Warning in Case of Gas Attack Siren Tests at Hendon An electric double trumpet, as used by French fire brigades, being demonstrated at Hendon this morning. Tests are being made at Hendon, London, N.W., to discover the best type of siren for warning the public in the event of a gas attack from the air. This morning a number of selected sirens were sounded from Hendon police station. A ring of police observers was stationed in pairs at distances of 100, 200, 300 and 400 yards with sound-recording instruments to determine the power and penetration of the different siren notes over variousd distances. March 10, 1938. (Photo by Topical Press).