592 
ON THE DEFLECTION OF THE PLUMB-LINE AT ARTHUR’S SEAT, 
Calton Hill. The following communication has been drawn up by him, and I trust it 
will prove acceptable to the Royal Society, and do him credit as a mathematician. 
I have myself examined the geological structure of Arthur’s Seat and the whole of 
the county of Edinburgh, and have had the specific gravity of all the rocks ascer- 
tained, with the view of estimating the mean specific gravity of the whole mass ; 
but although the geological structure of Arthur’s Seat is well exposed, and we have 
deduced from its mean specific gravity (2'75) the mean specific gravity of the earth, 
viz. 5’316, it is not such a mountain as I should have selected for this special object. 
Since these observations were made, on examining the correspondence connected 
with the Survey, with the view of drawing up an historical sketch of its progress for 
publication, I was agreeably surprised to find that the late Dr. Macculloch had 
been employed for six years, from 1814 to 1819, in examining the whole of Scotland 
for the purpose of selecting a mountain which, from its homogeneous structure, size, 
and form, would be best suited for observations for the purpose of determining the 
mean specific gravity of the earth, and that he considered the Stack Mountain in 
Sutherlandshire admirably suited for the purpose. The transfer of the whole force 
of the Survey from the North of England and Scotland to Ireland, prevented the late 
General Colby from undertaking this investigation ; but as the Survey of Scotland is 
now in full progress, 1 purpose early in the spring to go down to the Stack Moun- 
tain, to have it and the surrounding country surveyed and contoured, and to have 
the observations taken for determining the attraction of its mass, and I trust at the 
close of the present year to lay the results before the Royal Soeiety, 
I forward herewith a model of Arthur’s Seat, made from the contoured plan on the 
seale of 6 inches to a mile, and also an impression of the plan itself, with sections 
showing the geological structure of Arthur’s Seat, and a table of the specific gravity 
of the rock of which it is composed. 
Henry James, 
Feb. 7, 1856. Lieut. -Col. R.E. 
In deducing from the observations made at the three stations on Arthur’s Seat, 
with the zenith sector, the latitudes of those stations, if we assign to the resulting 
latitude given by any one star a weight equal to the number of observations of that 
star, the final latitudes of the three stations will stand thus: — ■ 
Stations. 
Designated. 
Latitude. 
Number of 
observations. 
South Station 
S 
65 56 26-69 
427 
Artliur's Seat (summit*) 
A 
55 56 43-95 
425 
North Station 
N 
55 67 9*50 
411 
* The station on the summit of the hill was 14 feet from the Ordnance Trigonometrical Station, and bear- 
ing 18® North-west; the former is therefore 0"‘13 North of the latter. 
