at present, and such measurements are confined to laboratory practice; 

 however, development of a portable apparatus that shows great promise 

 is underway. In lieu of such an instrument, the pendulum apparatus is 

 considered to be sufficiently stable for establishing base control 

 stations. Current programs include thev establishment of secondary 

 pendulum stations that will be related to the few primary national bases 

 and supplemented by gravimeter readings made for the detailed surveys 

 relative to the secondary pendulum bases. This has not been a satisfac- 

 tory plan in the past, since old pendulums were inaccurate, bulky, 

 affected by other natural phenomena, and not usable as portable 

 instruments. New pendulum instruments more nearly satisfy the needs, 

 but they still are not as precise as gravimeters. However, gravimeters, 

 have problems of drift. Therefore, a portable, absolute (no drift) 

 measuring device should be developed to provide a means of establish- 

 ing secondary control points accurate to 0.1 milligal. 



5. Geodetic gravimeters 



Currently available at the Hydrographic Office are five com- 

 mercially produced land gravimeters: two Worden, one North Ameri- 

 can, and two LaCoste and Romberg. All of these instruments have 

 attainable accuracies of 0.1 milligal and meet the requirements of 

 this Office with respect to land gravity surveys. Each of the meters 

 incorporates certain desirable features, but each also has some disad- 

 vantages for world-wide surveys. 



a. Worden gravimeter 



This instrument is light-weight, compact, and very good for 

 local surveys, or for establishing connections between points that are 

 reasonably close together. The meter is temperature compensated 

 (not controlled) and is easily read, except that it must be lifted off 

 its tripod for reading the dials. The meter is calibrated in the factory 

 by a tilt-table method which is not as precise as are observations on 

 the ground over a known range of gravity. The calibration curves are 

 generally convex, but, as with all gravimeters, they can be changed 

 by rough handling of the instrument. Disadvantages of the Worden 

 gravimeter include the problem of drift (a slow but continuous change 

 in readings due to the relaxation of the spring system) and the lack 

 of a method to clamp the measuring element when it is not in use. 

 The two meters owned by this Office also are limited in their range 

 of use: one is set for use from the equator to approximately 45° north 

 and south latitude, and the other for use between about 30° and 90° 

 north and south latitude. Because of the inherent drift in these meters, 

 they usually run out of the reading range within four years and require 



XIII- 4 



