Charles Mason Biography
From the Dictionary of National Biography
MASON, CHARLES (1730-1786), astronomer, was James Bradley's assistant at Greenwich, with a salary of 26 pounds 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 until Dec 1761 in collecting tidal data.
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. lviii. 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 in 1769 on an astronomical mission at Cavan in Ireland. He observed the second transit of Venus on 3 June, partial solar eclipse of 4 June, the phenomona of Jupiter's satellites, and in August and September the famous comet which signalised the birth year of Napoleon Bonaparte. After a tour of 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. A catalogue of 387 stars, calculated by Mason from Bradley's observations, was annexed to the Nautical Almanac for 1773, and he corrected Mayer [German mathematician] "Lunar Tables', on behalf of the Board of Longitude (Maskelyne as serving head), in 1772, 78, and 80. Results of his comparisons with 1220 of Bradley's places of the moon were given in the "Nautical Almanac' for 1774, and finally revised 'Tables', printed in London in 1787, continued long to be the best extant. Payment of 1,000 pound for the work fell far short, according to Lalande, of Mason's expectations. He returned to America, and died in Philadelphia in 1787. His journals were almost tossed, also found a certificate of admission into American Society of Philadelphia. Dixon was reportedly born in a coal mine, died at Durham in 1777.
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