Lost Copyright:
All works published before January 1, 1978 that did not contain a valid copyright notice may be considered to be in the public domain.
Owners of works published between 1978 and March 1, 1989 that did not contain a valid copyright notice were given a five year grace period in which to correct the problem of publication without notice before their work was unceremoniously tossed into the public domain.
Expired Copyright:
The public domain contains all works for which the statutory copyright period has expired.
Additionally, you are free to copy any work published before 1964 in which the copyright owner failed to renew his copyright. See the section on Duration of Copyrights for more detailed information on effective copyright periods.
taken from:
http://www.benedict.com/info/info.asp