Mr. hunter on a new Method 
can fee a method by which the machine before defcribed may 
be made to anfwer this purpofe, I am almoft afraid to propofe 
it. I mean, that within the fcrew de another ftill fmailer 
fliould be made to play, and be connedted with the fcrew cd, lb 
as to move round along with it. It muft have a~ + a -f i threads 
in an inch, and they muft be in tire contrary direction to thofe 
of cd, fo that when they are both turned together, and cd 
moves upwards, this other one may move downwards. At one 
turn of the handle this will move upwards x — — - 
rr — — - of an inch, and at the fame time will move 
round in a circular diredtion. For example, let cd have 5 
threads ( = a) in an inch, de 6 ( = #1), and a third fcrew within 
de, but connedted with cd fo as to partake of its motion, ji 
( = a + 1). At one turn of the handle, this fcrew will rife 
upwards 4 x 4 x tt - T b °f an inch ; but this appears too 
complicated for ufe, and the lcaft inaccuracy in the conftruc- 
tion would hinder it from moving. 
But, on the other hand, if while the point e rifes it is of 
confequence that it be kept from going round, the machine 
under confi deration will beft anfwer thi purpofe* On this 
principle it may be ufeful in leveral refpedts : fot inftance, let 
A (fig. 2.) reprefen t a magnifying lens, and let be moveable 
upon the fcrew bc of 1 6 threads in an inch, which turns within 
the larger fcrew cd of 15 threads in an inch, and that again 
moves within the plate ef in the end of the cylinder gf 
To ufe the inftrument, fix the object to be magnified upon the 
pin gl, and then turn the lens A upon the fcrew bc, till it be 
* The fcrew ec is refrained from moving along with cd by the fmall pillar hk, 
which flides backwards and forwards in a groove in the cylinder gf. 
nearly 
