of applying the Screw* 63; 
nearly at the proper diftance from the pin, and oppofite to it. 
You may then adjuft the diftance more accurately by turning 
the fcrew DC, at each turn of which the lens will recede from, 
or approach to, the pin _^th of an inch. This it will do 
and not turn afide, but ftill remain oppofite to the pin 
lg. A double microfcope might be fitted 011 in the place of the 
lens A. The whole inftrument may be furniftied with a handle, 
as at M ; or, if larger, it may have three feet to ftand on a 
table. 
On the laft principle it muft be owned, the common 
fcrew has the advantage, as two fcrews will produce 
more friction than one; and, befides, in the compound engine 
there is an additional friftion from the piece eg (fig. x.) 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 dire&ion cd. Within the fcrew 
ab is the fmaller one af of 51 threads in an inch, retained from . 
moving round by the bar gfh. The piece af is continued to k, 
where it forms a fine point. • To ufe the Inftrument, let it be 
adjufted to the telefcope or microfcope by which you are to view 
a ftar, or fome fmall objedt, and let the point k appear juft to 
touch one edge of the object. Then turn the index c, and the 
point k will advance upwards till it appears to cover the other 
edge of the objedt, and thus you can. determine its fize. The 
point k wfill advance at each complete turn of the Index -p T _ 
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 will advance 
•5To T W o' of an inch.. 
1 
Thus, 
