for National Geographic News
At first glance, the manuscript appears to be a medieval Christian prayer book.
But on the same pages as the prayers, experts using a high-tech imaging system have discovered commentary likely written in the third century A.D. on a work written around 350 B.C. by the Greek philosopher Aristotle.
The discovery is the third ancient text to emerge from the layers of writing on the much reused pages. In 2002 researchers had uncovered writings by the mathematician Archimedes and the fourth-century B.C. politician Hyperides.
Last year one of the pages was found to contain a famous work by Archimedes about buoyancy that had previously been known only from an incomplete Latin translation.
Project director William Noel, curator of manuscripts at the Walters Art Museum in Baltimore, Maryland, called the latest discovery a "sensational find."
The findings were presented today at a general meeting of the American Philosophical Society in Philadelphia, Pennsylvania.
The book, known as the Archimedes Palimpsest, was first analyzed in 1906, when a Danish researcher recognized that it contained works by the ancient mathematician.
In the 10th century a scribe had copied the ancient Greek manuscripts from papyrus scrolls onto parchment—thin leaves of treated animal skin.
Later the writing was washed out using a solvent such as orange juice and overwritten with new text—a process known as palimpsesting.
"In those days, parchment writing materials were so valuable that they were commonly reused when the book was considered out of date or if the subject was judged inappropriate or less valuable," Roger L. Easton, of the Rochester Institute of Technology, wrote in an email.
By the 12th century, pages from five different earlier works had been erased, overwritten, and compiled into a Christian prayer book, the Euchologion—what is now called the Archimedes Palimpsest.
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