MASON, Charles, astronomer, born in England in 1730; died in Philadelphia, Pennsylvania, in February, 1787. He served for several years as assistant to the astronomers royal at the Greenwich observatory, and was associated with Jeremiah Dixon in the observation of the transit of Venus on 6 June, 1761, at the Cape of Good Hope. In 1763 both gentlemen were commissioned to survey the boundary-line between Pennsylvania and Maryland by the respective proprietors of these colonies. They arrived in Philadelphia in November, 1763, and began their work, which was continued along the parallel of latitude 39º 43'*** 26"3" N., 244 miles west from the Delaware river, beginning at the northeast corner of Maryland, until they reached a point within thirty-six miles of the entire distance to be determined, when they were compelled to suspend operations in consequence of opposition by the Indians. At the end of every fifth mile a stone was planted, graven with the arms of the Penn family on one side and those of Lord Baltimore on the other. The intermediate miles were marked with smaller stones having a P on one side and an M on the other. All of these stones were sent out from England. Mason and Dixon returned to Philadelphia and were discharged in December, 1767. This line, known as "Mason and Dixon's line," became famous in the history of the United States as marking the northern limit, with the exception of portions of Delaware and Virginia, of the slave-states. Mason and Dixon devoted a month during 1766, at the request of the Royal astronomical society, to determining " the precise measure of a degree of latitude in America in the neighborhood of Pennsylvania," the particulars of which are printed in vol. lviii, of the "Transactions" of that body. In the same volume are to be found "Astronomical Observations made at the Forks of the Brandywine" for the purpose o5 " determining the going" of a clock sent thither by the Royal society in order to find the difference of gravity between the observatory at Greenwich and the spot where the clock was set up in Pennsylvania." Mason was a trained observer, and has recorded in his private journal, mingled with the original field-notes of the survey, not only the incidents of each day as they occurred, with the name of every person whose hospitality he shared, but accounts of the flora and fauna, the geological structure and the agricultural capabilities of the country, interspersed with notices of the Mohawk, Seneca, Delaware, and other Indians who served as his escort or whom he encountered on his route. He dwells with enthusiasm on the beauties of the scenery as viewed from the Alleghanies, and gives a tolerably correct account of the Mississippi valley that he obtained from an Indian chief. These journals were accidentally discovered at Halifax, Nova Scotia, in 1860, in a pile of waste paper in the cellar of the government-house where they had been thrown. Messrs. Mason and Dixon sailed for England, 9 September, 1768, and the following year Mason observed the transit of Venus on 3 June at Cavan, Ireland. He was subsequently employed by the Bureau of longitudes to verify the lunar tables of Tobias Mayer, and they were published after his death by Nevil Maskelyne under the title of " Mayer's Lunar Tables improved by Charles Mason" (London, 1781). Mason returned to this country, but at what date is unknown.