another input of the surface comparator circuit 151 by means of the conductor 152 the output of the surface comparator circuit 151 is connected into the cable hoist control circuit 149 which drives the motor and gear box 148 which in turn drives the hoist drum 33 in the operation of the apparatus and circuitry illustrated in fig2 it should be appreciated that as the subsurface accelerometer 140 measures the acceleration of the well logging instrument and which consequently produces a signal indicative of the subsurface velocity through the circuit 141 the subsurface velocity is thus compared with the velocity of the cable at the surface by means of the subsurface comparator 142 whenever a significant difference is exhibited between the two velocities the actuator 143 is actuated to thus make or at least attempt to make the well logging instrument proceed through the borehole at a greater velocity the subsurface velocity of the well logging instrument is also compared with the velocity of the cable at the earth 39 s surface in the surface comparator circuit 151 to control the movement of the hoist drum 33 thus if the well logging instrument continues to slow down even though the actuator 143 has been providing additional motivation for the well logging.