JB65 | On Local Attraction, 41 



Postscript. 

 [Received 29th April, 1865.] 

 If the raw or uncorrected results of the Surveys in India and 

 Europe (I mean uncorrected for local attraction) are made use of, 

 they bring out meridians of a slightly different curvature in these 

 different paits of the earth. If these were the true forms of the seve- 

 ral meridians the result would be that the equator could not be a circle 

 and the figure of the earth not a spheroid of revolution. A few years 

 ago, General T. F. de Schubert calculated the form of an ellipsoid of 

 three unequal axes which would best suit the observations^ Captain 

 Alexander Clarke, R. E. (Memoirs Boy. As. Soc. Vol. XXIX, for 

 I860,) went through the same calculation, following Bessel's method. 

 His result was that the equatorial radius in longitude 14° or there- 

 abouts is one mile longer than that in longitude 104°. He speaks 

 with hesitation regarding the result, on the ground that the data are 

 far too scanty to lead to a conclusion to be relied upon. He appears, 

 however, not to shrink from the hypothesis on which he works, from 

 the true grounds of distrust, viz. (1) the d priori improbability 

 that the earth's mean figure is not one of revolution, as the evidence of 

 the fluid-origin of that figure is overwhelming* and (2) that the effect 

 of local attraction is altogether overlooked by him. General de Schubert 

 x ndeed in a subsequent paper (See Monthly Notices of Boyal Astrono- 

 mical Soc. for 1860, p. 264, where it is noticed) does anticipate that 

 local attraction may modify and altogether destroy the data on which 

 he rested the argument of an ellipsoidal figure. The Paper which I 

 have sent to the Society and have noticed in this letter gives, for the 

 first time, a method for estimating the effect of local attraction and 

 proves (in the third section) that so very moderate an allowance as 1" 

 or 2" for local attraction will altogether destroy the disparity between 

 the curvature of the different meridians. When the arguments in this 

 paper are impartially weighed I feel convinced that the improbable 

 ellipsoidal theory will be abandoned altogether. 



* The evidence, with full details, is given in the third edition of my treatise 

 on the " Figure of the Earth" now passing through the press at Cambridge 

 and a copy of which when published.! purpose sending to the Society, 



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