Early geophysical papers of the Society of Exploration Geophysicists . earlygeophysical00soci Year: 1947 = of the beam from light-source to seismograph effected the actual registration. In Fig. 2, the upward jog in all seismograph lines marks the deflection caused by closing the firing circuit, and the downward jog a few hundredths of a second later, the instant of the explosion. IV. Firing Positions At Quincy and Rockport, several abandoned water-filled quarries were used as firing locations. The charges were placed against the quarry walls and 204

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