SECT, 1] GRAVITY AT SEA 145 



the gravity reading in the case of a static instrument with the beam set on the 

 reading hne. 



Equating (o) and (6) and integrating both with respect to time over the 

 interval Ti to T2 : 



f ' {I4, + Bcj> + kcf>) (It = l{ ' {g + z + B -C) dt. (7) 



Of the terms on the right-hand side of (7), J^' [g + z) dt = g{T2 —Ti), where g is 

 the average value of g. {T2—T1) must be long enough to average out the 

 vertical acceleration z. j^'^ B dt^ B{T2 -Ti) and j'^^^C dt = R{T2 -Ti) + 

 B{T2 —T\), where R is the gravity setting. {B computed in the accelerometer 

 control box is added to R in the differential gear box and the sum positions the 

 measuring screw.) The right-hand side of (7) thus becomes 



L{g + B-R~B){T2-Ti) = L{g -R){T2 -Ti), (8) 



so that 



g = R-\IL{T2-Ti) \ "^ {Icf+Bcf> + kcf>)dt. (9) 





The beam deflection, (f), is measured by a photo-electric system and the 

 quantity {Icf + ^(j) + k(f)) dt is computed by an analogue computer and recorded 

 on a pen recorder. (/^ + ^(f> + ^4) di is read from the graph and g computed from 

 equation (9). The various settings of R are recorded in digital form on an Ester- 

 line Angus 20-pen recorder and R can be determined from this record. A 12-min 

 averaging time has been found to be generally convenient under normal sea 

 conditions, although shorter intervals can be used under very calm conditions. 



In reading a spring-type gravity meter on land, g = g and R is normally 

 adjusted to bring the beam to the zero point {<f) = 0). In this case the beam is 

 still, so that ^ = <^' = and {I(f>-\-^^-[-k4>) dt = 0. Hence g=R. It is also possible 

 to read the meter with the beam off the null position and to make an appro- 

 priate correction, in which case g = R —L-'^k(f). When reading the meter on terra 

 firma, and </» are always zero, but on a moving ship or in an airplane this is 

 not true because of the disturbing accelerations and the constantly changing 

 value of gravity. Corrections need to be applied for the velocity and acceleration 

 of the beam, in addition to the correction for the departure of the beam from 

 the null position. If these corrections are not made, it is necessary to wait until 

 the mean values of and (f> are small before making a valid gravity reading, 

 with the result that the meter reading lags behind the changes of gravity and 

 that rapid variations are not read. This feature makes it possible to use the 

 LaCoste-Romberg meter in an aircraft, where the changes of gravity are very 

 rapid. 



The meter was first used on the I.G.Y. cruise of the Texas A and M College 

 ship Hidalgo (a 136-ft converted mine-sweeper) in the spring of 1958 (LaCoste, 

 1959) with encouraging results at a number of check stations. Further tests 

 were made on board the University of California Research Vessel Horizon 

 (150 ft length, 505 gross tons) in the fall of 1958, Horizons track passed close 



6 — s. Ill 



