Mason`s biography

[Daniel O'Hara]

Also from the OUP Dictionary of National Biography, here`s Mason`s
write-up; Dixon follows.

Dan O`Hara



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. 
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 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.