of applying the Screw, 63 
nearly at the proper diftance from the pin, and oppofite to it. 
You may then adjud the didance more accurately by turning 
the fcrew dc, at each turn of which the lens will recede from, 
or approach to, the pin -,Y- 0 th of an inch. This it will do 
and not turn afide, but dill remain oppohte to the pin 
lg. A double microfcope might be fitted on in the place of the 
lens a. The whole inflrument may be furnifhed with a handle, 
as at M ; or, if larger, it may have three feet to Hand on a 
table. 
On the lad principle it mud be owned, the common 
fcrew has the advantage, as two fcrews will produce 
more fridtion than one ; and, betides, in the compound engine 
there is an additional friction from the piece fg (fig. i.) upon 
the pillars between which it moves. 
Another cafe in which this machine may be employed is in 
the micrometer. Thus, let the fcrew ab (fig. 3 ) of 50 threads 
in an inch be turned round by the index c, which moves upon 
the graduated circle ecd in the direffion CD. Within the fcrew 
ab is the frnaller one af of 51 threads in an inch, retained from 
moving round by the bar gfii. The piece af is continued to k, 
where it forms- a fine point. To ufie the indrumenf, let it be 
adjuded to the telefcope or microfcope by which you are to view 
a dar, or fome fmall objed, and let the point n appear jud to. 
touch one edge of the objedb. Then turn the index c, and the 
point k will advance upwards till it appears to cover the other 
edge of the object, and thus you can determine its fize. The 
point k will advance at each coniDlete turn of the index 
of an inch ; and if the circle be divided into 80 equal parts, one 
of which, if it is an inch in diameter, will be very obfervable, 
while the index moves over one of thefe, the point k willadvance- 
of an inch. 
•a "o 4 o *0 o 
