HERON, or HER°, a mathematician and natural philosopher of Alexandria, was the pupil of Ctesibius, and flourished probably about a century or a century and a half before Christ. His name has been preserved in the well-known experiment of Hero's fountain, in which, by means of condensed air, water is made to spring from a jet in a continuous stream. Several of Heron's writings are entirely lost, and of those that remain some have never been printed. His most valuable work is that on Pae2CaaliCS, in which are given his experiments on the elasticity of the air and of steam. His Mechanics and Barnleus treat of the subjects which would now he comprised in an elementary book on dynamics. At the end of his Catapeltica or Belopoietica, which are probably the same, occurs Heron's solution of the ancient and much discussed problem to find two mean proportionals between two given straight lines. Cheirobalistra, Cambestria, Camarica, Automata, are the titles of some of his other physical works. His mathematical works (see Hultsch's Heron's Alexandrini Geometricorum et Stereometricorum Relignice, Berlin, 1864) are very fragmentary, and it is difficult to determine whether several of them are not to be attributed to later and anonymous writers. Heron seems to have been the first to show how the area of a triangle may be found from its three sides. See two papers by Vincent and Boncompagni in Bullettino di bibliogr. e di storia delle scienze matenz. e fisiche, iv.