40 OLDHAM-: THE STRUCTURE OF THE HIMALAYAS, ETC. 



In my earlier investigation of the effect of the Gangetic Alluvium 

 on the Plumb-line, 1 only those masses lying within 50 miles of each 

 station were considered, but in this more extended investigation 

 the limit has had to be enlarged and in each case a strip running 

 transverse to the range and extending 100 miles on cither side of 

 the station, and so much of this strip as lies within 100 miles of it, 

 has been taken into consideration. In other words each station 

 successively has been conceived as lying in the centre of a 200-mile 

 square, and everything outside this limit has been put out of con- 

 sideration. It will be shown, further on, that the effect of this 

 limitation, of the area considered, is so small, in proportion to the 

 effect produced by the area actually considered, that it may be 

 left out of consideration for the purpose of this investigation. 



We are now ready to take up the effect of different hypotheses 

 of compensation as applied to the Imaginary Range and, as a stalling 

 point, the tabular statement No. 4 (page 41) gives the deflections 

 which would be produced by the Range proper, excluding the effect of 

 the Siwalik Hills, at a series of stations 10 miles apart, the masses 

 within a square of 200 miles a side being alone considered, and as 

 they would result (1) from the uncompensated effect of the visible 

 mass and (2) allowing for the effect of compensation according to Mr. 

 Hayford's factors for uniform compensation to 1137 km. 



Before proceeding further it will be useful to consider what 

 modification in these figures would result from including a larger 

 area in the calculations and, as an extreme case far exceeding 

 anything to be met with in nature, I have supposed the stations 

 to be situated in the centre of a square of 2,400 miles on the side, 

 and the plateau, slope, and plain of the Imaginary Range, to be 

 extended over the whole of the area thus taken into consideration. 

 In these circumstances the uncompensated deflection at the station 

 0, on the edge of the hills, would be increased by 78" ; at station 

 10, or at a distance of 100 miles from the edge of the hills, in either 

 direction, the deflection would be increased by 69" ; the difference 

 would, therefore, be increased by 9". Jf, however, the effect of 

 the Hayford compensation be taken into account, the increased de- 

 flections would be reduced to just under 3" at the edge of the hills 

 and just over 2" at 100 miles away, and the difference would be but 



1 Proc. Roy. Soc, Series A, XC, pp. 32-41 (1914). 

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