MECHANICS. 
liiOTeable round a pin passing through its 
centre. Pullies are of two kinds ; fixed, 
which do not move out of tlieir places ; and 
moveable, which rise and fall with the 
weight. 
When a pulley is fixed, as Plate II. Me* 
chanics, fig. 11. two equal weights suspended 
to the ends of a rope passing over it wdll 
balance each other, for they stretch the 
rope equally, and if either of them be 
pulled down through any given space, the 
other will rise through an etjual space in the 
same time ; and consequently, as the velo- 
cities of both are equal, they must balance 
each other. This kind of pulley, therefore, 
gives no mechanical advantage ; but its use 
consists in changing the direction of the 
power, and sometimes enabling it to be ap- 
piliedwith more convenience. By it, a man 
may raise a weight to any point, as the top 
of a building, without moving from the 
pilace he is in ; whereas, otherwise, he 
would have been obliged to ascend with the 
weight; it also enables several men toge- 
ther to apply their strength to the weight by 
means of the rope. The moveable pulley 
represented at A (fig. 12.) is fixed to the 
weight W, and rises and falls with it. In 
comparing this to a lever, the fulcrum must 
be considered as at A, the weight acts upon 
the centre c, and the power is applied at the 
extremity of the lever D. The power, 
therefore, being twdce as far from the ful- 
crum as the weight is, the proportion be- 
tween 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. Again, every moveable 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 pidley is as 2 to 1 . When the upper 
and fixed block contains two pullies, which 
only turn upon their axis, and the lower 
moveable block contains also two, which 
tiot only turn on their axis, but rise with the 
weight F (fig. 13.) 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 infcrcase of power is 
gained, the force by wltich F may be sus- 
tained will be equal to half the weight di- 
vided by the number of loWer ptillies ; 
that is, as twice the mimher of lower pullies 
is to 1, so is the weight suspended to the 
j)ower. But if the extremity C (fig. 14.) 
he 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 
pullies adding unity is to 1, so is the weight 
to tlie power. These rules hold good,wliat- 
ever may be the number of pullies in the 
blocks. If) instead of one rope going round 
all the pullies, the rope belonging to each 
pulley be made fast at top, as in fig. 15, a 
different proportion between the power and 
the weight will take place. Here it is evi- 
dent, that each pulley doubles the power j 
thus, if there are two pullies, the power 
will sustain four times the weight ; if tliree 
pullies, eiglit times the weight ; if four put- 
lies, sixteen times ; and so on: thatis, the 
power P of lib. will sustain a w'eight W of 
1616. 
Wljen pullies in blocks are placed per- 
pendicularly under each other, on separate 
pins, they occupy considerable space, and 
would not in general answer ; it is, there- 
fore, common to place all the pullies in 
each block on the same pin, by the side of 
each other, as in fig. 16. but the advantage 
and rule for the power, are the same here as 
in fig. 13 and 14. A pair of blocks with the 
rope fastened round it, is commonly called 
a tackle. 
To avoid, in a great measure, the friction 
of several pullies rmmingon different pivots, 
Mr. James White, a very able mechanic, 
invented the concentric pulley, (fig. 17.) for 
which he obtained a patent. O and R are 
two brass pullies in which grooves are cut ; 
round these a cord is passed, by which 
means the two answ'er the same purpose of 
so many distinct pullies a.s there are grooves; 
and the advantage gained is found by 
doubling the number of grooves in tlie 
lower block. In this case the advantage 
gained is 12, that is, a power of 1216. will 
balance a weight of 144. The concen- 
tric pulley removes very consideiably the 
shaking motion of the common pulley as 
well as the fi iclion. 
The inclined plane is of very great use in 
rolling up heavy bodies, .such as casks, 
wheel-barrows, Ac. It is formed by placing 
boards, or earth, in a sloping direction. The 
force with which a body descends upon an 
inclined plane, is to the force of its absolute 
gravity, by' which it would desftend perpen- 
dicularly in free space, as the height of the 
plane is to its length. -For suppose the 
