365 



inotrical stations ; but in almost every case the physical cause of such 

 irregularity may be with probability inferred. 



In the case of the station at the Edinburgh Observatory, and on 

 the summit of Arthur's Seat, whei-e the latitudes inferred geodeti- 

 cally in consistency with the entire survey arc compared with the 

 direct astronomical determinations, a deviation of the plumb-line 

 towards the south, to the extent of between 5" and 6" is manifested. 

 The exact latitudes are as follow : — 



Observed. Calculated. Difference. 



Observatory, Calton Hill, SS^* 57' 23"-20 55" 57' 17"-57 5"-63 



Arthur's Seat, summit, . 55°56'43"-71 55°56'38"-44 5"-27 



From this it is evident that the discrepancy occurring at the Ob- 

 servatory cannot be ascribed to the deflecting attraction of Arthur's 

 Seat, where it exists almost equally. Colonel James attributes it in 

 both cases to the effect of the hollow of the Firth of Forth to the 

 north, together with the mass of high ground to the south, includ- 

 ing the Pentland and Lammermoor ranges. On actually calculat- 

 ing the effect of the configuration of the ground within a radius of 

 15 miles, about 2"-6 of the deviation is accounted for; and the 

 writer believes that the mountainous country beyond may farther 

 sensibly increase the effect. 



With a view to determine the strictly local attraction of Arthur's 

 Seat, three stations were fixed nearly on a common meridian line, 

 passing through the summit of the hill. These are marked N, A, 

 and S. The station N (most northerly) is in the vicinity of St 

 Anthony's Chapel, A is almost on the highest point of the hill, S 

 is situated on the knoll above Samp.son's Ribs. 220 double obser- 

 vations of stars were made at each station in September and October 

 1855 with Airy's Zenith Sector. 



The difference of astronomical latitude of the stations N and S is 

 42" 56. 



The difference of the geodetical latitudes is 38"'46. 



The difference of these numbers, or 4"- 10, measures the double 

 deflection of the plumb-line at the two stations due to the attraction 

 of the interposed hill. 



The accurate system of contours which have been carried round 

 the hill allows the calculation of the attraction of all its parts at 

 the two stations N and S, to be performed with the utmost nicety, 

 on the supposition of its being of homogeneous material. By in- 



voL. ni. 2 H 



