A curious kind of document, which links the classical times with the middle ages, in respect to the we of parchment, is afforded by the "palimpsests," or manuscripts from which old writing had been erased in order to make way for new. A well-prepared leaf of parchment was so costly an article in the middle ages, that the transcribers who were employed by the monastic establishments in writing often availed themselves of some old manuscript, from which they scraped off the writing; such a doubly-used piece of parchment was called a "palimpsest." This practice seems to have been followed long before, but not to so great an extent as about the fourteenth and fifteenth centuries, at which time there were persons regularly employed as "parchment-restorers." The transcribers had a regular kind of knife, with which they scratched out the old writing, and they rubbed the surface with powdered pumice stone, to prepare it for receiving the new ink. So common was this practice that when one of the emperors of Germany established the office of imperial notary, it was one of the articles or conditions attached to the holding of the office that the notary should not use "scraped vellum" in drawing deeds. Sometimes the original writing, by a careful treatment of the parchment, has been so far restored as to be visible, and it is found to be parallel, diagonal, and sometimes at right angles to the writing afterwards introduced. In many cases the ancient writing restored beneath is found to be infinitely more valuable than the monkish legends written afterwards. Cicero's De Republica was discovered by Angelo Mai in the Vatican library written under a commentary of St. Augustine on the Psalms; and the Institutions of Gains, in the library of the chapter of Verona, were deciphered in like manner under the works of St. Jerome. Papyrus, parchment, and vellum were sometimes used together in the MSS. books. Thompson, author of "Greek and Latin Palaeography," observes: "Examples, made up in book form, sometimes with a few vellum leaves incorporated to give stability, are found in different libraries of Europe. They are: The Homilies of St. Avitus, of the 6th century, at Paris; Sermons and Epistles of St. Augustine, of the 6th or 7th century, at Paris and Genoa; works of Hilary, of the 6th century, at Vienna; fragments of the Digests, of the 6th century, at Pommersfeld; the Antiquities of Josephus, of the 7th century, at Milan; an Isidore, of the 7th century, at St. Gall. At Munich, also, is the register of the Church of Ravenna, written on this material in the 10th century."