ON ASTRONOMY. 95 



This whole distance, measured with singular ability, extending from 

 Cape Comorin to the foot of the Himalay mountains, is 1,477 miles. 



Two of Everest's bases of verification, of about 7^ miles in extent, 

 differed by computation, the one 4 and the other 7 inches from the 

 measured length. 



13. The third is theRussian arc, extending from Ismail on the Danube 

 to the Arctic sea near the North Cape, being more than 25 degrees 

 (25° 20'.) The work was under the direction of the able astronomer 

 Strove. It was begun in 1816, and has but recently been completed. 

 The complete discussion of the observations has not yet been given to 

 the public. This arc is about 1,750 miles in extent, and is believed 

 to have been measured with eminent ability. 



14. Any sketch of the geodetic operations which serve to determine 

 the figure of the earth would be incomplete without a notice of the 

 United States Coast Survey. 



The great interest with which this question was regarded in early 

 times arose from its importance as an astronomical problem; but in 

 the attempts to devise more accurate methods for the measurement, of 

 arcs upon the earth's surface, it was found that, for the construction 

 of accurate maps of any extended portions of the earth, a knowledge 

 of its precise figure was not only essential, but that the method of 

 triangulation which was adopted to connect the extreme points of 

 the arc to be measured, together with the astronomical positions of 

 those points, formed the best possible basis of a map of the region 

 traversed by the system of triangles. 



This was the origin of the science called geodesy or geodetic sur- 

 veying. The interest which was felt in the solution of an impor- 

 tant scientific question was, therefore, soon merged in that which 

 attended its practical benefits to the human race, in the construction 

 of accurate maps, showing the boundaries of States and provinces, 

 and the configuration of sea coasts. 



The French arc was first made the basis of a complete map of 

 France, and most of the measurements of arcs in modern times have 

 been incidental to the prosecution of extended surveys of the territo- 

 ries through which they pass. 



The figure of the earth having been approximately determined, the 

 more accurate measurement of its dimensions, from having been the 

 primary object of the great geodetic operations of the globe, has, 

 therefore, become secondary or incidental to geodetic surveys. Thus 

 the Indian arc, the Russian arc, and numerous measurements in Eu- 

 rope, would, perhaps, not have been undertaken, except as the result 

 of extended trignometrical surveys for national purposes. 



Every improvement in the construction of instruments for geodetic 

 and astronomical purposes, and in the methods of observation em- 

 ployed, gives additional value and weight to subsequent measure- 

 ments, and hence it is that the data derived from the geodetic opera- 

 tions of the United States Coast Survey for the more accurate solution 

 of the question of the figure of the earth may be looked for with great 

 interest. 



The important objects of this survey, united with the great extent 

 of the coast, requires the highest attainable accuracy in the great 



