“Zones of silence” on a much more extensive scale than those that disturb the operation of fog signals have been frequently observed, in recent years, in connection with great explosions, cannonading, and volcanic eruptions. The first case of this kind to attract scientific notice was that of a dynamite explosion at Förde, Westphalia, on December 14, 1903, the acoustic phenomena of which were investigated by Dr. G. von dem Borne; and among the many cases that have since been studied was that of the bombardment of Antwerp in October, 1914. Without describing these various cases separately, we may state that when reports were collected from the surrounding country to determine the places at which the sounds were audible and these reports were entered on a map, it was found that there was a large and usually very irregular area of audibility surrounding the source of sound, beyond which lay a broad, more or less circular zone of inaudibility, and finally, beginning about 100 miles from the source, there was a second large region of audibility, extending perhaps 150 miles from the source. In some cases a single sound at the source gave multiple reports (double, triple, or quadruple), chiefly in the outer zone of audibility. In his attempt to explain these curious silent zones, Von dem Borne pointed out that the atmosphere at very high levels is supposed to consist mainly of hydrogen, in which sound travels nearly four times as fast as in the common gases of the lower air, and that sound waves ascending to such heights along a slanting course would be bent strongly toward the earth. Another student of this phenomenon, Dr. A. Wegener, who is the champion of the idea that the atmosphere contains an unknown gas lighter than hydrogen (called “geocoronium” or “zodiacon”), sees in the prevalence of this gas at high levels the cause of a similar quasi-reflection of sound waves. Probably the majority of investigators, however, believe that the effect is due chiefly or entirely to the refraction of sound by wind.