THE tLNDERGROUND FORM OF FLOOR OF OAXGETIC TROUGH. 6? 



sation, at a station situated at a corresponding distance, reckoned 

 from the main boundary ; at the stations nearest to the hills a 

 small additional correction was included, for the effect of the 

 attraction of the foot-hills of the Sub-Himalayan region. The figure 

 allowed in this way, for the effect of the attraction of the Hima- 

 layas, is a little less than the reality, but the difference would not 

 exceed one second of arc, or at most a couple, at any of the first 

 six stations, included in the table No. 19, and is negligible at 

 the rest. 1 



Allowing for this effect, a first attempt at calculation, on the 

 supposition that the slope of the floor of the trough continued 

 regularly up to the main boundary, showed that this would give too 

 small northerly deflections at the northern stations, nor were 

 matters much improved by supposing that the maximum depth 

 was continued outwards from the main boundary for some con- 

 siderable fraction of the width of the trough, before the upward 

 slope commenced. It became evident, therefore, that the maxi- 

 mum depth of the trough could not be at the northern edge, but 

 must be somewhere out towards the centre, though nearer to the 

 northern than the southern edge ; a supposition was accordingly 

 adopted, that the trough had a depth of 15,000 feet at the main 

 boundary, increasing to 20,000 feet at 50 miles away and then 

 decreasing to nothing in 150 miles. The result of this calculation 

 is given in the column headed I, but at a later period, when the 

 study of other sections had revealed a possibility that the trough 

 attains its greatest depth close to the outer edge of the visible hills, 

 another assumption was made, that the maximum depth was 

 25,000 feet at the outer edge of the hills, that the floor sloped re- 

 gularly upwards from this to the southern edge of the plain, and 

 on the north rose abruptly upwards to a depth of 20,000 feet 

 diminishing to 15,000 feet at the main boundary. The result of 

 this supposition is given in the column headed II. 



1 Wehavo a check on the correctness of method of arriving at the allowance to he 

 made for the effect of the trough, and any other invisible influence, in Major Crostli- 

 wait's calculation of the residuals at I'athardi and Nimkar. On the same basis of 

 reference as is hero used in the text, the residuals, after allowing for the effect of 

 visible topography and its compensation, are — 12" and -|-5" respectively, the 

 values derived by using the Imaginary Range were — 9" and +4". Tho use of 

 the Bcssel-Clarko spheroid would introduce a change of 1" in tho values of the 

 residuals. Evidently the Imaginary Range gives a larger deflection than the actual 

 topography of this part of the actual range, but it must be remembered tluit the Hima- 

 layas in Nepal territory are quite unsurveyed. 



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