returning to the United States in 1928, Birch went back to Harvard to pursue physics. He was awarded his Master of Arts (A.M.) degree in 1929, and then commenced work on his 1932 Doctor of Philosophy (Ph.D.) degree under the supervision of Percy Bridgman, who would receive the Nobel Prize for Physics in 1946. For his thesis, Birch measured the vapor-liquid critical point of mercury. He determined this as 1460±20 °C and 1640±50 kg/cm2, results he published in 1932 in the Physical Review. Around this time, there was an increased interest in geophysics at Harvard University, and Reginald Aldworth Daly established a Committee for Experimental Geology and Geophysics that included Bridgman, astronomer Harlow Shapley, geologists Louis Caryl Graton and D. H. McLaughlin and chemist G. P. Baxter. William Zisman, another one of Bridgman's Ph.D. students, was hired as the committee's research associate, but, having little interest in the study of rocks, he resigned in 1932. The position was then offered to Birch, who had little interest or experience in geology either, but with the advent of the Great Depression, jobs were hard to find, and he accepted. On July 15, 1933, Birch married Barbara Channing, a Bryn Mawr College alumna, and a collateral descendant of the theologian William Ellery Channing. They had three children: Anne Campaspe, Francis (Frank) Sylvanus and Mary Narcissa. Frank later became a professor of geophysics at the University of New Hampshire. World War II In 1942, during World War II, Birch took a leave of absence from Harvard, in order to work at the Massachusetts Institute of Technology Radiation Laboratory, which was developing radar. He worked on the proximity fuze, a radar-triggered fuze that would explode a shell in the proximity of a target. The following year he accepted a commission in the United States Navy as a lieutenant commander, and was posted to the Bureau of Ships in Washington, D.C. Later that year he was assigned to the Manhattan Project, and moved with his family to Los Alamos, New Mexico. There he joined the Los Alamos Laboratory's Ordnance (O) Division, which was under the command of another Naval officer, Captain William S. Parsons. Initially the goal of the O Division was to design a gun-type nuclear weapon known as Thin Man. This proved to be impractical due to contamination of the reactor-bred plutonium with plutonium-240, and in February 1944, the Division switched its attention to