听力原文： Robert Spring, a 19th century forger, was so good at his profession that he was able to make his living for 15 years by selling false signatures of famous Americans. Spring was born in England in 1813 and arrived in Philadelphia in 1858 to open a bookstore. At first he made some money by selling his small but genuine collection of early U. S. autographs. Discovering his ability at copying handwriting, he began imitating signatures of George Washington and Ben Franklin and writing them on the title pages of old books. To make less the chance of detection, he sent his forgerics to England and Canada for sale and circulation. Forgers have a hard time selling their products. A forger can't approach a respectable buyer but must deal with people who don't have much knowledge in the field. Forgers have many ways to make their work look real. For example, they buy old books to use the aged paper of the title page, and they can treat paper and ink with chemicals. In Spring's time, right after the Civil War, Britain was still fond of the Southern states, so Spring invented a respectable maiden lady known as Miss Fanny, whose economic problems forced her to scll a great number of letters and manuscripts belonging to her famous father. Spring had to work very hard to satisfy the demand. All this activity did not prevent Spring from dying in poverty, leaving sharp-eyed experts the difficult task of separating his forgeries from the originals. Why did Spring sell his false autographs in England and Canada?