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II. On the Attraction of the Himalaya Mountains, and of the elevated regions beyond 
them, upon the Plumb-line in India. By the Venerable John Henry Pratt, M.A., 
Archdeacon of Calcutta. Communicated by the Rev. J. Challis, M.A., F.R.S. <^c. 
Received October 23, — Read December 7, 1854. 
1. It is now well known that the attraction of the Himalaya Mountains, and of the 
elevated regions lying beyond them, has a sensible influence upon the plumb-line in 
North India. This circumstance has been brought to light during the progress of 
the great trigonometrical survey of that country. It has been found by triangulation 
that the difference of latitude between the two extreme stations of the northern 
division of the arc, that is, between Kalianpur and Kaliana, is 5 ° 23' 42"’294, whereas 
astronomical observations show a difference of 5° 23" 37"'058, which is 5"'236* less 
than the former. 
2. That the geodetic operations are not in fault appears from this ; that two bases, 
about seven miles long, at the extremities of the arc having been measured with the 
utmost care, and also the length of the northern base having been computed from 
the measured length of the southern one, through a chain of triangles stretching 
along the whole arc, about 370 miles in extent, the difference between the measured 
and the computed lengths of the northern base was only 0‘6 of a foot, an error whicli 
would produce, even if wholly lying in the meridian, a difference of latitude no greater 
than 0"'006. 
3. The difference 5"'236 must therefore be attributed to some other cause than 
error in the geodetic operations. A very probable cause is the attraction of the 
superficial matter which lies in such abundance on the north of the Indian arc. This 
disturbing cause acts in the right direction ; for the tendency of the mountain mass 
must be to draw the lead of the plumb-line at the northern extremity of the arc more 
to the north than at the southern extremity, which is further removed from the 
attracting mass. Hence the effect of the attraction will be to lessen the difference of 
latitude, which is the effect observed. Whether this cause will account for the error 
in the difference of latitude in quantity, as well as in direction, remains to be con- 
sidered, and is the ‘question I propose to discuss in the present paper. 
4. But if mountain attraction have any sensible influence at the stations on the 
arc, how is it that the geodetic operations are not affected by it ? How is it that such 
a remarkable degree of exactness between the measured and computed lengths of the 
* This is the difference as stated by Colonel Everest in his work on the Measurement of the Meridional 
Arc of India, published in 1847. See p. clxxviii. 
MDCCCLV. I 
