An investigation of the nature of the ground motion necessary to produce a sinusoidal record starting suddenly from rest reveals that Benndorf's formula for the dynamic magnification of a mechanical seismograph always yields a value for the computed maximum ground displacement that is smaller than the true value. The apparent period taken from the record is shorter than the true period. The computed ratio of amplitude to period, a useful damage criterion, is always greater than the true value. The discrepancies in both amplitude and period become small as the ratio of the ground frequency to the instrument frequency increases.

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