596 ON THE DEFLECTION OF THE PLUMB-LINE AT ARTHUR’S SEAT, 
of the Other two points, shown in the following Table, in contrast with the observed 
latitudes : — 
Station. 
Astronomical 
Latitude. 
Geodetical 
Latitude. 
A-G. 
s 
55 56 26’69 
55 56 24-25 
2-44 
A 
55 56 43-69 
55 56 38-44 
5-25 
N 
55 57 9-22 
55 57 2-71 
6-51 
It might have been anticipated, that, on account of the attraction of the hill at the 
South Station, the deflection of the plumb-line would have been to the north, which 
by throwing the zenith to the south would have caused the observed latitude to be 
less than its true value. The contrary, however, takes place, for the observed lati- 
tude is greater than the geodetical. On proceeding next to the second station, 
namely, that on the summit of the hill, a similar anomaly is observed ; there is an 
attraction or deflection to the south of more than five seconds, which can by no 
means be attributed to the hill, as its attraction upon any object at its vertex is very 
nearly equal north or south. A similar anomaly is visible at the North Station ; 
there is a deflection to the south of 6"'5, which is considerably more than that due to 
the mass of the hill, as will appear hereafter. 
It is clear, therefore, that there is some other disturbing force acting at each of 
these stations besides the attraction of Arthur’s Seat, and which appears to produce 
a general dejlection to the south of about five seconds. 
The comparison of the observed and calculated latitudes of the observatory on the 
Calton Hill serves to corroborate this fact. The latitude of this observatory, as 
determined by observation, is 
55° 57' 23"-20. 
The latitude of the Trigonometrical Station on this hill, when referred to the same 
spheroidal surface we have before mentioned, namely, that agreeing most nearly with 
all the astronomically determined points in Great Britain, is 55° 57' 17"'51 : the 
difference of latitude of these two points (taking the centre of the Altitude and Azi- 
muth instrument of the observatory as the point whose latitude is above given) is 0"’06, 
so that the calculated latitude of the Calton Hill observatory is 
55° 57' 17"-57, 
which is less than the astronomical by 5"‘63 ; showing a deflection to the south of 
that amount in existence at the Calton Hill. Now the attraction of the mass of 
Arthur’s Seat upon the Calton Hill is easily calculated to be between 0"T and 0"'2, 
consequently the deflection here visible is certainly not due to Arthur’s Seat. 
It seems therefore very probable that the general deflection of five seconds to the 
south, brought out at all these stations, is due to one and the same cause. 
An explanation of this phenomenon immediately offers itself in the existence of the 
hollow of the Forth to the north, and the Pentland Hills and other high ground to 
