5^4- Mr. Dalby’s Remarks on Gen . Roy’s Account 
try, ufing the chords of the meafured bafes, and the angles 
formed by the chords of the other tides of the triangles (jvhich 
angles might then be found from the horizontal ones) inftead 
of the obferved angles. 
In the application of plane trigonometry to the obfervations, 
that part of the earth's furface to which the operation has been 
confined, is confidered as a plane, and the meafured bales as 
right lines on that plane ; but whether the computations are 
made on this principle, or attempted on that of taking the 
bafes and the other fides of the triangles as chords, there feems 
to be no certain rule for reducing the obferved angles of each 
triangle to 180°, fo as to give the diftances the moft correct, 
which would alfo be the cafe if the angles of each triangle had 
been taken in the fame plane; hence it is evident, that the 
method of correction has been in fome degree arbitrary ; 
for, though the fum of the three obferved angles of each tri- 
angle is in general very near what it ought to be (taking the 
earth as a fphere) ; yet, when that fum is not exaClly i8o°, 
in reducing them to plane ones, each obferved angle maybe 
taken as a plane one, and the other two augmented in cafe of a 
defe£t ; but each ought to be diminifhed when there is an ex- 
cefs in the obferved fum. In making thefe reductions, however, 
it is evidently neceffary to confider whether each of the obferved 
angles is equally well afcertained, and correCt them accordingly; 
but this muft be left to the judgement of the obferver. 
From the foregoing confiderations, it follows, that the an- 
gles of the triangles taken as plane ones may be varied to cer- 
tain limits, and confequently the oppofite fides deduced there- 
from muft vary to certain limits alfo ; but it is evident, that a 
mean of the extreme refults, obtained In this manner, will be 
very near the truth \ and therefore this method of making the 
comparifon 
