The geophone, an instrument invented by the French during the War to detect sapping operations of the Germans underground, has been developed by U. S. Engineers for rescue work in mines, gauging tunnel borings, etc. It contains diaphragms like a seismograph, which transmit the most delicate vibrations to the observer's ears. Sledge pounding can be detected 3,000 feet through solid rock, and ordinary talking through 50 feet of coal.
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