Nerida Ellerton trembled as she held a page from a "ciphering book" written by a teenage Abraham Lincoln as she did research in the archives of Houghton Library at Harvard University.

However, the Illinois State University mathematics professor did not immediately recognize the significance of what she had in her hand - among the oldest manuscripts written by Lincoln known to still exist.

When Ellerton and her husband, fellow ISU mathematics professor Ken Clements, discovered the document in 2009, "we assumed that its existence was common knowledge," she said Friday at a press conference announcing their findings.

They recently determined it is a missing "leaf" from Lincoln's school workbook, from which it was previously believed only 10 leaves survived.

A "leaf" is a two-sided page and Lincoln's former law partner, William Herndon, had given away these individual leaves as gifts about 10 years after he received the book from Lincoln's stepmother, Ellerton explained.

This particular leaf is believed to be from late 1825, when Lincoln was 16 years old. According to a news release on the Illinois State University website, Lincoln attended schools in Indiana from 1820 to 1826.

Ellerton and Clements not only reconstructed the order of the pages, but also double-checked Lincoln's math, which they declared to be accurate. It showed Lincoln was much more accomplished at math than anyone - including Lincoln himself - gave him credit for, the researchers said.

Stowell said, "As to Lincoln understating his accomplishments, that was fairly Lincolnian."

Lincoln the politician emphasized his railsplitter image, working with his hands, not just sitting behind an office desk, he said.

"That was part of his appeal in 1860," Stowell said of the 16th president.

To read the complete article, see:Illinois State profs find missing page from Lincoln's 'ciphering book'(www.chicagotribune.com/news/local/breaking/chi-abraham-lincoln-ciphering-book-illinois-state-professors,0,6687905.story)