44 



OLDHAM: THE STRUCTURE OF THE HIMALAYAS. ETC, 



hills, which have to be included in making a comparison between 

 the effect of actual and imaginary topography. These are absent 

 in the Sikkim area ; elsewhere they lie south of the main boundary, 

 and belong properly to the region of the Gangetic trough, but must 

 be considered as part of the topography so far as they affect the 

 deflection of the plumb-line. They will be simplified into a plateau 

 of 20 miles in width and 1,500 feet in height above the plains, 

 dimensions which conveniently, and approximately, represent the 

 actual topography ; and, as the mean density of the Siwalik rocks 

 is about 2*2, and of the rocks of the main range about 2'7, the deflec- 

 tions will be estimated at eight-tenths of those which would result 

 from Mr. Hayford's figures. In table No. 6 the deflections so 

 obtained are given on the assumptions respectively, of no compensa- 

 tion, and compensation according to Mr. Hayford's tables ; the 

 difference does not in any case exceed one second of arc, and though 

 there seems some reason, in this area, for not using the hypothesis 

 of compensation, it will be safer to use the figures in the second 

 column, which must be added to those obtained from other tables, 

 when it is necessary to consider the attraction of the hills of the 

 Sub-Himalayan region. 



Table G. — Deflections due to an Imaginary plateau, representing 

 the Sub-Himalaya or Siwalik Hills, assumed 20 miles broad by 

 1,500 feel in height and of density '8 of average rock. 





Station. 



Uncompensated. 



Compensated. J 



■ 



— 5 



+ I 1 









— 4 









— 3 



+ 1 





— 2 



+ *2 



+ 1 





— 1 



+ 8 



+ 2 









+ 14 



+ 13 





+ 1 













+ 2 



— 14 



— 13 





+ 3 



— 3 



— 2 





+ 4 



— 2 



— 1 





+ 5 



— 1 



— 1 





+ 6 



— 1 









+ 7 



— 1 









+ 8 











Having formed an estimate of what the effect of compensation 

 would be, if it is given the average value determined by observa- 



[ 192 ] 



