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IX. 

 GEODETIC OBSERVATIONS. 



All great national surveys have for many years past contributed 

 data for the determination of the figure of the earth. At first these 

 consisted mainly of measurements of the distances between successive 

 points situated on a common meridian, and of the astronomical arcs 

 of amplitude between these points, but it is only of late years that 

 similar measurements have been attempted in the case of points 

 situated on a common parallel of latitude. The reason of this is that 

 the required astronomical data are very readily obtained in the case 

 of meridional arcs by determining the latitudes of the geodetic 

 points, which is one of the simplest problems of practical astronomy ; 

 but on arcs of parallel, longitudes are required instead of latitudes, 

 and the precise astronomical determination of an absolute longitude 

 under such circumstances is well nigh impracticable. When, how- 

 ever, places are connected telegraphically, their differences of longi- 

 tude can be determined with great precision, and this has been done 

 both in America and Europe. In the former country the operations 

 were utilised for geographical purposes only, but in Europe, which 

 is covered with a net of triangulations, of which there is as yet no 

 counterpart beyond the Atlantic, their chief interest lies in the light 

 they throw on the figure of the earth. 



At the same time these investigations have benefited the Indian 

 Survey in a way that may be shortly explained. In order that 

 the true latitudes, longitudes, and azimuths of the stations of a 

 system of triangulation on the earth's surface may be computed, 

 it is necessary that the polar and equatoreal axes of the terrestrial 

 spheroid should be correctly known, as they are involved in the 

 formula? by which such computations are effected. The value 

 of those elements that have since 1830 been used in the calculations 

 of the Indian Survey are those known as " Everest's Constants, 

 1st set." Any error in these adopted elements will, of course, 

 produce an error in the geodetical latitudes and longitudes of the 

 stations, increasing with the distance from the originating station. 



1 Y 20321. Q 



