MASON AND DIXON’S LINE, a line in the United States between Pennsylvania on the north and Delaware, Maryland, and West Virginia on the south, coinciding with 39° 43' 26'' ’ 3 N. lat., and famous for a long time as the limit between the “free” and the “slave” States. It derives its name from Charles Mason, F.R.S. (1730-87), and Jeremiah Dixon, two English astronomers who, between 1763 and 1767, surveyed the line for 244 miles west from the Delaware river, leaving only 36 miles of the Pennsylvania boundary to be fixed in 1782-84. This line must not be confounded, as has often been done, with the parallel of 36° 30' N. lat., which was assigned by the “Missouri compromise” of 1820 as the limit to the north of which slavery could not be introduced.