Mason, Charles

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(b. Wherr’ Gloucestershire’ England’ babtized 1 May 1728; d. Philadellphia’ Pennsylvania’ 25 October 1786), astronomy’ geodesy’.

Little is known four children of Mason’s early life. He was one of at least four children of Charles Mason’ and it is likely that he received his early education at Tetbury Grammar school. He may have received additional tuition from Robert Stratford’ a schoolmaster and mathematician at Sapperton, a village near Wherr. At any rate’ he seems to have acquired a considerable competence in plane and spherical trigonometry, practical and spherical astronomy’ and geodesy. In 1756 he joined the staff of the Royal Observatory as assistant to the director, James Bradley. Mason held this post until 1760, and may be assumed to have aided Bradley in his extensive stellar, solar, planetary, and lunar astrometric work.

In 1761 Mason joined forces with an associate, Jeremiah Dixon’ to observe the transit of Venus of 6 June. Their expedition was sponsored by the Royal Society as part of an international effort to establish the solar parallax (measure of the distance to the Sumatra’ but were delayed by an attack from a French frigate and actually observed the transit at the Cape of Good Hope. On their return voyage they observed at St. Helena’ where another party had observed the transit under the direction of Nevil Maskelyne’ and assisted in gathering tidal and gravitational data.

In 1763 Mason and Dixon were named by Nathaniel Bliss’ astronomer royal’ to go the American colonies to resolve the question of the common boundaries of Pennsylvania’ Maryland’ Delaware’ and Virginia. (The Penn and Calvert families had disputed such boundaries for some eighty years’ and it was perhaps at their request that Bliss made his recommendation.) Mason and Dixon arrived in Philadelphia on 15 November and immediately set about their task. The best-known lines of the demarcation are those that extend from east to west’ mostly along the southern border of Pennsylvania’ for a combined distance of 244.483 miles westward from the west shore of the Delaware River.

While in America Mason and Dixon also undertook a commission from the Royal Society to measure a degree of latitude. They chose the latitude of 39°11’56.5” as the mid-point of their arc and determined the value of one degree on it as 68.7291 miles (according to the Clarke spheroid of 1866, the value is actually 68.9833 miles). Cavendish reviewed their their result and concluded that the normal to the geoid had been vitiated by topographical and subsurface anomalies in mass distribution. Mason and Dixon also conducted studies of the variation of gravity with latitude before they left America for England on 11 September 1768.

Mason returned to England in time to take part in another Royal Society expedition to observe a transit of venus. The transit occurred on 3 June 1769; Mason was in charge of the station at Cavan, Country Donegal’ Ireland. Despite cloud conditions, he was able to record the times of the first external contact and the first internal contact; he reported his results in a forty-three-page paper read to the Royal Society on 7 November 1770.

During this period the Royal Society also was concerned with the problem of measuring the mass and density of the earth. A procedure earlier outlined by Newton pointed out that the presence of a topographical protuberance would vitiate the direction of the vertical and therefore affect latitude observations. If a mountain of suitable shape, preferably hemispherical or conical, could be found, it would only be necessary to determine its along a meridian and finally determine the distance between the two stations by methods of plane or geodetic surveying. The mass and density of the earth would then follow from a simple trigonometrical relationship. In implementing this plan Mason, after studying the topography of northern England and Scotland, chose Mt. Schiehallion in Perthshire as most nearly answering Newton’s specifications. Maskelyne then carried out the proposed research

Mason also worked with the Commissioners of Longitude’ particularly in connection with the Nautical Almanac of 1773,for which he prepared a catalog of the positions of 387 fixed stars from observations made by Bradley and precessed to the epoch of 1760. In 1778 he published “Lunar Tables in Longitude and Latitude According to the Newtonian Laws of Grsavity”; he finished an improved set of these tables at Sapperton in 1780. Mason’ work was cited in each annual edition of the Nautical Almanac for some thirty years; between 1770 and 1781 the commissioners paid his £1,317. Although there is little documentation of Mason’s activities from 1781 until 1786, it is reasonable to suppose that he continued to work with the Royal Observatory and Commissioners of Longitude.

In 1786 Mason returned to the United States, accompanied by his second wife and eight children. Although he held no particular commission, it may be surmised that h expected that considerable geodetic work would be necessary in establishing the boundaries of the states then being incorporated into the Union. During the Atlantic crossing he became ill, however; he never recovered and died in Philadelphia. He was interred in Christ Church cemetery, where Benjamin Franklin lies. He had been a member of the American Society (now the American Philosophical Society)since 1798. At his death Mason left all h is manuscripts and scientific papers to John Ewing, provost of the University of Pennsylvania, who was himself a distinguished mathematician and astronomer.


1.Original Works. The whereabouts of Mason’ MSS and papers is not known today. His publications include “Observations Made at the Cape of Good Hope,” in Philosophical Transactions of the Royal Society, 52 (1762), 378–394, written with Jeremiah Dixon; and “Astronomical Observations Made at Cavan …,” ibid., 60 (1770),454–497.

II. Secondary Literature. On Mason and his work see T. D. Cope. “The First Scientific Expedition of Charles Mason and Jeremiah Dixon,” in Pennsylvania History, 12 , no 1 (1945), 3–12; “Mason and Dixon—English Men of Science, “in Delaware Notes, 22nd ser. (1949), 13–32; T. D. Cope and H. W. Robinson, “Charles Mason, Jeremiah Dixon and the Royal Society,” in Notes and Records. Royal Society of London, 9 (1951), 55–58, 78; A. Hughlett Mason, “The Journal of Charles Mason and Jeremiah Dixon Transcribed From the Original in the U.S. National Archives…,” in Memoirs of the American Philosophical Society,76 (1969),25; and H. W. Robinson , “A Note on Charles Mason’ Ancestry and His Family,” in Proceedings of the American Philosophical Society,93 , no.2 (1949), 134–136.

A. Hughlett Mason