Why do you sometimes see “f”s instead of “s”s in old books?
Look carefully the next time you see a book over 200 years old. The letter that may easily be mistaken for an “f” is actually slightly different. The symbol “ƒ” is called a “long”, or “gothic ‘s’”. It is derived from the German language, which had separate letters for the long and short “s” sounds. The use of the long “ƒ” in English, however, was not based on phonetics but rather on where in a word that the letter appeared. Regardless, use of the long “ƒ” faded in the late 1700s, and by the 19th century only the now-familiar “s” character was commonly used.