Synge returned to Trinity College Dublin, in 1925, where he was elected to a fellowship and was appointed the University Professor of Natural Philosophy (the old name for physics). He was a member of the American Mathematical Society and the London Mathematical Society. He was treasurer of the Royal Irish Academy in 1929. He went back to Toronto in 1930, where he was appointed Professor of Applied Mathematics and became Head of the Department of Applied Mathematics. In 1940, he supervised three Chinese students, Guo Yonghuai, Chien Wei-zang and Chia-Chiao Lin, who later became leading applied mathematicians in China and the United States. He spent some of 1939 at Princeton University, and in 1941, he was a visiting professor at Brown University. In 1943 he was appointed as Chairman of the Mathematics Department of Ohio State University. Three years later he became Head of the Mathematics Department of the Carnegie Institute of Technology in Pittsburgh, where John Nash was one of his students. He spent a short time as a ballistic mathematician in the US Air Force between 1944 and 1945. He returned to Ireland in 1948, accepting the position of Senior Professor in the School of Theoretical Physics at the Dublin Institute for Advanced Studies. This school had been set up in 1940, and had several outstanding members, including Erwin Schrödinger (who contributed to quantum mechanics), who was also a Senior Professor. His contributions Synge made outstanding contributions to different fields of work including classical mechanics, general mechanics and geometrical optics, gas dynamics, hydrodynamics, elasticity, electrical networks, mathematical methods, differential geometry, and Einstein's theory of relativity. He studied an extensive range of mathematical physics problems, but his best known work revolved around using geometrical methods in general relativity. He was one of the first physicists to seriously study the interior of a black hole, and his early work was cited by both Kruskal and Szekeres in their independent discoveries of the true (so-called maximal) structure of the Schwarzschild black hole. Synge's later derivation of the Szekeres-Kruskal metric solution, which was motivated by a desire to avoid "using 'bad' [Schwarzschild] coordinates to obtain 'good' [Szekeres-Kruskal] coordinates," has been generally under-appreciated in the literature, but was adopted by Chandrasekhar in his black hole monograph. In pure mathematics, he is perhaps best known for Synge's theorem, which concerns the topology of closed orientable Riemannian manifold of positive sectional curvature. When such a