JEANS, SIR JAMES HOPWOOD (1877— ), English mathematician, was born in London on Sept. 11, 1877. He was educated at Cambridge, where he was second wrangler in 1898 and Smith's Prizeman in 1900. He has held the posts of Stokes lecturer in applied mathematics at Cambridge and professor of ap plied mathematics at Princeton university and he was secretary of the Royal Society (1919-29). Jeans has applied mathe matics to many branches of physics and astronomy with marked success. In the kinetic theory of gases he has given proofs for the law of equipartition of energy and for Maxwell's law of distri bution of the velocity of molecules. Jeans developed a formula for the distribution of energy emitted by a black body; he used classical methods and arrived at a formula similar to that of Rayleigh which only holds for long wave-lengths. He has also written on other aspects of radiation such as the interaction between radiation and free electrons. In astronomy Jeans has applied mathematics to such good effect that a number of original theories on cosmogony have resulted. As an example, his work on the stability of pear-shaped figures may be taken; he con sidered such bodies rotating in an incompressible fluid, later he considered the fluid to be compressible. The same mathematical
analysis was then applied to problems of splitting due to tides produced by the approach of a second body and this in its turn was applied to the break-up of stars. Jeans also showed that Laplace's theory of the origin of the solar system was incorrect. Another problem of stellar dynamics which has claimed his atten tion is the effect of the gravitational attraction on the motion of stars. He has also written papers on the formation of binary stars ; on the nature of spiral nebulae ; on his theory of giant and dwarf stars; on the source of stellar energy; and on the evolution and radiation of gaseous stars.
Jeans is the author of Dynamical Theory of Gases (1904), Mathematical Theory of Electricity and Magnetism (1908), Problems of Cosmogony and Stellar Dynamics (1919), Radiation and the Quantum Theory (1914 and 1924), Atomicity and Quanta (1926), Astronomy and Cosmogony (1928), The Universe Around Us (1929). He was knighted in 1928.