# Dictionary of National Biography, 1885-1900/Mason, Charles (1730-1787)

​MASON, CHARLES (1730–1787), astronomer, was James Bradley's assistant at Greenwich, with a salary of 26l. a year, from 1756 to 1760. He and Jeremiah Dixon were chosen by the Royal Society to observe the transit of Venus of 6 June 1761, at Bencoolen in the island of Sumatra; but H.M.S. Seahorse, in which they embarked in the autumn of 1760, was compelled by an attack from a French frigate to put back to Plymouth to refit, and they reached the Cape of Good Hope on 27 April, too late to proceed further. They, however, successfully observed the transit there, and on 16 Oct. reached St. Helena, where Mason co-operated with Nevil Maskelyne [q. v.] until December 1761 in collecting tidal data (Phil Trans., lii. 378, 534, 588, liv. 370). Mason and Dixon were next engaged by Lord Baltimore and Mr. Penn to settle the boundary between Maryland and Pennsylvania. Their survey, begun in 1763, extended 244 miles west from the Delaware River in latitude 39 43', and wanted only thirty-six miles of completion when stopped by Indian opposition in November 1767. 'Mason and Dixon's line' was long famous as separating the 'slave' from the 'free' States. They measured besides, at the expense of the Royal Society in 1764, an arc of the meridian in mean latitude 39 12'. No triangulation was employed; the line was measured directly with deal rods, the latitudes being determined with a zenith-sector by Bird. Notwithstanding great care in execution, the result was not satisfactory. The observations were presented to the Royal Society on 24 Nov. 1768, and were discussed by Maskelyne (ib. Iviii. 270, 323). Mason and Dixon observed in Pennsylvania in 1766-7 the variation of gravity from Greenwich, part of a lunar eclipse, and some immersions of Jupiter's satellites (ib. lviii. 329). They sailed from New York for Falmouth on 9 Sept. 1768.
Mason was employed by the Royal Society during six months of 1769 on an astronomical mission at Cavan in Ireland. He observed the second transit of Venus on 3 June (ib. lx. 488), the partial solar eclipse of 4 June, the phenomena of Jupiter's satellites, and in August and September the famous comet which signalised the birth year of Napoleon Bonaparte. After a tour in the highlands of Scotland under the same auspices in the summer of 1773, he recommended Schiehallion as the subject of Maskelyne's experiments on gravity (ib. lxv. 502). A catalogue of 387 stars, calculated by him from Bradley's observations, was annexed to the 'Nautical Almanac' for 1773, and he corrected Mayer's ' Lunar Tables,' on behalf of the board of longitude, in 1772, 1778. and 1780. The results of his comparisons of them with 1220 of Bradley's places of the moon were given in the 'Nautical Almanac' for 1774, and the finally revised 'Tables,' printed at London in 1787, continued long to be the best extant. The payment of 1,000l. for the work fell far short, according to Lalande (Bibl. Astr. p. 601), of Mason's expectations. He returned to America, and died at Philadelphia in February 1787. His manuscript journal and field-notes of 1763-7 were found in 1860 at Halifax, N.S., flung amidst a pile of waste paper into a cellar of Government House. With them was preserved a certificate of his admission in 1768 as a corresponding member of the American Society of Philadelphia. His associate, Dixon, said to have been born in a coal-mine, died at Durham in 1777. Mason's astronomical correspondence with Thomas Hornsby [q. v.] is preserved at the Radcliffe Observatory.
[Delambre's Histoire de l'Astronomie au xviiie Siècle, pp. 630, 634; Johnson's Universal Cyclopædia, iii. 333; Historical Magazine, v. 199, Boston, 1861 (an account of Mason's Journal by P. C. Bliss); Bradley's Miscellaneous Works, pp. lxxxix, xcii. (Rigaud); Philosophical Transactions, lii. 611 (Short); Mädler's Geschichte der Himmelskunde, i. 426, 490; Wolf's Geschichte der Astronomie, p. 619; Poggendorff's Biographisch-literarisches Handwörterbuch; Lalande's Astronomie, ii. 176; Watt's Bibl. Brit.; Bailly's Hist. de l'Astr. Moderne, iii. 41, 106.]
A. M. C.