OF THE MECHANICAL POWERS. 



75 



chine, because it is never used without the application 

 of a lever or winch to assist in turning it : and 

 then it becomes a compound engine of a very great 

 force either in pressing the parts of bodies close toge- 

 ther, or in raising great weights. It may be con- 

 ceived to be made by cutting 

 a piece of paper ABC into 

 the form of an. inclined plane 

 or half wedge, and then c 

 wrapping it round a cylinder 

 A B. And here it is evident 

 that the winch E must turn 

 the cylinder once round be- 

 fore the weight of resist- 

 ance JD can be moved from 



one spiral winding to another, as from d to c ; therefore, 

 as much as the circumference of a circle described by the 

 handle of the winch is greater than the interval or distance 

 between the spirals, so much is the force of the screw. 

 Thus, supposing the distance between the spirals to be 

 half an inch, and the length of the winch to be twelve inch- 

 es ; the circle described by the handle of the winch where 

 the power acts will be 76 inches nearly, or about 152 

 half inches, and consequently 152 times as great as the 

 distance between the spirals : and therefore a power at 

 the handle, whose intensity is equal to no more than a 

 single pound, will balance 152 pounds acting against 

 the screw ; and as much additional force, as is sufficient 

 to overcome the friction, will raise the 152 pounds ; and 

 the velocity of the power will be to the velocity of the 

 weight, as 152 to 1. Hence it appears, that the longer 

 the winch be made, and the nearer the spirals are to 

 one another, so much the greater is the force of the 

 screw. 



A machine for shewing the force or power of the 

 screw may be contrived in the following manner. Let 



LECT. 



ill. 



