Mechanic Powers. 



that is, to four hundred : I add another move- 

 able pulley, and that will lessen the remaining 

 four by one half, which is two hundred ; if I 

 still add a third, that will lessen the remaining 

 two by one hah , which is one ; so that if I use 

 three moveable pulleys in raising eight hundred 

 weight, I shall be able to raise it with as much 

 ease as one hundred without them. 



As systems of pulleys have no great weight, 

 and lie in a small compass, they are easily car- 

 ried, and can be used in many cases where more 

 cumbrous engines cannot. They have much 

 friction, however, because the diameter of their 

 axis bears a very considerable proportion to their 

 own diameter, because they are apt to rub 

 against each other, or against the sides of the 

 block, and because the rope that goes round 

 them is never perfectly pliant. Still they are 

 highly useful, and their combinations may be 

 varied at pleasure, to suit the case in hand, whe- 

 ther at land or sea. 



IV. The inclined plane is very justly regarded 

 as the fourth mechanic power, though some have 

 rejected it altogether. The advantage of this 

 machine (if you will admit of that term) is, that 

 by means of it a heavy body may be made to 

 ascend a given height with much less power than 

 it would require to raise it the same height if it 

 were perpendicular. This is a very common mode 

 of assisting human strength ; you will every day 

 see porters, when they have to roll a cask or 



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