176 



The following communications were read : 



I. " On the Attraction of the Himalaya Mountains, and of the 

 elevated region beyond them, upon the Plumb-line in 

 India/' By the Venerable JOHN HENRY PRATT, M.A., 

 Archdeacon of Calcutta. Communicated by the Rev. Pro- 

 fessor CHALLIS, M.A., F.R.S. &c. Received October 23, 

 1854. 



The author commences by observing that it is now well known 

 that the attraction of the Himalaya Mountains, and of the elevated 

 region beyond them, has a sensible influence on the plumb-line in 

 North India. This circumstance was brought to light during the 

 progress of the great trigonometrical survey of that country. It 

 was found by triangulation that the difference of latitude between 

 the two extreme stations of the northern division of the arc, namely 

 Kalianpur and Kaliana, is 5 23' 42"'294, whereas astronomical ob- 

 servations show a difference of 5 23' 37"'058, which is 5"-236 less 

 than the former. 



That the geodetic operations are not in fault, appears from this ; 

 that two bases, about 7 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 extending about 

 370 miles, the difference between the measured and the computed 

 lengths was only 0*6 of a foot, which would produce, even if wholly 

 lying in the meridian, a difference of only 0"'006 in the latitude. 



The difference 5"-236 must therefore be attributed to some other 

 cause. A very probable cause presents itself in the attraction of the 

 superficial matter which lies in such abundance on the north of the 

 Indian arc. It is easily seen that this disturbing force acts in the 

 right direction, that is, it diminishes the difference of astronomical 

 latitude between the two stations. Whether the cause here assigned 

 will account for the error in the difference of latitude in quantity as 

 well as in direction, is the question which the author proposes to 

 discuss in the present paper. 



