These acoustic mirrors were a WWI phenomenon, useful in the days before radar against slow-moving aerial attackers, and obsolete before WWII. However, I was aware of one that has survived—they are rather solid blocks of concrete or equally durable materials—and I wondered if there was any trace remaining of the Kilnsea mirror that Tolkien knew. One man’s passing curiosity is another man’s hobby: behold Andrew Grantham’s website with a list of acoustic mirrors including pictures of surviving examples. The Kilnsea mirror has survived:
Die-hard Tolkien pilgrims should note that it is not on public land, but there are footpaths that give a view of it.
(It is now too late to visit Fort Paull, whose anti-aircraft gun is also mentioned in the essay. The fort, which dates from Tudor times although the current fortifications were built in the 1860s, has more recently been a rather eclectic museum, which closed in January 2020. The exhibits, including the last surviving Blackburn Beverley, were auctioned off.)
