722 Col. roy’s Experiments for 
ference of height will be 1183.33 feet. Laftly, Mr. 
maskelyne’s refult, from the triangles on the North-fide 
of the mountain, makes it 1180. The mean of thefe 
three 1183.31 feet, is the height of the Weftern fummit 
of the mountain above the South obfervatory, which only 
exceeds my height by one-third of a foot. Here it is to 
be obferved, that from the vicinity of thefe triangles to 
the mountain, and the fhortnefs of the fxdes, the greateft 
curvature amounted only to 1 6 or 1 7 inches, which con- 
fequently made the effedt of refradtion next to nothing. 
This near agreement between the refults feems there- 
fore to prove, that the mode of computation for curva- 
ture and refradtion, made ufe of in the Tay bridge obfer- 
vations, is juft. 
By the firft angles of elevation, taken from the ftation 
of the barometer in Glenmore, the Weftern lummit of 
Schihallien is 2001.88 feet above it; from which, if we 
dedudt 1 1 83.33, there remains 815.55 for the height of 
the South obfervatory above the faid ftation : but if the 
laft angles of elevation at the ftation of the barometer 
are made ufe of, the height between it and the obferva- 
tory comes out 818.97, whereof the mean is 818.76 
feet. Though thefe inftances are of themfelves fuffi- 
cient to prove, that the geometrical heights may be fafely 
depended upon ; yet, as an example of the method that 
was 
