42 MECHANIC POWERS. 



therefore, being twice as far from the fulcrum as 

 the weight is, the proportion between the power 

 and weight, in order to balance each other, must be 

 as 1 to 2. Whence it appears, that the use of this 

 pulley doubles the power ; and that a man may 

 raise twice as much by it as by his strength alone. 

 Or it may be considered in this way; every move- 

 able pulley hangs by two ropes equally stretched, 

 and which must, consequently, bear equal parts of 

 the weight; but the rope A B being made fast at 

 B, half the weight is sustained by it; and the other 

 part of the rope, to which the power is applied, 

 has but half the weight to support; consequently, 

 the advantage gained by this pulley is as 2 to 1. 



When the upper and fixed block contains two 

 pulleys which only turn upon their axis, and the 

 lower moveable block contains also two, which not 

 only turn on their axis, but rise with the weight F 

 (Fig. 7-) the advantage gained is as 4 to 1 ; for 

 each lower pulley will be acted upon by an equal 

 part of the weight; and, because in each pulley 

 that moves with the weight, a double increase of 

 power is gained, the force by which F may be sus- 

 tained will be equal to half the weight divided by 

 the number of lower pulleys; that is, as twice the 

 number of lower pulleys is to 1, so is the weight 

 suspended to the power. 



if the extremity C (Fig. 8.) be fixed to the 

 lower block, it will sustain half as much as a pulley: 

 consequently here the rule will be, as twice the 

 number of pulleys adding unity is to 1, so is the 

 weight to the power. 



These rules hold good, whatever may be the 

 number of pulleys in the blocks. 



If, instead of one rope going round all the pulleys, 

 the rope belonging to each pulley be made fast at 



