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XXVIII. On the Deflection of the Plumhdine at Arthurs Seat, and the Mean Specific 
Gravity of the Earth. Communicated by Lieutenant-Colonel James, R.E., F.R.S., 
M.RJ.A. &^c., Superintendent of the Ordnance Survey. 
Received February 11, — Read February 21, 1856. 
The Royal Society has, from the very commencement of the Ordnance Survey of 
the United Kingdom, taken a deep interest in its progress. I have therefore great 
pleasure in announcing to the Society the completion of all the computations con- 
nected with the Primary Triangulation, the measurement of the Arcs of Meridians, 
and the figure and dimensions of the Earth. 
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. 
After determining the most probable spheroid from all the astronomical and 
geodetic amplitudes in Great Britain, we find that the plumb-line is considerably 
deflected at several of our 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"'25, and at the Royal Obser- 
vatory at Edinburgh it amounts to 5"'63 to the South. The unequal distribution of 
matter here, the great trough of the Firth of Forth being on the North, and the range 
of the Pentland on the South, presents a tangible cause for the deflection ; but as the 
contoured plans of the county of Edinburgh are published, and we have the most 
perfect data that it is possible to obtain for estimating the amount of local attrac- 
tion at Arthur’s Seat and the Calton Hill, and as it appeared to me that an investiga- 
tion of this matter was not only necessary to confirm and establish the results arrived 
at from the previous investigation of all the observed latitudes, but would also prove 
highly interesting to science, I decided on having observations taken with Airy’s 
Zenith Sector on the summit of Arthur’s Seat, and at points near the meridian on 
the North and South of that mountain, at about one-third of its altitude above the 
surrounding country. 
The observations were made by Serjeant-Major Steel of the Royal Sappers and 
Miners, during the months of September and October last; 220 double observations 
of stars were taken at each Station, and the I'esults have justified my confidence in 
him as an observer. 
To Captain Clarke, R.E., I entrusted all the reductions and computations con- 
nected with these observations, as well as the computations of the local attraction at 
4 I 
MDCCCLVI. 
