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By BARBARA MOFFETT
National Geographies Writer
WASHINGTON. D. C.—Late on the night of May 18 research meteorologist Walter Hoecker heard the news that Mount St. Helens finally had made good on her threats.
But several hours earlier—at 2:50 p.m. local time—an instrument that detects sound waves in Hoecker’s Silver Spring, Md., office already had found out. In three hours and 20 minutes the volcano’s voice had crossed the United States.
Just a Hobby
The instrument, a sensitive microbarograph, was developed by the U.S. Bureau of Standards to measure atomic bomb blasts, natural explosions, and weather phenomena. Hoecker and a co-worker, both employees of the National Oceanic and Atmospheric Administration, operate the microbarograph as a hobby, mostly using it to track thunderstorms.
But the waves that Hoecker saw on the instrument when he went to work the next morning weren’t made by any thupderstorm. On the paper that runs constantly under a pen were five irregularities, registered at five-minute intervals. Each irregularity represents a sound wave. Hoecker says, all caused by that one mighty blast.
"In a way it was a surprise that the blast was large enough to be recorded here," he said.
It was not a surprise to Dr. William Donn of the Lamont-Doherty Geological Observatory. Donn now estimates, based on sound waves, that the volcano’s force was equal to 10 to 15 million tons of TNT—500 times more powerful thatn the bomb dropped on Hiroshima.
It took the sound waves 10 minutes longer to reach the sensitive microbarograph at Donn’s Palisades, N. Y., laboratory. After studying the recorded waves, Donn and a colleague turned to the files
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