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Insular Gravity and Oceanic Isostasy
Published online by Cambridge University Press: 24 October 2008
Extract
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 instance, 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, 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.
- Type
- Research Article
- Information
- Mathematical Proceedings of the Cambridge Philosophical Society , Volume 23 , Issue 2 , April 1926 , pp. 130 - 135
- Copyright
- Copyright © Cambridge Philosophical Society 1926
References
* Cambridge Transactions (1849)Google Scholar: 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.Google Scholar
* Math, and Phys. Papers, vol. II, p. 153Google Scholar. 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. This analysis of Stokes in fact establishes as a general proposition that the effect of distant irregularities of surface mass consists of a direct vertical attraction, say g″, together with an indirect part due to change of level, equal to −4g″, thus countervailing four times: this influence, of wide range and presumably actually email, is superposed on the local effect here considered.Google Scholar
* In 1855–1859: cf. Clarke, A. R., Geodesy, pp. 96–8.Google Scholar
† Cf. the chapter in Jeffreys', H. recent treatise The Earth.Google Scholar
‡ In the case illustrated above, with radius of ocean about 500 miles and depth of compensation 100 miles, about 10 per cent, of the anomaly both of attraction and of potential would remain after compensation of the ocean.Google Scholar
* Cf. Clarke, A. R., Geodesy, p. 93Google Scholar; or Thomson, and Tait, , Nat. Phil. (1867); also infra for conical forms—and the remark added at the end.Google Scholar
† Proc. Washington Academy of Sciences (12. 4, 1925).Google Scholar
* Proc. Washington Acad. (01. 16).Google Scholar
* Cf. Cavendish, , Scientific Papers, vol. II, pp. 402–7.Google Scholar