HERO OF ALEXANDRIA, Greek geometer and writer on mechanical and physical subjects, probably flourished in the second half of the 1st century. This is the more modern view, in contrast to the earlier theory most generally accepted, according to which he flourished about 100 B.C. The earlier theory started from the superscription of one of his works, [Greek: Heronos Ktesibiou belopoiika], from which it was inferred that Hero was a pupil of Ctesibius. Martin, Hultsch and Cantor took this Ctesibius to be a barber of that name who lived in the reign of Ptolemy Euergetes II. (d. 117 B.C.) and is credited with having invented an improved water-organ. But this identification is far from certain, as a Ctesibius _mechanicus_ is mentioned by Athenaeus as having lived under Ptolemy II. Philadelphus (285-247 B.C.). Nor can the relation of master and pupil be certainly inferred from the superscription quoted (observe the omission of any article), which really asserts no more than that Hero re-edited an earlier treatise by Ctesibius, and implies nothing about his being an _immediate_ predecessor. Further, it is certain that Hero used physical and mathematical writings by Posidonius, the Stoic, of Apamea, Cicero's teacher, who lived until about the middle of the 1st century B.C. The positive arguments for the more modern view of Hero's date are (1) the use by him of Latinisms from which Diels concluded that the 1st century A.D. was the earliest possible date, (2) the description in Hero's _Mechanics_ iii. of a small olive-press with one screw which is alluded to by Pliny (_Nat. Hist._ viii.) as having been introduced since A.D. 55, (3) an allusion by Plutarch (who died A.D. 120) to the proposition that light is reflected from a surface at an angle equal to the angle of incidence, which Hero proved in his _Catoptrica_, the words used by Plutarch fitting well with the corresponding passage of that work (as to which see below). Thus we arrive at the latter half of the 1st century A.D. as the approximate date of Hero's activity.