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Abraham Lincoln

Another McClure's story meant to compete against Century Magazine was on US president Abraham Lincoln, on whom Century had published a series by his private secretaries, John Nicolay and John Hay. At first, Tarbell was reluctant to take up work on Lincoln as she later said, "If you once get into American history, I told myself, you know well enough that will finish France." At the same time, however, Tarbell had been fascinated with Lincoln since she was a young girl. She remembered the news of his assassination and her parents' reaction to it: her father coming home from his shop, her mother burying her "face in her apron, running into her room sobbing as if her heart would break."

When Tarbell first approached John Nicolay, he told her that he and Hay had written "all that was worth telling of Lincoln". Tarbell decided to begin with Lincoln's origins and his humble beginnings. Tarbell traveled the country meeting with and interviewing people who had known Lincoln—including his son Robert Todd Lincoln. Robert Lincoln shared with Tarbell an early and previously unpublished daguerreotype of Lincoln as a younger man. She followed up on a lost 1856 speech by Lincoln by tracking down Henry Clay Whitney —who claimed to have written down notes—and then confirming his notes via other witnesses. Whitney's version of the speech was published in McClure's, but has since been disproved by other historians.

Tarbell's research in the backwoods of Kentucky and Illinois uncovered the true story of Lincoln's childhood and youth. She wrote to and interviewed hundreds of people who knew or had contact with Lincoln. She tracked down leads and then confirmed their sources. She sent hundreds of letters looking for images of Lincoln and found evidence of more than three hundred previously unpublished Lincoln letters and speeches. Tarbell met with John H. Finley while visiting Knox College where Lincoln famously debated Stephen Douglas in 1858. Finley was the young college President, and he would go on to contribute to Tarbell's work on Standard Oil and rise to become the editor of The New York Times. Tarbell traveled abroad to Europe, discovering that a rumor that Lincoln had appealed to Queen Victoria to not recognize the Confederacy was, in fact, false.

By December 1895, the popular series by Tarbell once again helped boost McClure's circulation to over 250,000 which climbed to over 300,000, by 1900, making it higher than its rivals. This occurred even as the editors at Century's Magazine sneered, "They got a girl to write the Life of Lincoln." McClure would go on to use the money generated by Tarbell's articles to buy a printing plant and a bindery.

It was at this time that Tarbell decided to be a writer and not an editor. The articles were collected in a book, giving Tarbell a national reputation as a major writer and the leading authority on the slain president. Tarbell published five books about Lincoln and traveled on the lecture circuit, recounting her discoveries to large audiences.

The tight writing schedules and frequent travel eventually impacted Tarbell's health. On the verge of physical collapse, she checked into the Clifton Springs Sanitarium near Rochester, New York in 1896. Besides rest and relaxation, her treatment included taking the water cure. She would visit the Sanitarium numerous times over the next thirty years.