Abstract
During the prevalence of a severe local thunderstorm in the evening of July 12, 1910, the magnetic observatory of the United States Coast and Geodetic Survey, situated at Cheltenham, Maryland, was struck by lightning. The charge first struck the flagpole which was mounted on the southern end of the roof of the “variation building,” the one in which the photographically recording magnetic instruments are mounted. The top of the flagpole stood about twenty‐five feet (7.62 meters) above the roof of the building. The charge shattered the flagpole as it passed down it to within about a foot from its base, where it left it and jumped over to a copper nail in the roof, where a hole about two inches (5 cm) in diameter was pierced through the roof.
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