af applying the Screw . 61 
threads in an inch equal to or + 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 (a 1 -f a x 6 b — ) 2 1 60 to 1 ; whereas, to have the fame 
force by means of the common fcrew, it muft have 30 threads 
in an inch, and fo muft yield under a refiftance which the 
other fcrew would overcome without any difficulty. Upon this 
principle, the fcrew may be applied with advantage in preiles of 
different kinds, by fixing one of the plates of the prefs to the 
end of the fcrew at e. 
As to the fecond requifite, both methods may be equally 
adapted to it ; yet other circumftances wall determine us to apply 
the common fcrew where a final 1 increafe of power is necefiary, 
and the prefent contrivance, when we Hand in need oft a greater. 
This will follow from the third rule, as in the method now 
propofed a double number of fcrews is required, which makes 
the ftrudture more complicated, occafions more expence, and 
requires a greater accuracy of conftruftion, 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 fcrew enables us to ftiortep 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 fir ft .part of the 
fifth rule ; but as to the laft, the motion ultimately produced, it 
will depend on particular, circumftances which of them is moft 
fit for ufe in any cafe. Thus, if the fcrew de be intended to 
carry an index which muft turn round at the fame time that it 
rifes upwards, the common fcrew is preferable ; for although I 
can 
