46 MECHANIC POWERS. 



cation 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 

 closer together, or in raising great weights. It 

 may be conceived to be made by cutting a piece of 

 paper, ABC (Plate 3. fig. 6.) into the form of an 

 inclined plane or half wedge, and then wrapping 

 it round a cylinder (Fig. 7.) the edge of the paper 

 A C will form a spiral line round the cylinder, 

 which will give the thread of the screw. It being 

 evident, that the winch must turn the cylinder 

 once round, before the weight of resistance 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 of the spirals to be half an 

 inch, and the length of the winch 12 inches, 

 the circle described by the handle of the winch 

 where the power acts will be JG 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 is, and the nearer the spirals are to one 

 another, so much the greater is the force of the 

 screw. 



A machine for showing the force or power of the 

 screw may be contrived in the following manner. 

 '-Let the wheel C have a screw (Fig. 8.) on its axis, 



