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21 Hero of Alexandria
SUBJECT AREA: Architecture and building, Mechanical, pneumatic and hydraulic engineering, Photography, film and optics, Steam and internal combustion engines[br]fl. c.62 AD Alexandria[br]Alexandrian mathematician and mechanician.[br]Nothing is known of Hero, or Heron, apart from what can be gleaned from the books he wrote. Their scope and style suggest that he was a teacher at the museum or the university of Alexandria, writing textbooks for his students. The longest book, and the one with the greatest technological interest, is Pneumatics. Some of its material is derived from the works of the earlier writers Ctesibius of Alexandria and Philo of Byzantium, but many of the devices described were invented by Hero himself. The introduction recognizes that the air is a body and demonstrates the effects of air pressure, as when air must be allowed to escape from a closed vessel before water can enter. There follow clear descriptions of a variety of mechanical contrivances depending on the effects of either air pressure or heated gases. Most of the devices seem trivial, but such toys or gadgets were popular at the time and Hero is concerned to show how they work. Inventions with a more serious purpose are a fire pump and a water organ. One celebrated gadget is a sphere that is set spinning by jets of steam—an early illustration of the reaction principle on which modern jet propulsion depends.M echanics, known only in an Arabic version, is a textbook expounding the theory and practical skills required by the architect. It deals with a variety of questions of mechanics, such as the statics of a horizontal beam resting on vertical posts, the theory of the centre of gravity and equilibrium, largely derived from Archimedes, and the five ways of applying a relatively small force to exert a much larger one: the lever, winch, pulley, wedge and screw. Practical devices described include sledges for transporting heavy loads, cranes and a screw cutter.Hero's Dioptra describes instruments used in surveying, together with an odometer or device to indicate the distance travelled by a wheeled vehicle. Catoptrics, known only in Latin, deals with the principles of mirrors, plane and curved, enunciating that the angle of incidence is equal to that of reflection. Automata describes two forms of puppet theatre, operated by strings and drums driven by a falling lead weight attached to a rope wound round an axle. Hero's mathematical work lies in the tradition of practical mathematics stretching from the Babylonians through Islam to Renaissance Europe. It is seen most clearly in his Metrica, a treatise on mensuration.Of all his works, Pneumatics was the best known and most influential. It was one of the works of Greek science and technology assimilated by the Arabs, notably Banu Musa ibn Shakir, and was transmitted to medieval Western Europe.[br]BibliographyAll Hero's works have been printed with a German translation in Heronis Alexandrini opera quae supersunt omnia, 1899–1914, 5 vols, Leipzig. The book on pneumatics has been published as The Pneumatics of Hero of Alexandria, 1851, trans. and ed. Bennet Wood-croft, London (facs. repr. 1971, introd. Marie Boas Hall, London and New York).Further ReadingA.G.Drachmann, 1948, "Ktesibios, Philon and Heron: A Study in Ancient Pneumatics", Acta Hist. Sci. Nat. Med. 4, Copenhagen: Munksgaard.T.L.Heath, 1921, A History of Greek Mathematics, Oxford (still useful for his mathematical work).LRD -
22 Joy, David
[br]b. 3 March 1825 Leeds, Englandd. 14 March 1903 Hampstead, London, England[br]English mechanical engineer, designer of the locomotive Jenny Lind and of Joy's valve gear for steam engines.[br]By the mid-1840s Joy was Chief Draughtsman at E.B.Wilson's locomotive works at Leeds. During that period, attempts by engineers to design ever larger and more powerful locomotives were producing ungainly types, such as the long-boiler and the Cramp ton, which were to prove blind alleys in locomotive development. Joy rediscovered the proper route with his Jenny Lind 2–2–2, built in 1847. His locomotive had minimal overhang, with the firebox between the driving and trailing axles; the driving axle supported inside frames which stopped short at the firebox, allowing the latter to be wide, while leading and trailing wheels were held by outside plate frames which had a degree of elasticity. The boiler was low-pitched, the steam pressure high at 120 psi (8.4 kg/cm2). The result was a powerful locomotive which rode well and immediately became popular, a forerunner of many later designs. Joy subsequently had a varied career with successive railways and engineering firms. In the late 1850s he invented steam reversing gear for large, marine steam engines, a hydraulic organ blower and a pneumatic hammer. In 1879 he invented his radial valve gear for steam engines, which was adopted by F.W. Webb for the London \& North Western Railway's locomotives and was also much used in marine steam engines.[br]Bibliography1879, British patent no. 929 (valve gear).Further ReadingObituary, 1903, Engineering (20 March).Obituary, 1903, The Engineer (20 March).PJGR
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