The magnetic storm of August 11–12, 1911, as observed at the Cheltenham Magnetic Observatory

A magnetic storm of extraordinary magnitude occurred on August 11, 1919, at the magnetic observatory of the United States Coast and Geodetic Survey, latitude 38° 44′ N. and longitude 76° 50′.5 W. It began abruptly at 1 h 58 m 75th meridian time, A.M., August 11. The first movement in declination ( D ), was from 6° 18′.9 W to 6° 14′.9 W, followed by a sudden change to 6° 37′ W. The recording spot left the sheet at 2 h 56 m , at the value 7° 18′ W. and returned at 3 h 28 m , the maximum having occurred during this interval, probably reaching 7° 38′ W.