(b. Chester, England, 18 July 1689; d. Kew, England, 13 April 1728)
Samuel Molyneux was the only son of William Molyneux to survive infancy. His mother died in 1691 and he was raised by his father, who zealously undertook his education on Lockean principles. His father died in 1698, leaving him to the care of his uncle, Thomas Molyneux. He entered Trinity College, Dublin, when he was sixteen, and there formed a friendship with George Berkeley, who dedicated his Miscellanea Mathematica to him in 1707. Molyneux received the B.A. in 1708, and the M.A. in 1710. In 1717 he married Lady Elizabeth Capel, who inherited a large sum of money and a residence outside London, Kew House, in 1721. Molyneux was thus able to devote himself to the study of astronomy and optics. He was elected fellow of the Royal Society in 1712.
Molyneux’s most important astronomical investigations were undertaken in collaboration with his close friend James Bradley. In 1725 the two scientists decided to examine for themselves the validity of Robert Hooke’s supposed detection of a large parallax for γ Draconis. To this end they ordered a large zenith sector with a radius of twenty-four feet from George Graham, the distinguished London instrument maker. The sector was set up on 26 November 1725 at Molyneux’s residence, passing through holes in the ceilings and roof. Observations of γ Draconis on 3, 5, 11, and 12 December 1725 did not, however, reveal any change in the apparent position of the star.
Bradley observed the star again on 17 December “chiefly through curiosity,” and to his great surprise found that it had moved southward, in the opposite direction to that which would arise from the projected parallax. Observations performed throughout the next twelve months revealed that the star exhibited an annual circular movement. Anxious to ascertain the laws of this phenomenon and to discover its physical cause, Bradley had Graham construct a more versatile sector than Molyneux’s—one having a larger angular range. Molyneux helped set up this instrument at Bradley’s aunt’s residence at Wanstead on 19 August 1727. By 29 December 1727, Bradley had completed the observations necessary for his discovery of the aberration of light. The two scientists further worked together from 1723 to 1725 to improve methods of constructing reflecting telescopes; their efforts here did much to help bring reflecting telescopes into more general use.
In addition to his scientific activities, Molyneux pursued an active and noteworthy career in politics. He was a member of the English parliaments of 1715, 1726, and 1727, and a member of the Irish parliament of 1727. On 29 July 1727 he was appointed a lord of the admiralty, in which office he devised several schemes for the improvement of the navy. It was probably because of the pressure of public business arising from this appointment that he was unable to continue his astronomical observations after helping Bradley to set up his sector. Kew House was demolished in 1804 and a sundial, erected by William IV in 1834, now commemorates the observations made there.
I. Original Works. Molyneux’s writings include “A Relation of the Strange Effects of Thunder and Lightning, which Happened at Mrs. Close’s House at New-Forge, in the County of Down in Ireland, on the 9th of August, 1707,” in Philosophical Transactions of the Royal Society, 26 (1708), 36–40; “Sectio Oculorum Duorum Cataraetâ Affectorum,” ibid., 33 (1724), 149–150; “The Method of Grinding and Polishing Glasses for Telescopes, Extracted from Mr. Huygens and Other Authors,” in Robert Smith, A Compleat System of Optics (Cambridge, 1738), 281–301; “The Method of Casting, Grinding and Polishing Metals for Reflecting Telescopes, Begun by the Honourable Samuel Molyneux Esquire, and Continued by John Hadley Esquire, Vice-President of the Royal Society,” ibid., 301–312; “Sir Isaac Newton’s Reflecting Telescope Made and Described by the Honourable Samuel Molyneux Esquire, and Presented by Him to His Majesty John V. King of Portugal: with Other Kinds of Mechanisms for This and for Mr. Gregory’s Reflecting Telescope,” ibid., 363–368; “A Description of an Instrument Set up at Kew, in Surrey, for Investigating the Annual Parallax of the Fixed Stars, with an Account of the Observations Made Therewith,” in James Bradley, Miscellaneous Works and Correspondence of the Rev. James Bradley, S. P. Rigaud, ed, (Oxford, 1832), 93–115; and “Observations Made at Kew,” ibid., 116–193, which includes observations by James Bradley after 22 April 1726.
II. Secondary Literature. An excellent account of Molyneux’s education can be gleaned from the extensive exchange of letters between William and Thomas Molyneux and John Locke in The Works of John Locke, IX (London, 1823), 289–472. See also the article on Molyneux by Agnes M. Clerke in Dictionary of National Biography.