388 Proceedings of the Royal Institution, 



isoxne ideaiiiay be formed of the labour incurred in this work; while 

 tiie tests tio- which it was subjected afford some interesting proofs of 

 its accuracy. ^.j^; ,, 



The total length of the base was 41640.879 feet, or 7.886 miles. 

 Along the line at certain intervals, fixed points were left during the 

 measurement, and secured with great care, and then by means of a 

 triangulation, in which particular contrivances were employed to give 

 more than usual precision, these distances were compared against 

 each other. Thus, for instance, the distance between the second and 

 third points being by measurement 1050 feet, was deduced from the 

 distance between the first and second points, and found to be 

 1049.993 feet. Again, the distance between the third and fourth 

 points, being by measurement 2100 feet, was deduced from that be- 

 tween the first and third, and found to be 2099.995, and the compa; 

 risons of the other portions gave results equally satisfactory. 



The place selected for the measurement of this base was an exten- 

 sive plain on the east shore of Lough Foyle, in the north of Ireland, 

 and, generally speaking, was exceedingly favourable for such an ope- 

 ration. One obstacle, and that of rather a formidable nature, was 

 presented by the river Roe, which, crossing the plain, intersected the 

 line of the base. At the place of intersection, the breadth is 460 feet, 

 and the greatest depth about four. It was deemed advisable to mea- 

 sure this twice, as the only doubtful part of the whole work, and 

 the result of the two measurements was a difference of y^th of an 

 inch. On the whole base line, the probable error cannot exceed 1 J 

 inch. The extremities have been secured in such a way as to last for 

 ages ; and the government have, by the advice of the Royal Society, 

 purchased the ground on which they rest, witli a view to ensure their 

 preservation. 



While this operation was going on, the triangulation was at the 

 same time rapidly proceeding, and has now extended towards the 

 south as far as the Keeper Mountain, in the county of Tipperary, 

 , while Nephin may be considered its boundary in the west, and Kip- 

 pure in the east. The whole forms a beautiful series of triangles 

 resting on the principal mountains of Ireland, connectejd^witl^ those 

 of Cumberland, the Isle of Man, North and South Wales, and co- 

 vering the Irish Channel. These triangles are for the most part of 



