of applying the Sere W.. dr.- 
threads m an inch equal to a z y a, which would render it too- 
weak to refill: any confiderable violence. For example, if dc 
have five threads in an inch, and de fix, and if the handle ckl. 
is a foot in length, the power gained by the engine will be' 
nearly as (at -j- a x 6 b = ) 2 1 60 to 1 ; whereas, to have the fame 
force by means of the common ferew, it muff have 30 threads- 
in an inch, and fo muff yield under a refinance which the 
other ferew would overcome without any difficulty. Upon this 
principle, the ferew may he applied with advantage in preffes of 
different kinds, by fixing one of the plates of the prefs to the 
end of the ferew at e. 
As to, the fecond requifite, both methods may be equally 
adapted to it ; yet other circumftances will determine us to apply 
the common ferew where a fimalt increafe of power is necefiary, 
and the prefent contrivance, when we ffandinneed of a greater. 
This will follow from the third rule, as in. the. method now 
propofed a double number of ferews is required, which makes 
the ffrudiure more complicated, occafions more expence, and 
requires a greater accuracy of conffrudtion, fince, unlefs this is 
attended to, the machine will not move. 
However, the machine may, in fome cafes,, anfwer the 
fourth intention better than the common one, as the power 
gained by the additional ferew enables us to ffiorten the handle, 
which will tend to make the whole more portable. 
The power is here applied in the fame direction as. in the 
common fcrew~, fo that both equally anfwer the firff part of the 
fifth rule ; but as to the laff, the motion ultimately produced, it 
will depend on particular circumftances which of them is moff 
fit for ufe in any cafe. Thus, if the ferew. de be intended to 
carry an index which muff turn round at the fame time that it 
rifes upwards, the common ferew is preferable ; for although V 
can : 
