Page:Dictionary of National Biography, Third Supplement.djvu/468

 divided by 4 π. The line which represents in direction and magnitude the flow of energy at any point is now known as the ‘Poynting vector’ and is of fundamental importance in electromagnetic questions.

Poynting made important advances in our knowledge of the pressure of light. He established the existence of the tangential force produced when light is reflected from a surface at which there is some absorption, and the existence of a torque when light passes through a prism, and succeeded in demonstrating the recoil from light of a surface giving out radiation. These experiments, in which he was associated with William Henry Barlow [q.v.], are a good example of Poynting's skill in devising methods and apparatus. He had exceptional mechanical instincts and an excellent knowledge of the capabilities of instruments. The result was that the apparatus which he designed was always simple and effective.

Throughout his life Poynting was engaged on researches connected with gravitation. His first piece of experimental work was a determination of the density of the earth, using, instead of a torsion balance, a balance of the ordinary type. He also investigated the question whether the gravitational attraction between two crystals depends on the orientation of their axes, and whether this attraction is affected by temperature. He further made important contributions to the theory of the change of state in matter. He took great interest in the philosophical basis of physics, chose this as the subject of his presidential address to Section A of the British Association in 1899, and expressed views which, though now common, were then different from those accepted by the majority of the physicists of this country.

Poynting's Collected Scientific Papers were published by the Cambridge University Press in 1920. In addition to these he wrote On the Mean Density of the Earth (Adams prize essay, 1893), The Pressure of Light (1910), The Earth (1913), and, in conjunction with (Sir) J. J. Thomson, a series of textbooks on physics. Poynting was very successful as a teacher, and his sound judgement and conspicuous fairness and courtesy were of great service to the university of Birmingham. He became dean of the science faculty when Mason College was made the university of Birmingham, and held the office for twelve years. He died at Birmingham 30 March 1914.

Poynting married in 1880 Maria Adney, daughter of the Rev. J. Cropper, Unitarian minister, of Stand, near Manchester. They had one son and two daughters.

 PREECE, WILLIAM HENRY (1834–1913), electrical engineer, was born at Bryn Helen, Carnarvon, 15 February 1834. He was the eldest son of Richard Mathias Preece, stockbroker, of Bryn Helen, by his wife, Jane, daughter of John Hughes, shipbuilder, of Carnarvon. He was educated at King's College School and King's College, London, and received his early training in electrical engineering at the Royal Institution under Michael Faraday [q.v.], who directed his inherently scientific mind towards the many unsolved problems of applied electricity and telegraphic engineering. In 1852 Preece entered the office of Edwin Clark [q.v.], as a civil engineer; but in 1853 he was appointed to the Electric and International Telegraph Company, becoming superintendent of its southern district in 1856. From 1858 to 1862 he was engineer to the Channel Islands Telegraph Company. Preece is, however, best known on account of his long connexion with the Post Office, of which he first became an official in 1870. The various telegraphic companies were at that date taken over by the government and Preece was appointed divisional engineer for the southern district of the Post Office telegraphic system. In 1877 he was made electrician in chief, and in 1892 engineer in chief. He retired from the latter position in 1899 and from that time until 1904 was consulting engineer to the British Post Office and to the Colonies.

The scientific field explored by Preece in the course of his career was extremely wide and covered telegraphy, telephony, and radio-telegraphic communication. During his career at the Post Office he was responsible for many improvements and inventions in telegraphic work. He also applied his experience to the question of railway signalling; and he regarded the improvements which he made to secure the safe working of railways as among his most useful work. He introduced the Preece block system of working single lines and the electric system of communication between different parts of a train; he also took out a patent for reproducing by miniature signals in the signal box the positions of actual signals, and a system of locking signals. Preece was one of the earliest pioneers of wireless telegraphy, and in 1892 originated 442