Chap. 7.] 9Cbe perpetual Screw. 3 



from another. If the power is applied immediately 

 to the fcrew, the fpace it pafles through, or its quan- 

 tity of motion, is a c (fig. 5.) which is the meafure of 

 the circumference of the fcrew, and the motion of the 

 refiftance is meafured by c b, the width of one pace of 

 the fcrew. But as it is common to turn fcrews, efpe- 

 cially large ones, with levers or fomething equivalent, 

 in that cafe a c does not meafure the motive force of 

 the power; it is, on the contrary, meafured by the 

 circumference of the circle, of which the lever D E 

 is the radius. And as it is necefTaryj in order to main* 

 tain an equilibrium, that the powers mould be in the 

 inverfe ratio of their velocities, it may be eflablifhed 

 as a general rule in ufing fcrews, if we make no ac-* 

 count of the friction, thap the power is ta the refift- 

 ance as the height of the pace of the fcrew is to the 

 circumference which the power defcribes. 



The perpetual fcrew differs in many refpecls from 

 that which has now been delcribed. It is a cylinder, 

 which always turns in the fame direction, its two ex- 

 tremities A and B (fig. 6.) being carried upon folid 

 pivots, fo that its a6tion is perpetuated, whence it 

 derives its name. The threads z h of this fcrew, 

 which are generally fquare, coincide with the teeth of 

 a vertical wheel C h, which carries upon its axis a rol- 

 ler or windlafs T with a cord, to which is fattened the 

 burden P, which i$ required to be elevated. A very 

 fmall force, therefore, applied to the handle M E is 

 fufikient to raife a very 'confiderable burden at P, 

 but it requires confiderable time, from the invariable 

 rule in mechanics, that whatever is gained in force is 

 l,ofi: in velocity. 



In order to find the relation between the weight P 



and the force or power Q^, it mull firft be confidered 



that the weight P is counterbalanced immediately by 



G 2 the 



