Jordanus de Nemore (fl. 13th century), also known as Jordanus Nemorarius and Giordano of Nemi, was a thirteenth-century European mathematician and scientist. The literal translation of Jordanus de Nemore (Giordano of Nemi) would indicate that he was an Italian. He wrote treatises on at least 6 different important mathematical subjects: the science of weights; “algorismi” treatises on practical arithmetic; pure arithmetic; algebra; geometry; and stereographic projection. Most of these treatises exist in several versions or reworkings from the Middle Ages. We know nothing about him personally, other than the approximate date of his work. Life No biographical details are known about Jordanus de Nemore. Cited in the early manuscripts simply as “Jordanus”, he was later given the sobriquet of “de Nemore” (“of the Forest,” “Forester”) which does not add any firm biographical information. In the Renaissance his name was often given as "Jordanus Nemorarius", an improper form. An entry in the nineteenth-century manuscript catalogue for the Sächsische Landesbibliothek in Dresden suggested that Jordanus taught at the University of Toulouse, but the text in question was not written by Jordanus and this possible association is without foundation. A fourteenth-century chronicle of the Order of Preachers by the Englishman Nicholas Trivet (or Triveth, 1258–1328) suggested that the second master-general of the Dominican Order, Jordanus of Saxony (d. 1237) wrote two mathematical texts with titles similar to two by Jordanus de Nemore, but this late suggestion is more likely a confusion on the part of Trivet, rather than any proof of identity. Jordanus of Saxony never uses the name “de Nemore” and is nowhere else credited with mathematical writings – in fact he had lectured in theology at the University of Paris. Likewise the name of Jordanus of Saxony is never found with a mathematical text. This identity, popular among some in the nineteenth and twentieth centuries, has been for the most part abandoned. It is assumed that Jordanus did work in the first part of the thirteenth century (or even in the late twelfth) since his works are contained in a booklist, the Biblionomia of Richard de Fournival, compiled between 1246 and 1260. Writings Mechanics: scientia de ponderibus (the science of weights) The medieval “science of weights” (i.e., mechanics) owes much of its importance to the work of Jordanus. In the Elementa super demonstrationem ponderum, he introduces the concept of “positional gravity” and the use of component forces. Pierre Duhem (in his