This article provides the basic design for a laboratory instrument that may detect the Earth’s time-retarded transverse vector potential [Hafele J.C. Zelm. Jour., 2012, v.5, 134]. The instrument is based on the compound pendulum used by N.A.Kozyrev to measure the change in weight of a suspended aircraft navigation gyroscope [Kozyrev N.A. Zelm. Jour., 2012, v.5, 188]. If such an instrument is developed to measure the strength of the Earth’s vector potential with a precision of about 1 part in 1000, the neoclassical causal theory can be worked backwards to calculate the speed of the Earth’s gravitational field.