The original term of copyright protection in England was 14 years. In the US it began, in 1790, at, potentially, 28 years (a 14 year term that could be renewed once), then went to a system of two terms of 28 years, so that a renewed copyright lasted for 56 years. In 1976 we changed our law dramatically. Copyright became automatic as soon as a work was “fixed in tangible form,” and the copyright term was based on the life of the author. After another term extension in 1998, copyright in the US now lasts for the life of the author plus 70 years. For works created anonymously, as works for hire, or by a corporate author the term is 95 years from first publication or 120 years from creation, whichever is shorter. These changes mean that the public domain is barely growing at all in the US, since everything is protected automatically and it is now protected for a very long time.
