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[chap. 9 



proportional to the correction for the horizontal accelerations of the instrument. 

 It is averaged and converted into a shaft rotation which is continuously added 

 to the measuring-screw position through a differential gear-box (see Fig. 7). 

 The ratio of shaft rotation to accelerometer output is adjusted in the laboratory 

 before each trip during tests on a wave-simulating machine. In these tests the 

 meter is adjusted to give the same reading with the machine in motion as it 

 does when still. 



The meter itself is very stable as regards drift and other spurious changes of 

 reading. The reading of meter #3 at the Marine Physical Laboratory, San 

 Diego, changed by only 2.3 mgal between 21 February and 21 December, 1959, 

 during which period it was used on five separate surveys on four different 

 ships. The reading method depends on the equation of motion of the beam (see 



\m m\ 



Spring 



tension 

 adjustment 



<p = Beom deflection 



Fig. 8. LaCoste-Romberg gravity-meter suspension. 



Fig. 8). A couple acting on the beam causes it to move according to an equation 

 of the type : 



couple acting on beam = I^ + ^(f) + k(j), (">) 



where ^ is the deflection of the beam and /, ^ and k are instrumental constants. 

 This couple can also be equated to 



L{g + z + B-C), (0) 



where z is the vertical acceleration of the meter, B is the difference between 

 the total acceleration acting on the meter and the vertical component (i.e. the 

 instantaneous Browne correction), and L is an instrumental constant. C is 

 proportional to the spring tension set by the measuring screw and would be 



