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it is able to iuftain by the help of fuch an engine. And 
fince the radii of two contiguous wheels, whofe teeth 
are applied to each other, are as the number of teeth 
in each, or inverfely as the number of revolutions, 
which they make in the fame time; we may, in the 
computation, inftead of the ratio of thefe radii, put 
the ratio of the number of teeth on each wheel ; or 
the inverfe ratio of the number of revolutions they 
make in the fame time. 
Some writers have thought the nature and effects of 
the pulley might be belt explained by confidering a 
fixed pulley as a lever of the firft, and a moveable pulley 
asone of the fecond kind. But tho’ the pulley may bear 
being confidered in that light; yet, I think, the beft 
and moll natural method of explaining its effects (that 
is, of computing the proportion of any power to the 
weight it can fuftain by means of any fyftem of pulleys) 
is, by confidering that every moveable pulley hangs by 
two ropes equally ftretched, which muft bear equal 
parts of the weight : and therefore when one and the 
fame rope goes round feveral fixed and moveable pullies, 
fince all it parts on each fide of the pullies are equally 
firetched, the whole weight mud be divided equally 
amongft all the ropes by which the moveable pullies 
hang. And confequently if the power which ads on 
one rope be equal to the weight divided by the number 
of ropes, or double the number of moveable pullies, 
that power mufl fufiain the weight. 
Upon this principle, the proportion of the power 
to the weight it fuftains by means of any fyftem of 
the pullies, may be computed in a manner fo eafy and 
natural as muft be obvious to every common ca- 
pacity. 
The 
