Measurement of a Bafe on Hounflow- Heath. a.i q 
found to be within a few inches of the picket correfponding 
with the hypothenufe, as determined by the chain. But with 
the rods of twenty feet three inches, the day’s work was al- 
ways ended, with a fractional rod, by fufpending a plummet 
from fome convenient part of the Hem, marked for the pur- 
pofe, , and which confequently became the point of commence- 
ment next morning. 
The brafs cup, made ufe of on thefe occafions, is of the 
figure of an inverted truncated cone, whofe mean diameter is 
four inches, and its depth about five, with a very fmall incli- 
nation in the fides.: It was placed in a hole dug for it in the 
earth, immediately under the point of fufpenfion of the plum- 
met, ferving only to hold the water in which it vibrated. 
The nature of the tripod will be belt conceived from the 
plan , and elevation of it in tab. XVIII. It confifis of two 
ffrong pieces of beech wood, mortoifed into each other, fo as 
to referable a half crofs, or the letter T inverted, having three 
flrong iron prongs, about twelve inches in length, which pafs 
through the ends of the wood, and are faftened toit by fquare 
nuts at top. On the furface of the tripod lies a fimilar half 
crofs of mahogany, moveable by means of grooves in the di- 
rection of the longeft fide, and fixable by its proper Icrews, 
when biought to the defired pofition. This mahogany half- 
crofs carries on its furface a brafs ruler, moveable at right-angles 
to the former diredlion, fixable alfo by means of its own 
fcrews, and on. whofe end .is cut a very fine interfedlion. 
i hus any day s operation having been finilhed, the tripod was 
placed near the cup, with it longefi; fide parallel to the line of 
meafurement, and its prongs driven into the ground, fo as to 
be rendered perfectly immoveable without great violence. The 
plummet being then fiufpended by a fine gilt wire, at any part - 
of 
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