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The tsunami-warning system developed since the 1946 disaster in Hawaii relies mainly on a simple and ingenious instrument devised by Commander C. K. Green of the Coast and Geodetic Survey staff.

It consists of a series of pipes and a pressure-measuring chamber which record the rise and fall of the water surface.

Ordinary water tides are disregarded.

But when waves with a period of between 10 and 40 minutes begin to roll over the ocean, they set in motion a corresponding oscillation in a column of mercury which closes an electric circuit.

This in turn sets off an alarm, notifying the observers at the station that a tsunami is in progress.

Such equipment has been installed at Hilo, Midway, Attu and Dutch Harbor.

The moment the alarm goes off, information is immediately forwarded to Honolulu, which is the center of the warning system.