Lieut.-Col. James on the Figure and Dimensions of the Earth. 131 



The Hounslow Heath hase was measured with Ramsden's 100 ft. 

 steel chains, and only differs 0-1/3 ft., or about 2 inches, from its 

 computed length from the mean base. 



The Belheh-ie base in Aberdeenshire, also measured with the steel 

 chams, differs only 0-24 ft., or less than 3 inches, from the computed 

 length. 



The difference between the measured and computed length of the 

 Misterton Carr base, near Doucaster, also measured with the steel 

 chains, is only 0-157 ft., or less than 2 inches ; and it will be ob- 

 served that the difference between the computed and measured 

 lengths of these three bases (measured with chains) is not greater 

 than the difference between the measured and computed length of 

 the Lough Foyle and Salisbury Plain bases (measured with the com- 

 pensation bars), from which it may be inferred, that bases measured 

 with steel chains are deserving of the greatest confidence ; and when 

 the great simplicity, portability, and cheapness of the chains are 

 compared with the complex, heavy and expensive apparatus of the 

 compensation bars, it may be anticipated that they will be more 

 generally employed than they have been of late years, especially in 

 the colonies, and in countries where the transport of heavy articles is 

 effected with difficulty. . 



The length of the base on Rhuddlan Marsh in North Wales, which 

 was measured with steel chains, differs 1-596 ft. from the computed 

 length ; but from the circumstance that the extremities of the base 

 are very badly situated with reference to the surrounding Trigono- 

 metrical stations, the angles being very acute and not well observed, 

 little confidence has been placed in the result of the comparison of 

 its computed and measured length. 



One of the first practical results arising from the completion of 

 the triangulation is, that it is now possible to engrave the latitude 

 and longitude on the marginal hues of the old sheets of the one-inch 

 Map of England, and this is now beii\g done. 



The following account of the Trigonometrical operations and cal- 

 culations has been drawn up by Captain Alexander R. Clarke, R.E.; 

 this account may be considered an abridgement of that more detailed 

 account which is now in the press, and will be shortly published. 



It will be seen that the equatorial diameter of the earth, as derived 

 from the Ordnance Survey, is 7926-610 miles, or about one mile 

 greater than it is given by the Astronomer Royal in his ' Figure of 

 the Earth,' and that the ellipticity is ^^, or as the Astronomer 

 Royal conjectured, something " greater than 3-^," which he gives in 

 the same paper. 



The mean specific gravity of the earth, as derived from the obser- 

 vations at Arthur's Seat, was stated in a former paper to be .'i- 1 4 ; the 

 calculations have since been revised, and it is now found to be 5-316. 

 The mean specific gravity of the earth, as derived from the only 

 other observations on the attraction of mountain masses on which 

 any reliance has been j.laced, viz. the Schehallieu observations, is, as 

 finally corrected by Ilutton, |^, or almost 5 0. 



K2 



