Silent Zones

Collier's New Encyclopedia (1921)

a phenomenon in atmospheric acoustics especially important in connection with fog signals at sea. Such signals are sometimes practically inaudible in areas comparatively near their sources, while perfectly audible at greater distances. On land, zones of silence may be explained by topographical peculiarities, in which case they are permanent and unchanging. But in other cases they are found to be variable, and scientists are not yet agreed as to their causes. Tyndall attributed variations in audibility to what he called acoustic clouds, or areas of irregular temperature and humidity in the atmosphere, but this explanation has not been universally accepted as sufficient. More recently, zones of silence on a larger scale have been observed and studied in connection with the carrying of sounds from explosions and scenes of heavy cannonading such as those of the World War. The bombardment of Antwerp in 1914 was found by investigators to have been heard in irregular zones described around that city, a zone of silence intervening between the inner circle of audibility and an outer circle whose average distance was about 125 miles from the source. The phenomenon is at present generally regarded as being due to the refraction of sound by , although several investigators winds have proposed theories of layers of gases in the upper atmosphere which reflect such sound waves as attain high levels. SILESIA. Plesbiscites held in Upper Silesia, March 21, 1921, resulted in 716,- 408 votes cast for union with Germany, 471,406 votes for union with Poland. Important coal districts returned Polish majorities, however, and Polish insurgents took possession of these districts. (See Disagreements between Great Britain and France as to the final disposition of Silesia led to a tense international situation, but on August 12 both sides agreed to refer the matter to the League of Nations. The League Council named an international commission, consisting of representatives of Belgium, Brazil, China, and Spain, to submit a plan of settlement (September 1). The commission reported October 20, and its plan was approved by the League and the Council of Ambassadors. Silesia was partitioned between Poland and Germany, but was to remain for fifteen years under the rule of a mixed commission intrusted with the duty of making the necessary readjustments in a way to cause least inconvenience and hardship.