1859.] Reply to Mr. Pratt's letter to the Asiatic Journal. 21 



13. The next poiufc requiring notice is the note to page 11, 

 where I submit Mr. Pratt is in error. The surface on which the arc 

 is projected is that which is modified by local attraction or C n of 

 his figure and not C B which represents that attained after eliminat- 

 ing the local attraction's effects. The stilting process is actually 

 carried on and the reductions to the lengths of the arcs are 

 necessary. 



14. From the figure of the earth, the astronomer seeks to deter- 

 mine the radius or the length of the line joining his place to the 

 earth's centre and the angle which this line (or its projection on the 

 plane of the meridian) makes with the perpendicular to the surface. 

 Mr. Pratt's figure being confessedly only very local cannot give 

 these data. 



15. The Geodesist again seeks to determine from one station 

 whose latitude is known and also the azimuth and distance of a 

 second, the latitude of the second, their difference of longitude and 

 the azimuth of the first at the second. Mr. Pratt's figure cannot 

 give these, as I have shown in the case of the longitude and reverse 

 azimuth at Karachee. 



16. In my second paper noticed by Mr. Pratt I say, " I have 

 shown in my former paper that the ellipse given by Mr. Pratt for 

 the Indian meridian is useless for Geodesical purposes. I have 

 now, I think, shown reason to believe that there is no evidence 

 which will warrant our considering that any real departure from the 

 mean form exists," and again " the only figure of any Geodesical or 

 astronomical importance is, that determined as usual from the con- 

 sideration of a number of arcs situated in various circumstances as 

 regards sources of probable local disturbance, I think I have 

 supported this. 



17. "Were we possessed of a large number of observations of 

 latitude and longitude extending over India or the whole world, the 

 true step to take, after the astronomical precedent, would be to 

 determine (which of course is theoretically possible) the size and 

 position of the deflecting masses requisite to reconcile the observed 

 places with those geodesically determined. Whether the requisite 

 observations will ever be available, or the Analytical Giant who 

 must use them, is almost beyond even speculation. Meanwhile, let 



