114 



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

 steel chains, and only differs 0*173 ft., or about 2 inches, from its 

 computed length from the mean base. 



The Belhelvie base in Aberdeenshire, also measured with the steel 

 chains, 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 Doncaster, 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 lines of the old sheets of the one-inch 

 Map of England, and this is now being 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 



