THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



AUGUST 1878. 



XII. On the Figure of the Earth. 

 By Colonel A. R. Clakke, C.B., F.R.S* 



THE fraction g-J-Q, which, in round numbers, is taken to 

 express the ellipticity of the earth, has apparently a ten- 

 dency, as far as it is deduced from the measurement of terres- 

 trial arcs, to increase as the data of the problem are added to. 

 The ^^g-, obtained by Airy and Bessel from the very imperfect 

 data of forty years back, was replaced, on the completion of 

 the Russian and English arcs in 1858, by 2^4 ; and the geo- 

 detic work recently completed in India indicates a further 

 increase of the fraction, and so an assimilation to that obtained 

 from pendulum observations. The data of the Indian arc of 

 21°, as used in 1858, were vitiated by a serious uncertainty as 

 to the unit of length used by Colonel Lambton in the mea- 

 surement of the southern half of that arc. It appears from 

 the Annual Reports of Colonel Walker, C.B., F.R.S. , Surveyor- 

 General of India, who has been for many years Superintendent 

 of the Great Trigonometrical Survey of India (reports which 

 are replete with scientific interest), that this southern portion 

 of "the Great Arc," as Colonel Everest delighted to call it, 

 has been completely remeasured and the latitudes of a great 

 number of stations in it determined. A complete meridian 

 chain of triangles has also been carried from Mangalore on the 

 west coast, in latitude 12° 52' and longitude 75° E., to a point 

 in latitude 32°. As this triangulation is rigidly connected 

 with the arc from Cape Comorin to Kaliana, in 78° E. longi- 

 tude, it may be considered that the Indian Arc is now 24° in 

 length. 



* Communicated by the Author. 



Phil. Mag. S. 5. Vol. 6. No. 35. Aug. 1878. G 



