Black females, some of them children, were forced into sexual relationships for their white owners' pleasure and profit: attempting to keep the slave population growing by his own doing, and not by importing more slaves from Africa. Even Thomas Jefferson, 3rd President of the United States, is believed to have fathered six mixed-race children (four survived to adulthood) with one of his female slaves, Sally Hemings, a woman three-quarters white and half-sister to his late wife, who served as the widower's concubine for more than two decades. In the case of Harriet Ann Jacobs, author of "Incidents in the Life of a Slave Girl," her master, Dr. James Norcom, had sexually harassed her for years. Even after she had two children of her own, he threatened to sell them if she denied his sexual advances. Although Harriet Jacobs managed to escape to the North with her children, the Fugitive Slave Act of 1850 still put their livelihood at risk due to Dr. Norcom's family continuing to pursue her.