tag:blogger.com,1999:blog-11787852.post-64911698741231974222007-05-11T08:46:00.000-07:002007-05-11T08:46:00.000-07:00#7 is a Aucostic Location Device. It was used to detect airplanes before radar. I saw one in England a few years back. You can also learn about it here: http://www.damninteresting.com/?p=486Anonymousnoreply@blogger.com