Forbes, James David

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Forbes, James David

(b. Edinburgh, Scotland, 20 April 1809; d. Clifton, Scotland, 31 December 1868)

physics, geology.

Forbes’s discovery of the polarization of radiant heat strengthened the belief in the identity of thermal and luminous radiation and contributed to the development of the concept of a continuous radiation spectrum. His detailed studies of glaciers aided in the establishment of modern theories of their formation and movement.

Forbes was the youngest son of William Forbes, seventh baronet of Pitsligo, and Wilhelmina Belches, only child and heiress of John Belches Stuart of Fettercairn. Forbes was educated privately until the age of sixteen and developed an early interest in science. He entered the University of Edinburgh in 1825 to pursue legal studies in accordance with the wishes of his father. He distinguished himself in the natural philosophy courses offered by John Leslie and contributed several anonymous papers to David Brewster’s Philosophical Journal. Upon revealing his authorship, he was encouraged in his scientific work by Brewster, who proposed his membership to the Royal Society of Edinburgh. Forbes was elected to this body upon reaching the age of twenty-one. He received a modest inheritance at his father’s death in 1828 and abandoned law in favor of science. He rapidly became acquainted with prominent British scientists and was elected to the Royal Society of London in 1832. In the same year he participated actively in the formation of the British Association.

Forbes was appointed as Leslie’s successor to the chair of natural philosophy at Edinburgh in 1833, despite his youth and relative scientific inexperience. This controversial election, in which Brewster was his principal opponent, caused a rift in their friendship which lasted several years. In 1830 Forbes had begun to investigate radiant heat phenomena without significant results. After learning of Melloni’s successful detection of the refraction of thermal radiation, he visited Paris in 1833 to confirm Melloni’s results, whereupon he requested Melloni to supervise the manufacture of a thermopile for use in his own investigations.

With this instrument Forbes, in November 1834, discovered the polarization of radiant heat by transmission through tourmaline and thin mica plates and by reflection through the latrer. Using mica for dipolarization, he was successful in demonstrating the double refraction of thermal radiation, and in 1836 he found that heat could be circularly polarized by two reflections in a Fresnel rhomb of rock salt. He received the Rumford Medal of the Royal Society of London in 1838 for these discoveries and in 1845 was granted a royal pension for his scientific work.

After 1840 Forbes’s central interest was geology, although he continued sporadically to study heat conduction in solids and various types of soils, as well as the effects of the atmosphere on solar radiation. He had become an experienced mountaineer during geological field trips in the Pyrenees, in southeastern France, and in the Pennine chain. He turned his attention to the glaciers of the Alps of Savoy when invited there by Agassiz in 1841 on an exploratory expedition. Forbes’s early publications on glaciers, claiming priority in noting their veined structure and in demonstrating that the center of a glacier moves faster than its sides, occasioned controversy with Agassiz and others that did not terminate even at Forbes’s death.

Forbes pursued detailed studies of the glaciers of the Alps and of Norway during many subsequent summers. He determined that the surface of a glacier moves faster than the ice vertically beneath it and that the velocity of a glacier increases directly with the steepness of its bed. He postulated that a glacier is a viscous body whose movement is due to the mutual pressure of its parts.

Forbes energetically supported the scientific institutions of his time. He served as secretary of the Royal Society of Edinburgh from 1840 to 1851, and he was obliged to decline its presidency in 1867 and that of the British Association in 1864 owing to ill health. He was a corresponding member of many European scientific societies, and he carried on a voluminous correspondence with the leading scientists of the British Isles and Europe. During his tenure at Edinburgh, Forbes sought successfully to reform the Scottish system of higher education by instituting examinations for degrees. He resigned from his chair at Edinburgh in 1860 after his election as principal of the United College of St. Andrews, a position he held until his death.


I. Original Works. Forbes’s works include Address to the British Association, 4th General Meeting at Edinburgh (Edinburgh, 1834); Travels Through the Alps of Savoy (Edinburgh, 1843); The Dangers of Superficial Knowledge (London, 1849); Norway and Its Glaciers (Edinburgh, 1853); “A Review of the Progress of Mathematical and Physical Science in More Recent Times, and Particularly Between the Years 1775 and 1850,” in Encyclopaedia Britannica, 8th ed. (London, 1853; repr. separately, Edinburgh, 1858); and Occasional Papers on the Theory of Glaciers (Edinburgh, 1859).

Forbes also published over 100 scientific papers. His extensive incoming and outgoing correspondence, several journals and notebooks, and his competent watercolors of Alpine landscapes are in the archives of the University of St. Andrews.

II. Secondary Works. See J. C. Shairp, P. G. Tait, and A. Adams-Reilly, The Life and Letters of James David Forbes (London, 1873); John Tyndall, Principal Forbes and His Biographers (London, 1873); and George E. Davie, The Democratic Intellect: Scotland and Her Universities in the Nineteenth Century (Edinburgh, 1961).

John G. Burke