. The two radii which are opposite,
may be considered as a lever of the first sort, where the centre is in
the middle of the lever; as each end moves through an equal space,
there is no mechanical advantage. But if this skeleton-pulley be
employed as a common _block_ or _tackle_, its motions and properties
will be entirely different.

EXPERIMENT X. PLATE 2. FIG. 9.

Nail a piece of girth-web to a post, at the distance of three or four
feet from the ground; fasten the other end of it to one of the radii.
Fasten another piece of web to the opposite radius, and let a boy hold
the skeleton-pulley suspended by the web; hook weights to the strap
that hangs from the centre. The end of the radius to which the fixed
girth-web is fastened, will remain immoveable; but, if the boy pulls
the web which he holds in his hand upwards, he will be able to lift
nearly double the weight, which he can raise from the ground by a
simple rope, without the machine, and he will perceive that his hand
moves through twice as great a space as the weight ascends: he has,
therefore, the mechanical advantage which he would have by a lever of
the second sort, as in Experiment III. Let a piece of web be put round
the under radii, let one end of it be nailed to the post, and the
other be held by the boy, and it will represent the application of a
rope to a moveable pulley; if its motion be carefully considered, it
will appear that the radii, as they successively apply themselves to
the web, represent a series of levers of the second kind. A pulley is
nothing more than an infinite number of such levers; the cord at one
end of the diameter serving as a fulcrum for the _organ_ during its
progress. If this _skeleton-pulley_ be used horizontally, instead of
perpendicularly, the circumstances which have been mentioned, will
appear more obvious.

Upon the wooden road lay down a piece of girth-web; nail one end of it
to the road; place the pulley upon the web at the other end of the
board, and, bringing the web over the radii, let the boy, taking hold
of it, draw the loaded sledge fastened to the hook at the centre of
the pulley: he will draw nearly twice as much in this manner as he
could without the pulley.[24]

Here the web lying on the road, shows more distinctly, that it is
quiescent where the lowest radius touches it; and if the radii, as
they tread upon it, are observed, their points will appear at rest,
whilst the centre of the pulley will go as fa