An Account of a Trigonometrical Survey of Mayo, one of 
the Maritime Counties of Ireland. By William Bald, Civil 
Engineer, Member of the Geological Society, London, and 
M. R.I. A. 
Read April 30th, 1821. 
IN the year 1809 I was employed by the Grand Jury of Mayo to 
make a map and survey of their county, which was to be drawn and 
laid down from a scale of three inches to the Irish mile. The whole 
of the summer of 1809 was occupied in constructing a geometrical 
map of the environs of Castlebar, and in obtaining a knowledge of 
the face of the country, with a view to the planning of the triangles, 
and the measurement of a base as the foundation of the general 
geometrical survey and map. After a diligent search, no plain 
could be found that would admit of a base of more than two miles 
in length, except by going through bog. One of these was adopted, 
and extended to thirty-one thousand nine hundred feet, which was 
measured three times carefully over with a fifty feet chain, using 
iron pins. Ihave drawn out a separate diagram of the triangles 
calculated from this base as a comparison with the results obtained 
from more exact data. I ordered Mr. Edward Troughton, of Lon- 
don, to make a chain of one hundred feet in length, which might be 
applied to a flat surface of soil, not having the means of resorting 
to the more exact method of using coffers in measuring the bases, 
and he accordingly sent me one, consisting of twenty links, each five 
feet long, of a simple but accurate construction. An extract from 
VOL, XIV. L 
