[ 514 ] 
afterwards fet up for the like purpofe. I propofed to de- 
termine the diftance of the two earns by connecting 
them by angles with a bafe, to be meafured in a level 
fpot in the vale below the hill, and then to make ufe of 
the faid diftance as a fecondary bafe for determining the 
tides of the polygon, and the diftance of the two ftations 
of the obfervatory. Had the two earns been vifible from 
the two ftations of the obfervatory, two triangles would 
have fufiiced for connecting the two ftations together. 
But, notwithftanding that this was not the cafe, and that 
only the two earns were vifible from one another, yet all 
the angles of thefe two triangles were meafured by Mr. 
BURROW in the following method, fuggefted by himfelf. 
He went with the theodolite to the neighbouring hill on 
the South fide of Schehallien, which runs parallel to it ; 
and, by varying his fituation, found a point whence the 
Weftern earn and Southern obfervatory appeared by the 
theodolite to be in one vertical plane, and removing the 
theodolite he planted a pole there. In like manner he 
planted another pole on the fame hill, in a vertical plane 
with the Southern obfervatory and Eaftern earn. Then 
returning to the obfervatory, he took the horizontal angle 
contained between the two poles,, which it is evident is 
equal to its oppofite angle, or that contained between the 
earns. And going to the Weft carn^ he took the angle 
contained between the Eaft earn and the pole planted on 
the oppofite hill, in a line with the Southern obfervatory 
and Weft earn., which is the fame with the angle be- 
tween the Eaft earn and Southern Obfervatory. And 
iaftly, going to the Eaft earn., he took the angle con- 
5 tained 
