Mode of access: Internet from the National Park Service web site. Address as of 9/20/06: http://www.cr.nps.gov/hps/tps/briefs/brief21.htm; current access via PURL.
Title from title screen (viewed on Sept. 20, 2006).
"The web versions of the Preservation Briefs differ somewhat from the printed versions. Many illustrations are new, captions are simplified, illustrations are typically in color rather than black and white, and some complex charts have been omitted."