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The gravity instruments previously described and referred to in the discussions of measuring and reducing gravity observations are designed for operation on land and on a firm base. Gravity measurements have been extended to the water bottom and to moving ships.

Underwater (Boitom) Gravity Meters

The underwater gravity meter commonly used is basically a land gravity meter in a water-proof shell, and with mechanical and electronic accessories arranged so that the same operations are done remotely through an electric cable to a control box on the ship as would be done if the instrument were manipulated by hand. The instrument is mounted on gimbals within the waterproof case and small motors move the instrument on these gimbals until remote indicators at the control box show that it is level. In some instruments this function is automatic and the leveling motors are operated through a servodevice to keep the instrument level. The position of the beam of the moving system is indicated at the control box and the instrument can be balanced and read in much the same way as it would be manually. All of the operations of leveling, releasing the clamps to free the moving system, balancing the meter, obtaining the gravity value, and clamping the instrument at the end of the observation are carried out from the control box on the surface in a few minutes. The ship is riot anchored but is held within a short horizontal distance from the meter by manipulation of engines and rudders.

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