Abstract
1. A CENTURY ago geodetic and gravitational universal surveys were mainly concerned with determining the effective (gravitational) ellipticity of the earth, after due allowance had been made for local anomalies, with especial view to the exact purposes of physical astronomy. One of the chief of these anomalies was exhibited by a remark of Airy, after scrutiny of the available data in his treatise (1830) on figure of the earth in the “Encyclopedia Metropolitana,” that the observations show gravity to be abnormally in excess on island stations. It appeared, for example, that this cause might make the mass of the moon uncertain up to 2 per cent. A very refined explanation of this anomaly of island stations (which will be seen presently to be only partially effective) was offered by Sir George Stokes, from whom this last remark is quoted, in the course of a memoir,2 fundamental for theoretical geodesy, in which he demonstrated that no outside survey could lead to any certain knowledge of the distribution of mass inside the earth, even in its outer crust, except as a matter of probability when backed up by geological knowledge.
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Abstracted, with Sections 2 and 3 added, from Proceedings of the Cambridge Philosophical Society, Feb. 8, 1926.
Cambridge Transactions (1849): reprinted in "Math. and Phys. Papers", vol. ii. Some idea of the great debt owed by the Indian and other gravitational surveys to the continuous amateur advice of Sir G. G. Stokes, spread over half a century of their development, may be gleaned from the collection of his "Scientific Correspondence" (Camb. Univ. Press), vol. ii. pp. 253–325.
"Math. and Phys. Papers", vol. ii. p. 153. Stokes did not make any correction in this reprint in 1883; but Dr. Bowie states (loc. cit. infra) that there is no generally accepted explanation other than compensating excess of density beneath the ocean.
In 1855–59: cf. A. R. Clarke, "Geodesy", pp. 96–98.
Cf. the chapter in H. Jeffreys' recent treatise "The Earth".
In the case illustrated above, with radius of ocean about 500 miles and depth of compensation 100 miles, about io per cent, of the anomaly both of attraction and of potential would remain after compensation of the ocean.
For recent special estimates see a note by W. Bowie, Proc. Washington Acad., Dec. 1925.
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LARMOR, J. Oceanic Isostasy in Relation to Geological Tectonic1. Nature 118, 375–377 (1926). https://doi.org/10.1038/118375a0
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DOI: https://doi.org/10.1038/118375a0