A Letter from William Edgeworth, Esq. Civil Engineer, to Alex- 
ander Nimmo, Esq. Civil Engineer, M.R.I.A. communicated by 
Mr. Nimmo. 
Read January 1, 1823. 
Dublin, January 1, 1823. 
Dear Sir, 
AT your request I send you an account of a few of the principal 
triangles upon which the map of the county of Roscommon was 
constructed. 
Ihave calculated the sides from a base measured on the road 
from Longford to Edgeworthstown, an account of which was pub- 
lished in my father’s Report on the Bog District, No. 7, and, with 
a diagram of some triangles, is to be found in the Second Report 
of the Commissioners for improving Bogs in Ireland. 
On my map of the county of Longford I have published the 
principal triangles upon which it was founded; and the latitude 
was determined from observations of the pole star, with an instru- 
ment of sixteen inches diameter, that Major Taylor most kindly lent 
to me, 
The longitude of Edgeworthstown was determined from the 
Observatory of Dublin, by the valuable assistance of Dr. Brinkley, 
who was so good as to make corresponding observations of white- 
lights and explosions of gunpowder ; four in one night agreed with 
the mean, within a second of time. 
