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 R.V. 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 voltage is also 



