GRAVITY DETERMINATIONS ON THE CARNEGIE 



Early Gravity Observations at Sea 



The importance of gravity measurements for deter- 

 mining the figure of the earth long has been realized and 

 observations for this purpose have been in progress for 

 at least a century. Within the last few decades the num- 

 ber of gravity stations has rapidly increased to several 

 thousand. Most of these are located in Europe, Asia, 

 and North America, and therefore are not well distri- 

 buted geographically for the best determination of the 

 figure of the earth. Likewise, since the land area com- 

 prises only twenty-eight per cent of the total surface of 

 the earth, it is obvious that many observations over 

 ocean areas are needed. Many of the values determined 

 on islands cannot be used for this purpose because they 

 are locally disturbed and are not representative for a 

 greater area [see (1) of Literature Cited at the end of 

 this paper]. 



The large number of gravity values determined for 

 the purpose of deriving the accurate figure of the earth 

 have been used in testing the isostatic condition of the 

 earth's crust and the results obtained are found to agree 

 completely with those obtained from studies of the de- 

 flections of the vertical as obtained from triangulation 

 and astronomical data (2). The importance of gravity 

 researches for studies of the stresses in the earth's 

 crust, which must exist wherever there are deviations 

 from isostatic equilibrium, has been pointed out (3). 



The relation between these deviations from isostatic 

 equilibrium and tectonic disturbances in the earth's 

 crust has been demonstrated beautifully by a remarkable 

 research done by Dr. F. A. Vening Meinesz, of the 

 Netherlands Geodetic Commission, on "The gravity- 

 anomalies of the East Indian Archipelago" (4). This 

 research is based on the determination, on board a Dutch 

 submarine, of gravity at nearly three hundred stations, 

 practically all of which were made with the vessel sub- 

 merged. A similar research, made in the West Indies in 

 1928 by Dr. Vening Meinesz in cooperation with the U. S. 

 Navy and the Carnegie Institution of Washington (1), sub- 

 sequently (1932) was extended through further observa- 

 tions by Dr. Vening Meinesz on a submarine placed, by 

 the U. S. Navy, at the disposal of the International Ex- 

 pedition to the West Indies under the direction of Profes- 

 sor Richard M. Field, of Princeton University. 



Until Dr. Vening Meinesz conceived the idea of 

 swinging two practically identical and nearly isochronous 

 pendulums in the same vertical plane and recording pho- 

 tographically the difference in their angles of elongation, 

 no satisfactorily accurate method of making gravity ob- 

 servations at sea was available. This was owing princi- 

 pally to the fact that the horizontal accelerations on a 

 ship at sea so disturb the motion of a single pendulum 

 that it is not possible to obtain any accurate value of 

 gravity by this means. If, however, the difference in the 

 angles of elongation of two practically identical and near- 

 ly isochronous pendulums swinging in the same vertical 

 plane be recorded, then this difference is, within limits, 

 quite independent of horizontal accelerations of the appa- 

 ratus, for the simple reason that however this horizontal 

 acceleration may affect the angle of elongation of one of 

 the pendulums, it will affect that of the other in the same 

 way. This is the fundamental idea of the apparatus of 



Dr. Vening Meinesz and it is this principle which has 

 made possible the success of several hundred observa- 

 tions which he has made on board submerged submarines. 



The experience of Dr. Vening Meinesz with his ap- 

 paratus on board the Dutch submarines, H. M. S. KXni, 

 Kn, and KXI, and in particular on board the U. S. sub- 

 marine S-21 (1), indicated that it might be possible to ob- 

 tain gravity measurements on board surface vessels. In 

 view of the character and extent of cruise VII of the Car- 

 negie , of the Department of Terrestrial Magnetism of the 

 Carnegie Institution of Washington, there seemed to be 

 an ideal opportunity for obtaining gravity measurements 

 at sea, in various harbors, and near several islands, 

 which results would have been most desirable for a bet- 

 ter determination of the figure of the earth and for fur- 

 ther studies in isostasy. 



Installation and Tests of Apparatus 



Accordingly, it was decided to install a Meinesz 

 gravity apparatus on the Carnegie at San Francisco, in 

 August 1929, for this was the nearest port to Washington 

 in the remainder of the cruise, and therefore required 

 the least amount of transportation of apparatus and men 

 to install it and to make the necessary adjustments and 

 tests. Because of the great amount of detail and preci- 

 sion required in building such an apparatus and to the 

 fact that the makers, Nederlandsche Seintoestellen Fa- 

 briek, at Hilversum, Holland, were dependent on other 

 firms for special parts, it would have been impossible 

 to complete the instrument in time to standardize it at 

 the U.S. Coast and Geodetic Survey gravity base station 

 and install it on the Carnegie at San Francisco, without 

 the cooperation of interested persons and organizations 

 as well as of the manufacturers. 



Through the efforts of Dr. Vening Meinesz and the 

 fine spirit of generous cooperation on the part of Dr. N. 

 E. Norlund, Director of the Geodetic Service of Denmark, 

 it was arranged that the apparatus which had been ordered 

 tor the Geodetic Service and which was nearly completed, 

 be delivered to Washington, although this meant no incon- 

 siderable delay to the investigations of Dr. Norlund in 

 this direction. A set of invar pendulums kindly was 

 loaned by the Netherlands Geodetic Commission. This 

 cooperation made possible the delivery of an instrument 

 in time for the standardization in Washington and the in- 

 stallation on the Carnegie in San Francisco. (fig. 1). 



The standardization at Washington and the prelimi- 

 nary records on the Carnegie at San Francisco were 

 made under the supervision of Dr. F. E. Wright, of the 

 Geophysical Laboratory at Washington. His experience 

 in working with Dr. Meinesz on the gravity-measuring 

 cruise of the U. S. submarine S-21 made his assistance 

 invaluable. The instrument was placed in the main cabin 

 of the Carnegie about seven feet abaft the chronometer 

 cases. This was about as near to the center of oscilla- 

 tion of the ship as it was possible to determine that point. 



The adjustments of the apparatus were completed 

 before the end of August. They included an installation 

 of a special time-signal amplifier, which had been used 

 in Washington for the standardization, and which provided 

 for the recording of wireless time signals automatically 



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