the development of the Little Boy, a smaller device using uranium-235. Birch used unenriched uranium to create scale models and later full-scale mock-ups of the device. Birch supervised the manufacture of the Little Boy, and went to Tinian to supervise its assembly and loading it onto Enola Gay, the Boeing B-29 Superfortress tasked with dropping the bomb. He devised the 'double plug' system that allowed for actually arming the bomb after Enola Gay took off so that if it crashed, there would not be a nuclear explosion. He was awarded the Legion of Merit. His citation read: Birch was promoted to commander and released from the Navy in 1945. Post-war Birch returned to Harvard after the war ended, having been promoted to Associate Professor of Geology in 1943 while he was away. He would remain at Harvard for the rest of his career, becoming a professor in 1946, and Sturgis Hooper Professor of Geology in 1949, and professor emeritus in 1974. Professor Birch published over 100 papers. He served as president of The Geological Society of America in 1964 and was awarded their Penrose Medal in 1969. In 1947, he adapted the isothermal Murnaghan equation of state, which had been developed for infinitesimal strain, for Eulerian finite strain, developing what is now known as the Birch-Murnaghan equation of state. Albert Francis Birch is known for his experimental work on the properties of Earth-forming minerals at high pressure and temperature, in 1952 he published a well-known paper in the Journal of Geophysical Research, where he demonstrated that the mantle is chiefly composed of silicate minerals, the upper and lower mantle are separated by a thin transition zone associated with silicate phase transitions, and the inner and outer core are alloys of crystalline and molten iron. His conclusions are still accepted as correct today. The most famous portion of the paper, however, is a humorous footnote he included in the introduction: Unwary readers should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth. A few examples of equivalents follow: In 1961, Birch published two papers on compressional wave velocities establishing a linear relation of the compressional wave velocity Vp of rocks and minerals of a constant average atomic weight with density as: This relationship became known as Birch's law. Birch was elected to the American Academy of Arts and Sciences in 1942,