he foremost end of the wedge,
and pass it through the pulleys (P 4. and P 3.) as in the fifth
experiment. Let a boy draw the sledge by this rope over his shoulder,
and he will find, that as it advances it will raise the weight
upwards; the wedge is five feet long, and elevated one foot. Now, if
the perpendicular ascent of the weight, and the space through which he
advances, be compared, he will find, that the space through which he
has passed will be five times as great as that through which the
weight has ascended; and that _this_ wedge has enabled him to raise
five times as much as he could raise without it, if his strength were
applied, as in Experiment I, without any mechanical advantage. By
making this wedge in two parts hinged together, with a graduated piece
to keep them asunder, the wedge may be adjusted to any given
obliquity; and it will be always found, that the mechanical advantage
of the wedge may be ascertained by comparing its perpendicular
elevation with its base. If the base of the wedge is 2, 3, 4, 5, or
any other number of times greater than its height, it will enable the
boy to raise respectively 2, 3, 4, or 5 times more weight than he
could do in Experiment I, by which his power is estimated.

_The Screw._

_The screw_ is an inclined plane wound round a cylinder; the height of
all its revolutions round the cylinder taken together, compared with
the space through which the power that turns it passes, is the measure
of its _mechanical advantage_.[27] Let the lever, used in the last
experiment, be turned in such a manner as to reach from its gudgeon to
the shaft of the Panorganon, guided by an attendant lever as before.
(Plate 2. Fig. 8.) Let the wheel rest upon the lowest _helix_ or
thread of the screw: as the arms of the shaft are turned round, the
wheel will ascend, and carry up the weight which is fastened to the
lever.[28] As the situation of the screw prevents the weight from
being suspended exactly from the centre of the screw, proper allowance
must be made for this in estimating the force of the screw, or
determining the mechanical advantage gained by the lever: this can be
done by measuring the perpendicular ascent of the weight, which in all
cases is better, and more expeditious, than measuring the parts of a
machine, and estimating its force by calculation; because the
different diameters of ropes, and other small circumstances, are
frequently mistaken in estimates.

The space passed t