Forbes, JAMES DAVID

Forbes, JAMES DAVID, physicist, was the son of Sir William Forbes of Pitsligo, and grandson of the Sir William given below. Born at Edinburgh, April 20, 1809, he studied in the university there from 1825 until 1830, when he was called to the Scottish bar. But the physical sciences were from an early age serious rivals to the law in his affections. From 1833 he held the chair of Natural Philosophy in Edinburgh University, exchanging it in 1859 for the principalship of the United College in the university of St Andrews. Among his contributions to science are his investigations on the polarisation of radiant heat by the tourmaline, and also by reflection, and its circular polarisation (1834)—discoveries forming some of the strongest proofs of the identity of calorific and luminous rays; the unequal polarisation of heat from different sources (1844); the conductivity of heat by iron; the refrangibility of heat; the depolarisation of heat; underground temperatures, &c. He is, however, best known by his researches on the motion of glaciers, in connection with which subject he wrote Travels through the Alps (1843), Norway and its Glaciers (1853), Tour of Mont Blanc and Monte Rosa (1855), and Occasional Papers on the Theory of Glaciers (1859). He was the first to establish the great fact that glacier ice moves in its channel like a viscous fluid, the middle moving faster than the sides, and the upper portions faster than the lower. In meteorology Forbes, among other things, improved Wollaston's application of the thermometer to the determination of heights, and verified Fourier's theoretical results concerning the temperature of the ground at different depths and in different kinds of soil and rock. He also contributed numerous papers on astronomy and other subjects to the Transactions of the Royal Societies of London and Edinburgh, to the Edinburgh Philosophical Journal, and similar periodicals. He died December 31, 1863. See his Life and Letters, edited by Shairp, Tait, and Adams Reilly (1873).