the ’Trigonometrical Operation. 12? 
left marks the end of a chain. The other eight pofts in the 
arrangement, that is to fay, the right and left pofts of each of 
the four groups, are fuppofed to ftand twelve or fifteen inches 
from thole in the center. By referring to the elevation near 
the top, and the plans and feftion in the middle part of Plate 
1 . it will be perceived, that thefe pofts, together with certain 
other iron parts of the apparatus fixed to them, hereafter to be 
defcribed, fupport the ends of the coffering for each chain, 
free and independently of the central pofts, to which laft the 
brafs fcales alone are attached. 
Art. HI. Deal Coffers. 
Fifteen deal coffers, numbeied from one to fifteen, were 
neceffary for the length of three chains, being five to each. 
Six of them, that is to fay, the firft and fifth, the fixth and 
tenth, the eleventh and fifteenth, being the firft and laft of 
each chain, were only nineteen feet four or five inches in length. 
The other nine, being the three in the middle of each chain, 
were of the complete length of twenty feet. Thefe coffers 
perfe&ly refembled in fhape, and nearly in dimenfions, the 
cafes of the glafs rods, being ten inches broad in the middle, 
and uniformly of that depth throughout their whole length. 
But from the middle they became gradually narrower, in a 
curvilinear manner, towards each end, where they were only 
two inches wide. The two cheeks or fides were about half an 
inch thick, and the bottom, which entered into a fhallow 
groove in the middle of the cheeks, was an inch in thicknefs. 
Thus the cheeks being thin, bent and applied eafily to the 
bottom, to which they were firmly nailed, and the whole was 
fortified by fmall blocks of wood fattened at intervals in the 
mfide, fometimes above and fometimes below the bottom. 
R 2 From 
