616 CoL Henry James , on the [April 23P, 



WEEKLY EVENING MEETING, 



Friday, April 23. 



The Duke of Northumberland, K.G. F.R.S. President, 

 in the Chair. 



Col. Henry James, R.E. F.R.S. 

 On the Geodetic Operations of the Ordnance Survey. 



The Geodetic operations of the survey include the triangulation 

 and levelling, which extends over the whole United Kingdom, the 

 measurement of arcs of meridians, and the determination of the 

 figure, dimensions, and mean specific gravity of the earth. 



The special object for which these operations are required was 

 first described, and then, the methods employed in performing 

 them were briefly sketched. 



The triangulation, consists, 1st of a primary series of tri- 

 angles, the sides of which are some of them upwards of 100 

 miles in length, and the stations of which are placed on the highest 

 mountains ; such for example, as Snowdon, in Wales, Sea Fell, in 

 Cumberland, and Slieve Donard, in the county of Down, in Ireland ; 

 the sides of this triangle are each upwards of 100 miles long. This 

 great triangulation extends over the United Kingdom ; — a series of 

 stations are then selected to form a secondary triangulation, the 

 sides of which are from 10 to 15 miles long ; and again another 

 series of stations are selected to form the minor or tertiary triangula- 

 tion, the sides of which are about three-quarters of a mile in length ; 

 and thus the whole country is covered with a connected network of 

 triangles, to form the basis of the detailed survey which is now in 

 progress. Upon the accuracy with which this portion of the work 

 is executed, mainly depends the character of the national survey 

 for accuracy in its most important features. 



The minor triangulation is that which is immediately used for the 

 detailed survey : the field surveyors actually measure the length of 

 each side with their chains, and cross lines are also measured within 

 the triangles. As the length of each side is previously known, 

 the correctness with which the surveyors perform their work is 

 tested in the office, and accuracy insured in every part. The 

 establishment of a general triangulation also enables the engineer 

 to employ large numbers of surveyors at the same time, and without 

 any fear of the work being distorted in any direction, as every 



