Mechanic Powers. 



the length of both handles twelve inches. My 

 hands placed upon them in going round will de- 

 scribe a circle, which, upon calculation, will be 

 found to be seventy-six inches nearly, and con- 

 sequently this will be an hundred and fifty-two 

 times greater than half an inch, which was the 

 distance between two of the spirals. Thus, if a 

 bodyjis to be pressed down with this machine, 

 one man will press it, with this assistance, as 

 much as an hundred and fifty-two men without 

 it. Or if the screw were so contrived as to raise 

 the weight instead of pressing it, which sometimes 

 is the case, the human force would be assisted in 

 the same proportion with the same instrument. 

 But we here only speak as if the handles of the 

 screw were but twelve inches across, and the 

 spirals a whole half inch distant from each other ; 

 what if we suppose the handles ten times as long, 

 and the spirals five times as close ; the increase 

 of the human force then would be astonishing. 



The power of the screw may, however, be still 

 more correctly estimated by t what is called the 

 perpetual screw. To explain this, let the wheel 

 C (fig. 103) have a screw db on its axle, work- 

 ing in the teeth of the wheel D, which suppose 

 to be forty-eight in number. It is plain, that 

 for every time the wheel C and screw ab are 

 turned round by the winch A, the wheel D will 

 be moved one tooth by the screw; and, there- 

 fore, in forty-eight revolutions of the winch, the 



