not to the people in between. In 1904 a large explosion at Förde in western Germany was accompanied by similar results. There was a small area in which the sound arrived directly, then a zone of silence, and finally the sound was heard again, at a distance of 100 to 200 km from its source. G. von der Borne explained this as the return of an acoustic wave reflected from a hydrogen or helium zone in the upper atmosphere, and he was supported by van Everdingen, who observed large explosions in the early stages of the World War. It was remarked that the noise of guns firing in Flanders could often be heard in the east of England, especially in the summer, but rarely in winter. When explosionswere later made for the explicit purpose of investigating this effect, it was found that the apparent time of propagation showed that the waves reaching the zone of abnormal audibility had travelled on a path considerably longer than the direct one. It became clear that the sound was being reflected in the upper atmosphere.

A munitions factory at Silvertown, an industrial area in the East London Docks on the north bank of the Thames, exploded in January 1917. It could be heard in an elliptical area with a major axis of about 150 km in the NW-SE direction, and a minor axis of about 50 km, centred on the site of the explosion. This is the zone of normal audibility. Beyond, in the zone of silence, the explosion was