1859] Reply to Mr. Pratt's letter to the Asiatic Journal, 17 



Reply to Mr. Pratfs letter to the Asiatic Journal on the Indian Arc 

 of Meridian. — By Captain F. P. Tenn/VNt, Bengal Engineers, 

 F. R. A. S. 1st Assistant G. T. Survey of India. 



A couple of days ago, I received the copy of the letter from Mr. 

 Pratt to the Secretary of the Asiatic Journal. As I am not a member 

 of the Society T should be much obliged by your communicating the 

 following answer. 



1. I must explain ; that the direction of the plumbline at any 

 point of the earth's apparent surface is determined by combined 

 action, of the centrifugal force resulting from the earth's revolution 

 round its axis, and the attraction of every particle of the matter con- 

 stituting the earth. These are the only forces in action ; and the 

 result would be ; were all the matter in the earth free to arrange 

 itself; that the figure would be rigorously an ellipsoid of revolution, 

 whose ellipticity would depend on the law of the earth's density in 

 approaching the centre. 



2. The earth however is not fluid. The position of every particle 

 of by far the greatest portion of matter is almost unchangeable. In 

 addition to this, there are many projections from the general surface, 

 and depressions below it, as well as internal irregularities of struc- 

 ture. All these are small with reference to the enormous mass of 

 the earth itself which may therefore be most simply considered 

 as an ellipsoid of revolution -f* superfluities of matter in certain 

 positions — certain deficiencies. So also the total attraction of the 

 earth is the resultant of the attractions of the ellipsoid and the 

 separate irregularities. 



3. Were the ellipsoid alone existent, the plumbline would be 

 every where normal to its surface, that being one of the conditions 

 of equilibrium, but in consequence of these irregularities the 

 direction of the plumbline is changed and there is hardly any point 

 of the earth's apparent surface where it is perpendicular to the sur- 

 face of the fundamental ellipsoid. 



4. In geodesical operations we project all our stations, and con- 

 sequently the arcs joining them on a surface which is always' 

 perpendicular to the plumbline;. that surface being selected which 



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