Acoustic Mirrors

For centuries Great Britain could rely upon its position as an island nation to keep a barrier between them and any invaders from continental Europe. They cultivated a large and powerful navy to maintain control of the waterways, and this and good fortune enabled them to withstand or discourage attacks from Spain, France, and Germany. But with the development of aircraft at the beginning of the 20th century, potential invaders had a way to project force to the islands without having to go by sea, and a method of detecting any such attacks had to be found so that they could be targeted. Before radar had been developed, one method was to use huge hemispherical cavities to concentrate faint noises onto a microphone placed near the focus. Many of these were constructed, carved out of stone, or cast in concrete, and, being large and robust, most of them still exist.