6 2 Mr. hunter on a now Method 
.Can fee a method by which the machine before defended may 
be made to anfwer this purpofe, 1 am aim oft afraid to propofe 
it, I mean, that within the ferew de another ftili fmaller 
fhould be made to play, and be connected with the ferew cd, fq 
as to move round along with it. It mud have a + a-\- 1 threads 
in an inch, and they muft be in the contrary direction to thole 
of cd, fo that when they are both turned together, and cd 
moves upwards, this other one may move downwards. Atone 
turn of the handle this wall move upwards — x 
*■ a -j-a a“ + a + l 
~ -i : — - of an inch, and at the fame time will move 
round in a circular direction. For example, let CD have 5 
threads ( = a ) in an inch, de 6 ( = ai '), and a third ferew within 
de, but connected with cd fo as to partake of its motion, 31 
( — a* -p a + 1). At one turn of the handle, this ferew will rife 
upwards 4 x | x J T = _ 4 _ of an inch ; but this appears too 
complicated for ufe, and the leaf: inaccuracy in the conftruc- 
tion would hinder it from moving. 
But, on the other hand, if while the point e rifes it is of 
conference that it be kept from going round, the machine 
under confideration will belt anfwer this purpofe. On this 
principle it may be ufeful in feveral refpects : fot inftanCe, let 
a (fig. 2.) reprefen t a magnifying lens, and let it be moveable: 
upon the ferew bc of 16 threads in an inch, which turns within 
the larger ferew 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 ferew bc, till it be 
* The ferew bc is retrained from moving along with cd by the fmall pillar hk, 
which hides backwards and forwards in a gioove in the cylinder gf. 
nearly 
