Physicist and mathematician, born at Dalkeith; educated in Edinburgh; became senior wrangler at Cambridge, and Smith's prizeman in 1852; was in 1854 elected professor of Mathematics at Belfast, and in 1860 professor of Natural Philosophy at Edinburgh; has done a great deal of experimental work, especially in thermo-electricity, and has contributed important papers on pure mathematics; wrote, along with Lord Kelvin, "Treatise on Natural Philosophy," and along with Balfour Stewart "The Unseen Universe," followed by "Paradoxical Philosophy"; born 1831.