THE MECHANICAL POWERS. ¥s 
cable is wound ; while the longer arm, on which the power acts, is the — 
distance from the centre to the part of the spokes on which the sailors 
press. 
III.—The Pulley. 
A pulley consists of a small wheel, with a grooyed rim, fixed in a 
block so as to move freely on its axis, and having a gord passing over the 
rim, with weights attached to each end. There are two kinds of pulleys, 
one the fixed pulley, as in fig. 11, the other the movable pulley, as B, 
fig. 12. 
It is clear, that if P is to balance W in fig. 11, the weights must be 
equal, because the wheel simply acts like a lever with 
equal arms, as was seen in describing the wheel and axle; 
so that a fixed pulley does not give any increase of strength, 
but merely- changes the direction of application of the 
force. For example, if a man wished to raise something 
heavy to a considerable height, he might find the greatest 
difficulty in lifting what he could raise with ease by 
pulling a rope attached to the article over a pulley. 
By means of a movable pulley, however, as in fig. 
12, a man may exert a power greater than his own 
strength. But, just as in the case of the lever, it is a mistake to suppose 
that the pulley gives rise to any power in itself; it merely affords the 
means by which power may be concentrated. The parts of the string 
in the figure, between A and B, and between B and 
C, support each one half of the weight of W. And 
as the effect of C is simply to change the direction of 
the force, and the tension of the string being the 
same throughout, it is evident that the hand at P 
must be pulling with a force equal to half the weight 
of W. Thus, by means of a movable pulley, a weight 
or power at the end of a cord is able to balance a 
_weight twice as great. It will readily be seen how it is 
so. Suppose the cord to be pulled so as to raise W 
one inch. In order to do this, one inch of cord must 
be taken up on both sides of the pulley B. The 
inch on the side of AB will be pulled over the pulley B, and then two 
inches must be pulled over the pulley C; so that, in order to raise W 
one inch, the hand at P must descend two inches. Thus, exactly as in 
the case of the lever, although the work done at one end of the machine 
seems to be greater than that done at the other, it is not really so; 
for, at the other end, the work is merely spread over a larger 
space. 

Fig. 11. 


