314 



LIETTT.-COLONEL S. G. BURRARD ON THE 



northerly deflection, two show a southerly deflection, and three show the direction of 

 gravity to be vertical. The mean deflection throughout the trigon is 6"'4 North. 



Now if we are to accept these results as final, we shall have to believe that 

 throughout the third region the level surface is always inclined by 6"'4 to the 

 spheroidal surface. We know of no cause tending to produce such an extraordinary 

 deformation, and we are led to suspect the reasoning by which its existence has been 

 inferred. The only certain fact that has been brought to light by observation is that 

 the plumb-lines in the trigon have a northerly deflection greater by 6"'4 than the 

 plumb-line at Kalidnpur. We have, however, taken a step in advance of this safe 

 ground, and have assumed that the direction of gravity at Kalidnpur is vertical, and 

 that consequently the plumb-line throughout peninsular India is deflected 6"'4 towards 

 the north. Would it not be more reasonable to assume that the mean direction of 

 gravity throughout the third region is vertical, and that the plumb-line at Kalidnpur 

 is deflected G"'4 towards the south ? The assumption of a southerly deflection of 

 G"'4 at Kalianpur will lead then to the conclusion that throughout the third region 

 the level surface remains generally parallel to the spheroidal surface. 



From visible evidence Kalidnpur, situated as it is in flat plains, would be adjudged 

 a suitable datum station, but it unfortunately lies in the zone of southerly deflection, 

 and its plumb-line is thus exposed to the horizontal attractions of hidden masses. 



If our geodetic operations had been confined to the third region, and if our datum 

 station had been originally selected within this region, we should not have been led 

 to suppose that its whole area of 750,000 sq. miles was abnormally affected. If we 

 had subsequently extended our operations to Kalidnpur, we should have discovered 

 there a southerly deflection of about 6", and this we should have adopted without 

 question. 



TABLE showing the Number of Observed Deflections in the Third Region 



