[ 6q 5 } 
This upper part of the lever rifes above the lid of the 
ciftern, lb that it and the micrometer-fcrew are at 
all times clear of the water. The top of the lever 
is furnilh’d with an appendage which I call the feeler ; 
it is the extremity of this piece which comes in con- 
tad; with the micrometer-fcrew. The conftrudion 
and application hereof will better appear from the 
draught, than from many words. It hence appears, 
that, having the length of the lever from its fulcrum 
to the point of fulpenfion of the fee’er, the didance 
between the fulcrum and the point of contad: with 
the bar, the inches and parts that correfpond to a 
certain number of threads of the micrometer, and 
the number of divilions in the circumference of the 
index-plate ; the fradion of an inch exprefs’d by one 
divilion of the plate may be deduc’d : Thofe mea- 
fures are as follows. 
Inches. 
From the fulcrum of the lever to the feeler 5. 87 1; 
From the fulcrum to the plate of contad: 2.895" 
Length of 70 threads of theferew . . 2.4 ff 
Divilions in the circumference of the index-plate 
100. 
Hence the value of one divilion will be the tttsu 
part of an inch : But if the ferew be alter’d -f of one 
of thefe divilions, when the contad; between the ferew 
and feeler is well adjufted, the difference of contad 
(if I may fo call it) will be very perceivable to the 
flighted: obferver ; and, confequently, t’jtj part of 
an inch is perceivable in this inftrument. 
There is one thing dill remains to be Ipoke of, and 
that is, the verification of the micrometer-fcrew, 
which 
