British mathematician, astronomer, geophysicist, and philosopher Sir Harold Jeffreys (1891-1989) was one of the great original applied-mathematical thinkers of the 20th century. He is noted for his wide variety of scientific contributions.

Harold Jeffreys was born on April 22, 1891, in Durham, England. He graduated from Durham University and then carried out research in chemistry and photography. He won major prizes at the University of Cambridge, from which he graduated in 1917, in mathematics, astronomy, and geophysics. At Cambridge he was a lecturer in mathematics, reader in geophysics, and Plumian professor of astronomy and experimental philosophy from 1946 to his retirement in 1958. In 1940 he married Bertha Swirles, a talented mathematician and a fellow of Girton College, Cambridge.

## Scientific Contributions

The contributions for which Jeffreys is noted cover a wide range of fields. Much of his interest centered on the solar system and the theory of geophysics, fields in which progress demands the use of evidence and techniques from a variety of other fields, for example, statistical techniques and mathematical methods. It was characteristic of Jeffreys that when he found it necessary to refer to fields outside of astronomy and geophysics, he usually made important contributions to those fields as well. Noted examples are to be found in his books on the theory of probability, scientific inference, operational calculus, Cartesian tensors, and asymptotic approximations, as well as in a large treatise on the methods of mathematical physics written jointly with his wife. All of these works contain much original material inspired by the needs of his work in astronomy and geophysics. Additionally, he made significant contributions to the general theory of dynamics, aerodynamics, meteorology, relativity theory, and plant ecology.

To Jeffreys's credit are a number of outstanding achievements in astronomy and geophysics. In 1923, Jeffries calculated the surface temperatures of the four large outer planets—Jupiter, Saturn, Uranus, and Neptune—to be more than 100° below zero Centigrade. This was in sharp conflict with the then-prevailing view that these outer planets were red-hot. His findings were later verified by direct observation and led to a complete revision of theories on the composition and structure of the outer planets.

Using observations on the earth's bodily tides, Jeffreys, in 1926, gave the first quantitative estimate of the rigidity of the earth's core and established that most of the core is probably molten. He was the senior author of tables produced during 1930-1940, giving the travel times of earthquake waves through the interior of the earth. Since 1940 these tables have been used as the standard in calculating the epicenters and origin times of the world's earthquakes for the International Seismological Summary. Jeffreys also contributed notably to theories of seismic wave propagation, of mutations of the earth's axis, on mountain building, on convection currents inside the earth, on tidal problems, on the figure of the earth and the moon, and a theory on the internal structures of other terrestrial planets.

Knighted for his outstanding contributions to the scientific community in 1953, Jeffreys spent his career at Cambridge until his retirement in 1958. He died in Durham on March 18, 1989.

## Further Reading on Sir Harold Jeffreys

For references to Jeffreys and for background see Ruth Moore, *The Earth We Live On* (1956).