the 'Trigonometrical Operation. j 2 j 
under fide, which being fiackened, it was placed on its ferrule 
at right angles to the line of meafurement; and being turned 
round 90% the dovetails of the ferrule, Handing originally in 
the dire&ion of the bafe, came within the clamps, which were 
then tightened by four fcrews, turned with fquare keys adapted 
to the purpofe. 
It is fufficiently obvious, that fo many irons, with fuch a 
number of fcrews to each, could not fail of rendering this 
operation tedious ! The bufinefs would have been greatly expe- 
dited if there had been only two fuch fcrews, one on each end 
in a middle fituation ; and, inftead of the four fcrews, there 
Ihould have been four Heady pins, entering eaiily into holes 
prepared for them in the under fide. A fhort groove, of two 
or three inches in length, in each extremity of the bottom, 
would, 011 this fuppofition, have been neceflary to fuffer the 
fquare heads of the fcrews to pafs ; and it will be readily con- 
ceived, that the thicknefs of the bottom would have effectually 
fecured the chain from touching them, prevented the mutila- 
tion of its handles, and faved much lofs of time. Indeed the 
fame purpofe might have been effeded, but not fo advantage- 
oufly, by laying the original four fcrews lower in the iron, 
which its thicknefs eafily admitted off Finally, in order to 
avoid fuch like inconveniencies in future, there is ftill one im- 
perfection more, which it is incumbent on me to remark, 
namely, that cafl-iron ferrules will not anfwer ; for the force 
that was found to be neceflary to drive the polls into the 
ground, burft almoft the whole of them, fo that before the 
operation was compleated, they were obliged to be replaced 
with others made of hammered iron, forged for the purpofe. 
Of the fliort irons only three were neceflary, one for each 
end of the chain, and a fpare one in cafe of accident. They 
were 
