How did Dr Flint try to fool Linda"s family into thinking that she wanted to return to the south? What made him take that action?

Dr. Flint had not given up on finding Harriet. She decided that, since he thought she was in New York, that she should write a letter dated from that location. Her friend Peter was able to take the letters – she wrote one to her grandmother as well – to a person who was going to New York and would mail them on her behalf. This plan made her grandmother very nervous, as she felt sure "mischief would come of it." In the letter Harriet said she wanted replies directed to a street in Boston, as she did not live in New York but only went there occasionally.

When Dr. Flint received the letters (he was given the one for her grandmother), he immediately came to Aunt Marthy's house and read aloud the one addressed to her. From her hiding place, Harriet heard the letter and realized that he had changed the words to make it sound like she wanted to come home.