bertilak wrote, with respect to a work originally published in 1771 and reprinted in 1971: I agree that it would be prohibited ... to scan it in and post the images of the pages on the web.
No it wouldn't. Of course, nothing you read on Everything is legal advice, but I have read Title 17 of the U.S. Code (which contains the copyright law). Republishing a work unmodified does not create a new "original work of authorship"; the copyright date is still 1771. Even the facsimile is copyright 1771 because it is a verbatim copy of a literary work copyright 1771.
The rule for public domain is: If the work was first published before 1923, it's in the public domain; if the copyright holder has abandoned the copyright on the work (that is, released the work into PD), it's in the public domain; otherwise, it's under perpetual copyright due to a loophole in the U.S. Constitution.