the 'Trigonometrical Operation. 
247 
SECTION EIGHTH. 
1 
Secondary triangles , fubdivided into two fets, for the improve- 
ment of the maps of the country , and the plan of the City of 
London and its Environs • Plate XL 
IN the feries of great triangles, whereby the diflance be- 
tween the meridians of the Royal Obfervatories of Greenwich 
and Pans has been determined, the fame excellent inRrument 
havin been placed at every Ration on our fide of the Channel, 
and all the angles obferved with the utmoR care, it hath con- 
fequently followed, that the bafe on Hounflow Heath, and 
that in Romney Marfh, reciprocally meafure each other within 
a few inches of the truth, which is an inftance of fuch exa£l- 
nefs as probably never occurred in any former operation of this 
fort. The extreme fmallnefs of the error on the fum of the 
three angles of each triangle lufficiently proves that the ge- 
neral refult would not have differed greatly, if only two of the 
angles had actually been obferved. But in an operation of fo 
much importance, this could not have been depended upon ; 
nothing was to be left doubtful ; and therefore, in the execu- 
tion of the various parts, the moR minute attention was paid 
to every circumftance whereby the accuracy might be afFefted, 
and particularly to the placing of the lights and inRrument 
reciprocally over the fame point marking the Ration, that no 
poflible error might arife from parallax or excentricity. 
From 
