This is 'Portrait of a Girl Reading' by the American artist Thomas Sully (1783-1872) - he was actually born in Lincolnshire, England, but his family moved to the US when he was nine. There's an interesting article about him here, which tells a fascinating story of his trip to England in 1838 when he ended up painting a portrait of the young Queen Victoria. She apparently liked him because he was handsome and charming. He does look lovely (see below). He was also very taken with the queen. “I should be gratified,” wrote Sully, “if I were able to give an idea of the sweet tone of voice, and gentle manner of Queen Victoria! It was impressive of dignity and mildness, and at the same time I felt quite at my ease, as tho in company with merely a well bred lady.” He painted her several times.