Comparison of the Anglo-Gallic , Russian, and Indian Arcs. 145 



100° C. Carbonic acid is evolved, chloride, iodide, and acetate 

 of methyle being formed, whilst the residual solid is common 

 salt. 



A similar reaction has also been observed in the case of buty- 

 rate of soda, and also with benzoate of soda, iodide of phenyle 

 and biniodide of phenylene being formed in the latter case. 



XX. Comparison of the Anglo -Gallic, Russian, and Indian Arcs, 

 with a view to deduce from them the Mean Figure of the Earth, 

 By Archdeacon Pratt, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THROUGH the kindness of Lieut. -Colonel Gastrell, officia- 

 ting Surveyor- General of India, the formulae for the semi- 

 axes of the Mean Figure of the Earth, which I demonstrated in 

 my last communication to you, have been reduced to numbers 

 for the three long arcs — the Anglo- Gallic, the Russian, and the 

 Indian, which stretch over more than 22, 25, and 21 degrees 

 of latitude respectively. Three forms of the meridian are thus 

 deduced, corresponding to these three long arcs, involving re- 

 spectively the three unknown quantities (/j), (/ 2 ), (/ 3 ) — that is, 

 the sums of the unknown local attractions at all the stations in 

 the three separate arcs. 



I show that values of these three unknowns can be obtained 

 which make the three meridians almost precisely the same ; and 

 these values of (t^, (t 2 ), (t 3 ) are all very small; so that no ex- 

 travagant hypothesis has to be resorted to, regarding the amount 

 of local attraction, to produce this identity of form. 



The average of the three values of the semiaxes thus deduced 

 gives 



«=20926184, 6 = 20855304 feet, 



e = l~295-3; 



and none of the six values of a and b depart from these average 

 values by so much as 300 feet ; generally the departure is far 

 less. These, therefore, I consider to be the true values of the 

 semiaxes of the mean figure of the earth. They agree, as I said 

 they would, with the values deduced from my first method of 

 correcting BesseFs process, differing in fact by only the infinite- 

 simal quantities 5 and 12 feet (see No. 64, Proceedings of Royal 

 Society, p. 270; or my 'Figure of the Earth/ third edition, p. 134). 

 The mean figure has never before been determined, taking into 

 consideration the effect of local attraction. 



The forced hypothesis of an elliptic equator and an ellipsoidal 

 figure, which General de Schubert first suggested, and afterwards 



Phil. Mag. S. 4. Vol. 33. No. 221. Feb. 1867. L 



