ON THE ELLIPTICITY OF THE EARTH. 483 



laying undue stress on -j-^, the value assigned by M. La- 

 place in his " Exposition du Systeme du Monde, 1799," 

 we find in the second volume of Biot's " Astronomic 

 Physique," published in 1810, y^Ts staked, as the 

 precise result of a careful combination of all the arts of 

 the meridian which had been measured up to that date. 

 In 1830, Mr. Airy, after a careful consideration of all 

 the arcs which had been then measured, estimated their 

 most probable result at 29 g. 33 . (Encyc. Met. Figure of 

 the Earth.) In 1837, M. Bessel, whose acquirements and 

 habits specially qualified him for such a task, undertook 

 the revision and rectification of the earlier arcs, and 

 their combination with three additional arcs which had 

 been then recently completed. The general result was 

 ,,ry. The extension of the Eussian, and the later 

 revision of the Indian arc, have made another step in 

 the value of the denominator of the fraction, of nearly 

 the same amount as the preceding step between the 

 conclusions of M. Biot and of MM. Airy and Bessel. The 

 difference between the ellipticity which is now found 

 to result from the measurement of degrees, and that of 

 the pendulum, is reduced to 3 in the denominator 

 of the fraction which denotes the amount of the com- 

 pression ; or to about a fifth part only of the change 

 which additions and rectifications in the measures of 

 degrees have made since the early part of the century. 

 Viewing the obvious tendency of the two methods 

 the pendulum and the measurement of degrees to 

 unite in one and the same conclusion, we may surely 

 permit ourselves to regard the still subsisting very small 

 difference between them as comparatively, if not wholly, 

 insignificant. In regard, however, to this really slight 



II 2 



