1818: The U.S. and Great Britain established the boundary between the U.S. and Canada to be the 49th parallel. 1891: English Nobel Prize winner James Chadwick, discoverer of the neutron, was born. 1944: General Douglas MacArthur stepped ashore at Leyte in the Philippines, 2 1/2 years after he had said, ''I shall return.'' 1956: American aircraft designer Lawrence Bell, founder of Bell Aircraft (maker of the Bell X-1 the first broke the sound barrier), died. 1960: The length of the meter was redefined to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom. 1960: The first fully mechanized post office opened in Providence, RI. 1972: American astronomer Harlow Shapley, who discovered the sun's position in the Milky Way galaxy, died. 1983: IBM-PC DOS Version 2.1 was released. 1984: English physicist Paul Dirac, who shared a Nobel Prize with Erwin Schrödinger, and after whom the Dirac delta function is named, died.