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Then, from what was proved in the third cafe of the 
wedge, it will appear, that this force muft be to the 
weight of the body, as A D to B D, or rather in a 
proportion fomewhat greater : if it makes the plane 
move on and the body rife. 
From this laft obfervation we may clearly fhew the 
nature and force of the fcrew ; a machine of great 
efficacy in raffing weights or in preffing bodies clofely 
together. For if the triangle AB D be turned round 
a cylinder whofe periphery is equal to B D, then the 
length of the inclined plane B A will rife round the 
cylinder in a fpiral manner ; and form what is called 
the thread of the fcrew : and we may fuppofe it con- 
tinued in the fame manner round the cylinder from 
one end to the other j and A D the height of the in- 
clined plane will be every where the diftance between 
two contiguous threads of this fcrew, which is called 
a convex fcrew. And a concave fcrew may be formed 
to fit this exactly, if an inclined plane every way 
like the former be turned round the infideof a hollow 
cylinder,, whofe periphery is fomewhat larger than that 
of thenthpr.’ T et ns now fuppofe the concave fcrew 
to be fixed, and the cuuvca uuc iu be fitted into it, 
and a weight to be laid on the top of the convex fcrew : 
Then, if a power be applied to the periphery of this 
convex fcrew to turn it round, at every revolution 
the weight will be raifed up thro’ a fpace equal to the 
difiance between the two contiguous threads, that is to 
the line A D the height of the inclined plane BA; 
therefore fince this power, applied to the periphery, 
adts in a direction parallel to B D, it muft be to the 
weight it raifes as A D to B D, or as the diftance 
between two contiguous threads, to the periphery of 
the convex fcrew. 
N. B. 
