Sir George Stokes, 1st Baronet

George Stokes: The Waves That Nearly Drowned Him

His daughter recorded the origin story herself. "He was nearly carried away by one of these great waves when bathing as a boy off the coast of Sligo," Isabella Humphreys wrote of her father, "and this first attracted his attention to waves." George Gabriel Stokes spent the following seven decades on the problem, and by the end he had written the equations governing how fluids move, explained why clouds hang in the air, worked out why some substances glow under invisible light, and lent his name to a unit, a theorem, a lens, a spectral line and two craters. He held the Lucasian Professorship at Cambridge for fifty-four years — longer than anyone in the chair's history, Newton included.

A Rectory by the Sea

Stokes was born on 13 August 1819 at Skreen, County Sligo, the youngest son of the Reverend Gabriel Stokes, Church of Ireland rector of the parish, and Elizabeth Haughton. Three of his brothers entered the clergy; one, John, became Archdeacon of Armagh. He was especially close to his sister Elizabeth. The Atlantic coast of his childhood shaped his science as directly as any teacher.

He was schooled in Skreen, then Dublin, then Bristol, and matriculated at Pembroke College, Cambridge in 1837. In 1841 he graduated Senior Wrangler — top of the mathematical tripos — and Smith's Prizeman, and was immediately elected a fellow of Pembroke. He resigned the fellowship on marrying in 1857, as college statutes then required, was re-elected under revised statutes in 1869, and held it until he became Master of Pembroke in 1902, the year before his death.

The Physics of Moving Fluids

Between 1842 and 1851 Stokes essentially founded modern fluid dynamics. His papers on the motion of incompressible fluids, on friction in fluids and on internal friction converted a field of scattered empirical rules into a mathematical discipline. He used it to explain why clouds remain suspended and why waves subside, and to attack practical problems — the flow of water in rivers, the resistance encountered by ships.

Two results carry his name. The Navier–Stokes equations, to which he made seminal contributions, remain the fundamental description of fluid flow and one of the hardest unsolved problems in mathematics. Stokes' law gives the drag on a sphere moving slowly through a viscous medium, and thus its terminal velocity — the theoretical basis of the falling-sphere viscometer still used industrially to measure the thickness of fluids. The CGS unit of kinematic viscosity, the stokes, is named for him.

Light, and the Glow of Uranium Glass

From about 1845 Stokes turned to the wave theory of light. His 1849 paper on diffraction demonstrated that the plane of polarization must lie perpendicular to the direction in which light travels — a decisive constraint on what light physically is.

His most celebrated optical work came in 1852 with the paper "On the change of refrangibility of light." Observing fluorspar and uranium glass, Stokes described and named fluorescence: the conversion of invisible ultraviolet radiation into visible light of longer wavelength. The energy shift involved is called the Stokes shift, and the phenomenon underpins everything from fluorescent lighting to the labelled molecules that make modern biology visible.

He kept going for thirty years. He studied rainbows in the 1850s, the colours of thick plates in 1851, metallic reflection by non-metallic substances in 1853, light intensity through stacked plates in 1860, and in 1862 alone published on double refraction, the spectrum of electric light and the absorption spectrum of blood. In 1878 he established theoretical limits on microscope aperture. In 1849 he invented the Stokes lens, a cylindrical-lens device for detecting astigmatism.

One opportunity he let pass. Before the summer of 1852 Stokes taught Lord Kelvin how prismatic analysis of light could reveal the chemistry of the sun and stars — anticipating Gustav Kirchhoff by seven or eight years. But he had not grasped that emission at a given wavelength requires absorption at the same wavelength, and he later disclaimed "any part of Kirchhoff's admirable discovery." English scientists credited him with the principles anyway.

Mathematics, and Divergent Series

Stokes' theorem, the identity binding the integral of a field over a surface to the behaviour of that field on its boundary, bears his name because he popularized it; it became one of the load-bearing results of vector calculus. Investigating George Airy's treatment of rainbows, he expressed an unwieldy integral as a divergent series and truncated it cleverly to obtain accurate approximations — an early insight into asymptotic expansions, one of applied mathematics' most productive controlled cheats.

Bridges That Fell Down

Victorian Britain built its railways faster than it understood the materials, and Stokes was called in when they failed. After the Dee Bridge collapse near Chester in May 1847 he investigated the failure of a cast-iron beam and served on the resulting Royal Commission, calculating the forces a moving engine exerts on a bridge. After the Tay Bridge disaster of 28 December 1879, when a storm destroyed the High Girders section while an express train was crossing and more than seventy-five people died, he appeared as an expert witness on wind loading. The board of inquiry found the bridge "badly designed, badly built and badly maintained." Stokes was appointed to the follow-up Royal Commission on wind pressure, which measured storm winds across the country to establish what forces structures actually had to survive.

Faith Without Fudging

Stokes remained a conservative evangelical Protestant throughout his life. In 1886 he became president of the Victoria Institute, founded to defend evangelical Christianity against Darwinian evolution; in 1891 he delivered the Gifford Lectures on natural theology, and he was vice-president of the British and Foreign Bible Society. His position was more subtle than the affiliations suggest. "We all admit," he wrote as president of the Institute, "that the book of Nature and the book of Revelation come alike from God, and that consequently there can be no real discrepancy between the two if rightly interpreted." He stressed that scientific evidence is "probable only" and liable to revision — while warning equally against dismissing revelation. Unlike most Victorian evangelicals he rejected eternal punishment, holding instead to Christian conditionalism.

The Gatekeeper

Stokes was Lucasian Professor from 1849 to 1903, Secretary of the Royal Society from 1854 to 1884, its President from 1885 to 1890, and Conservative MP for Cambridge University from 1887 to 1892 — the first man to hold the professorship, the presidency and a parliamentary seat simultaneously. Thirty years as Secretary gave him, in one assessment, "an enormous if inconspicuous influence on the advancement of mathematical and physical science." He solved other people's mathematical difficulties and handed out research problems; historians call him a gatekeeper of Victorian science.

He was made a baronet in 1889 and won the Rumford Medal in 1852, the Hopkins Prize in 1870 and the Copley Medal in 1893. On 1 June 1899 his golden jubilee as Lucasian Professor drew delegates from across Europe and America; Kelvin presented marble busts by Hamo Thornycroft.

Family and End

On 4 July 1857 Stokes married Mary Susanna Robinson, daughter of the Irish astronomer Thomas Romney Robinson, at St Patrick's Cathedral in Armagh. Of five children, two daughters died in infancy; his son William, a physician, killed himself at thirty while temporarily insane. Stokes died on 1 February 1903, aged eighty-three, and was buried in Mill Road Cemetery, Cambridge, with a memorial at Westminster Abbey. The male line failed and the baronetcy became extinct — but the stokes, the Stokes shift, the Stokes line, craters on the Moon and Mars, and the equations still defeating mathematicians have proved harder to extinguish.