Silent Bolts

In 1885 a heavy bolt of lightning hit the Washington Monument. But nobody in the neighborhood heard any clap of thunder. The occurrence was recorded by the National Bureau of Standards as a bolt of thunderless lightning.

Of this unfamiliar phenomenon electrical engineers took little note until three years ago when Karl Boyer McEachron, General Electric Co.'s ace lightning researcher, started a study of bolts which strike Manhattan's 1,230-foot Empire State Building, a giant lightning rod. During his study Mr. McEachron observed that several strokes which hit the skyscraper made no noise. Last week he had an explanation:

"Thunder is the result of a pressure wave caused by the sudden expansion of air created by a quick lightning discharge. All flashes do not release energy with the same speed. ... In some cases the electrical current is built up and released slowly; that is, in one or two tenths of a second as compared to millionths of a second in other discharges. This so-called slow lightning produces no thunder."

* Summer "heat lightning" appears to be noise less, but it is simply the reflection along the horizon of lightning so far away that its thunder is usually not heard.

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