 29 July and continued diligently until 29 Sept., when the places of 3,150 stars had been recorded. Challis was arrested in his preparations to map them by the news of the planet's discovery at Berlin on 23 Sept. It was then found that, after only four days' observing, its varying positions among the stars had been twice unconsciously noted, 4 and 12 August. ‘I lost the opportunity,’ Challis wrote, ‘of announcing the discovery, by deferring the discussion of the observations, being much occupied with reductions of comet observations, and little suspecting that the indications of theory were accurate enough to give a chance of discovery in so short a time’ (Monthly Notices, xliii. 171). The elaborateness of his proceedings, in fact, while securing, postponed success, and left the prize to be grasped by a competitor, whose possession of Bremiker's map of that part of the heavens (Hora xxi.) rendered the planet's detection a matter of simple inspection and comparison. Three papers detailing the history of the discovery, by Airy, Challis, and Adams respectively, were read before the Royal Astronomical Society on 13 Nov. 1846, and printed in the sixteenth volume of their ‘Memoirs.’ Challis further drew up, at the request of the syndicate of the Cambridge observatory, a report on the subject, dated 12 Dec. 1846 (ib. xliii. 165); and a second, on his subsequent observations of Neptune, dated 22 March 1847 (Astr. Nach. xxv. 309).

The early sets of lectures delivered by Challis as Plumian professor (of which a syllabus appeared in 1838) were devoted to hydrodynamics, optics, and pneumatics, special attention being directed to the mathematical theories of light and sound. In 1843 he published a syllabus of a course on practical astronomy, which he continued to deliver until within a few years of his death, and issued from the University Press in 1879 with the title ‘Lectures on Practical Astronomy and Astronomical Instruments.’