THE NATURE AND INTERPRETATION OF OEODETIC EVIDENCE. 29 



force of gravity at the station, while a lesser depth of compen- 

 sation would have the reverse effect. 



Taking all the considerations into account we may conclude 

 that if, after allowing for the effect of the surrounding topography 

 and its compensation, there is left a positive, or a negative, 

 anomaly at any station, it may be due to one of three causes, and 

 may indicate 



(1) that the compensation of the elevated masses is in- 



complete, or in excess ; 



(2) that the real compensation is such that its centre of 



effect lies at a greater, or a lesser, depth than that 

 of the compensation assumed in the calculation ; or 



(3) that there is a local excess, or defect, of density in the 



matter lying below the level of the stations, inde- 

 pendent of the effect of the elevated masses and 

 their compensation. 



The form in which the gravity observations of the Survey of 

 India have been published has undergone greater changes than 

 in the case of the deflection of the plumb-line. The older calcula- 

 tions are based on Prof. Helmert's 1884 formula for the theoretical 

 variation of gravity with latitude, and on values of 5-6 and 2"8 

 for the mean densities of the earth, as a whole, and of surface rock, 

 respectively. All the published anomalies, making allowance for 

 the effect of height alone or of height and visible masses, were 

 calculated on this basis, but, with the introduction of the considera- 

 tion of the effect of compensation, different values for the density 

 of the earth and of surface rock were adopted, namely 5"576 and 

 2 - 67 respectively, and Helmert's 1901 formula replaced the earlier 

 one of 1884. The result is that the anomalies allowing for compen- 

 sation are not directly comparable with those in which it is not 

 considered. The difference in the densities used has but a small 

 effect, except in the case of the Himalayan stations, but the 1901 

 formula gives a larger value for the theoretical value of gravity 

 and, consequently, a negative change in the value of the anomaly 

 which amounts, in the stations dealt with, to from — - 022 to 

 —'027 dyne. * 



There are, fortunately, a sufficient number of stations for which 

 the Hayford anomalies have been calculated to serve most of the 

 objects of this investigation, and these will be made use of, so far 



[ 177 ] 



D 



