Showing how easy it is to build a “tin ear” to spot the approach of air raiders, Dr. Donald A. Wilbur of Rensselaer Polytechnic Institute, Troy, N. Y., hastily assembled a simple instrument from low-cost parts. It would be possible to construct similar locators for local air-raid spotters throughout the country without great expense. The detector, made up of a few boards, a large tin horn from an old phonograph, a microphone, a battery-operated amplifier and a pair of ear phones, is capable of picking up the sound of bombers five to ten miles distant.