12 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



explanation of this phenomenon has introduced two allied, (though 

 distinct, concepts of compensation and isostasy. 



The word compensation we owe to Archdeacon J. II. Pratt, 1 

 but the notion, though not the word, was suggested at an earlier 

 date, by Sir G. B. Airy. 2 Though the hypotheses regarding the 

 constitution of the earth, used by these two investigators, differed 

 radically from each other, the essence of the explanation was the 

 same, that under every great protuberance of the earth's surface, 

 such as a mountain range, there was a mass of density less than the 

 average at that depth, and that the plumb-line was not merely 

 alTected by the attraction of the visible mass of the mountain range, 

 but also by the defect in mass in the underlying portion of the 

 earth, which would cause an apparent repulsion of the plummet 

 and so neutralise, or compensate, in part or in whole, the direct 

 attraction of the mountain range. 



The most complete investigation of the effect of compensation, 

 which has been published, is that carried out by Mr. J. F. ITayford, 

 of the United States Coast and Geodetic Survey, in 1909. 3 Mr. 

 Hay ford adopted an hypothesis similar to that of Archdeacon 

 Pratt, and assumed that compensation took the form of a defect 

 of density, equal in amount to the excess of mass in the range and 

 distributed uniformly through some definite depth which would 

 be everywhere the same. The deflections which should be ex- 

 pected from the relief of the country surrounding each station, 

 up to a distance of 2,5G4 miles, were calculated, and compared 

 with the observed deflections, the difference being regarded as an 

 unexplained " residual," and it was found by a series of trials, 

 that these residuals were lowest if the depth of the layer, through 

 which the defect of mass was supposed to be uniformly distributed, 

 or " depth of compensation," was 113-7 km.; with a greater or less 

 depth the " residuals " were larger, and from this it was con- 

 cluded that the depth of compensation in the United States was 

 somewhere close to 113"7 km., or 7CVG7 miles. 



» J. II. Pratt, On tho Deflection of the Plumb-line in India, caused by the Attraction 

 of the Himalaya Mountains and of the elevated regions beyond; and its modification 

 l>y the Compensating effect of a Deficiency of Matter below the Mountain Mass. J'/nl. 

 Tran*.. OXLIX, 746-778 (1859). 



2 (J. Bi Airy. On the computation of the EfTect of the Attraction of Mountain Masses 

 as disturbing the Apparent Astronomical Latitude of Stations in Oeodetic Surveys. 

 PML Trans^ CXLV, 101-104*1865). . 



3 The Figure of the Earth and Isostasy, from measurements in the United Mates. 

 Washington? 19(H). 



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