Lieut.-Col. James on the mean specific gravity of the Earth. 315 



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"-l, 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 the 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 the matter, both on account of its scientific interest and with a 

 view to confirm the results arrived at from the previous investigation 

 6f all the observed latitudes. Col. James accordingly decided on 

 having observations taken with Airy's Zenith Sector on the summit 

 of Arthur's Seat, and at two other points near the meridian line on 

 the North and South of that mountain, at about one-third of its alti- 

 tude 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, and 220 double observations of stars were taken at each Station. 

 The reductions and computations connected with these observations, 

 as well as the computations of the local attraction at the Calton 

 Hill, were entrusted to Captain Clarke, R.E., by whom the account 

 now communicated of the mathematical investigation of the observed 

 data has been drawn up. 



Col. James has himself examined the geological structure of 

 Arthur's Seat and of the whole of the county of Edinburgh, and has 

 caused determinations to be made of the specific gravity of all the 

 different rocks, with the view of estimating the mean specific gravity 

 of the whole mass ; but he observes, that although the geological 

 structure of Arthur's Seat is well exposed, and its mean specific 

 gravity, 2*75, has been employed for deducing that of the earth, 

 viz. 5*1, still it is not a mountain he should have selected for this 

 special object. Accordingly he was pleased by discovering — ron re- 

 ferring, since these observations were made, to the correspondence of 

 the Survey, — that the late Dr. Macculloch had been employed 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 the purpose referred to, and that 

 he had pointed out the Stack Mountain in Sutherlandshire as 

 admirably answering the required conditions. 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, 

 Col. James purposes early in the spring to have the Stack Mountain 

 and the surrounding country surveyed and contoured, and to have 

 observations taken for determining the attraction of its mass, and he 

 trusts by the close of the present year to lay the results before the 

 Royal Society. 



