CXXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



feel bomid therefore to notice them in order to testify my gratitude 

 to Col. James for having extended the labours of his department 

 beyond the ordinary investigations of a survey, so as to embrace the 

 interesting question of the earth's density, and my high appreciation 

 of the ability of Capt. Clarke, R.E., who has made all the necessary 

 calculations requhed for the very intricate discussion of the results 

 obtained. It has long been a recognized fact that the plumb-line 

 of an instrument used for obtaining the altitudes of celestial objects 

 is often so deflected by local attractions, as to produce a material dis- 

 agreement between astronomical and geodetical amplitudes. Lieut. - 

 Col.Yolland, under the direction of the late General Colby, had in- 

 vestigated the circumstances of such attractions in respect to some of 

 the leading points of the Trigonometrical Survey, but Col. James 

 has gone further in Scotland, by taking advantage of the contoured 

 plans of the county of Edinburgh, and the known specific gravity of 

 the rocks, in order to determine the amount of local attraction which 

 would be exercised on the zenith sector placed upon Arthur's Seat, 

 near Edinburgh, and in what degree that attraction would account 

 for the deflection of 5"-25 at Arthur's Seat ; or of the 5"'63 at the 

 Royal Observatory on Calton Hill. 



The unequal distribution of matter, the great trough of the Frith 

 of Forth being on the north and the range of the Pentland Hills to the 

 south, affords a presumptive reason for such deflection, and in con- 

 sequence observations were taken both at Arthur's Seat, and at points 

 near the meridian, on the north ahid south of that mountain, the ob- 

 servation-duty having been confided to Sergeant-Major Steel of the 

 Royal Sappers and Miners, who performed it with great ability. 



After some preliminary investigations, Capt. Clarke states that the 

 geodetical amplitudes between the station on Arthur's Seat and those 

 north and south of it having been determined geodetically by a sub- 

 sidiary triangulation, the differences between the astronomical and 

 geodetical amplitudes were as follows : — 



Between the vertex of the Hill and South Station A-|- G. . 2"*81. 

 Between the vertex and North Station .... i\.-f-G. . 1"*26. 

 Between the North and South Stations A-|-G. . 4"-07. 



Comparing now the geodetical and astronomical latitudes, the astro- 

 nomical exceeds the geodetical at the Vertex 5"' 25, at South Station 

 2"-44, at North Station 6"'51 ; and Capt. Clarke observes that these 

 deflections cannot be accounted for by the attraction of Arthur's 

 Seat alone, as such attraction ought to have operated at the South 

 Station in an opposite direction, should have been 7iil at the Vertex, 

 and could not have produced so great a result as that of the North 

 Station. The comparison of the observed and geodetical latitudes 

 of Calton Hill gives similar results, as the deflection to the South is 

 5"*63, of which not more than 0"'l or 0"*3 can be ascribed to the 

 attraction of Arthur's Seat. Capt. Clarke then shows that the defi- 

 ciency of matter in the hollow of the Forth would be insufficient to 

 account for the deflection, as the attraction of the mass of water 

 would be about 0"'02, and that of a mass of rock of spec. grav. 2*5 



