. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] GRAVITY AT SEA 143 observations. Differences of less than 10 mgal were observed in nine cases and in two cases 50 mgal, probably due to errors in the submarine values. At six crossings of the ship's own track a root-mean-square difference of 5 mgal was observed. Two meters observing on the same stable platform showed differences less than 1 mgal. It is concluded that the gravity-meter values are reliable to + 1 mgal but that the gravity values measured on a surface vessel are reliable to + 5 mgal, due primarily to
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 1] GRAVITY AT SEA 143 observations. Differences of less than 10 mgal were observed in nine cases and in two cases 50 mgal, probably due to errors in the submarine values. At six crossings of the ship's own track a root-mean-square difference of 5 mgal was observed. Two meters observing on the same stable platform showed differences less than 1 mgal. It is concluded that the gravity-meter values are reliable to + 1 mgal but that the gravity values measured on a surface vessel are reliable to + 5 mgal, due primarily to the navigational limitations. Fig. 5 shows the G500 miles of track of a five-week cruise of Compass Island on which continuous gravity observations were obtained. In the past two years successful observa- tions have been made on Vema, a 700-ton auxiliary schooner, about 20% of the time on her world-wide cruises. D. The LaCoste-Romberg Surface-JShip Gravity Meter The LaCoste-Romberg instrument (Fig. 6) utilizes an adaption of the suspension used in their land meters with a beam of increased moment of inertia, damping and a photo-electric reading system. The instrument is (Second-order correction) Second-order correction counter Differential gearbox Gravity reoding counter I Accelerometer control box (Selsyn transmission controlling measuring screw) Counter box Measuring screw counter Gravity reading motor Moin control box. Timing indicotor _o o Gravity reading motor switch marker Gravity reading recorder r Assembly in shock mounts ond gimbals T Horizontal accelerometer Horizontal occelerometer Gravity meter 7" X^ I Fig. 7. Block diagram of LaCoste-Romberg surface-ship meter. suspended in gimbals and long-period pendulums are used to establish the direction of the true vertical. An electrical voltage, proportional to the square of the angle between the instrument in its gimbal suspension and the reference pendulums, is developed by the horizontal accelerometers. This v
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