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III. " Account of the Observations and Computations made for 

 the purpose of ascertaining the amount of the deflection of 

 the Plumb-Line at Arthur's Seat, and the Mean Specific 

 Gravity of the Earth ; with an account of the observed and 

 computed amount of the Local Attraction at Arthur's Seat 

 and at the Royal Observatory at Edinburgh." Communi- 

 cated by Lieutenant-Colonel JAMES, R.E., F.R.S. &c., 

 Superintendent of the Ordnance Survey. Received Fe- 

 bruary 11, 1856. 



(Abstract.) 



Col. James begins by observing, that as the Royal Society has, 

 from the very commencement of the Ordnance Survey of the United 

 Kingdom, taken a deep interest in its progress, he has great pleasure 

 in announcing to the Society that all the computations connected 

 with the Primary Triangulation, the measurement of the Arcs of 

 Meridians and the determination of the figure and dimensions of the 

 earth are now completed, and that the account of all the operations 

 and calculations which have been undertaken and executed is now in 

 the press, and will shortly be in the hands of the public. 



In the progress of these operations it has been found, on deter- 

 mining the most probable spheroid from all the astronomical and 

 geodetic amplitudes in Great Britain, that the plumb-line is consider- 

 ably deflected at several of the principal Trigonometrical Stations, 

 and at almost every station the cause of the deflection is apparent in 

 the configuration of the surrounding country. 



The deflection of the plumb-line at Arthur's Seat is 5"*1, and at 

 the Royal Observatory at Edinburgh it amounts to 5"'63 to the 

 South. The unequal distribution of matter in the vicinity of these 

 Stations the great trough of the Firth of Forth being on the North 

 and tbe range of the Pentland Hills on the South presents an 

 obvious cause for the deflection ; but as the contoured plans of the 

 county of Edinburgh have been published and the best attainable 

 data acquired for estimating the amount of local attraction at the 

 above-mentioned stations, it appeared desirable specially to investi- 

 gate tbe matter, both on account of its scientific interest and with a 



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