the _socket_; the middle of the
lever will be now opposite to the pulley, Pl. 2. Fig. 4.--hook to it
the rope that goes through the pulley P 3, and fasten to the other end
of the lever the rope by which the boy is to pull. This will be _a
lever of the second kind_, as it is called in books of mechanics; in
using which, _the resistance is placed between the centre of motion or
fulcrum, and the moving power_. He will now raise double the weight
that he did in Experiment II, and he will advance through double the
space.

EXPERIMENT IV.

Shift the lever, and the socket which forms the axis (without shifting
the lever from the place in which it was in the socket in the last
experiment) to the holes that are prepared for it at R R, Plate 2.
Fig. 5. The free end of the lever E will now be opposite to the rope,
and to the pulley (over which the rope comes from the scale-beam.)
Hook this rope to it, and hook the rope by which the boy pulls, to the
middle of the lever. The effect will now be different from what it was
in the two last experiments; the boy will advance only half as far,
and will raise only half as much weight as before. This is called _a
lever of the third sort_. The first and second kinds of levers are
used in quarrying; and the operations of many tools may be referred to
them. The third kind of lever is employed but seldom, but its
properties may be observed with advantage whilst a long ladder is
raised, as the man who raises it, is obliged to exert an increasing
force until the ladder is nearly perpendicular. When this lever is
used, it is obvious, from what has been said, that the power must
always pass through less space than the thing which is to be moved; it
can never, therefore, be of service in gaining power. But the object
of some machines, is to increase velocity, instead of obtaining power,
as in a sledge-hammer moved by mill-work. (V. the plates in Emerson's
Mechanics, No. 236.)

The experiments upon levers may be varied at pleasure, increasing or
diminishing the mechanical advantage, so as to balance the power and
the resistance, to accustom the learners to calculate the relation
between the power and the effect in different circumstances; always
pointing out, that whatever excess there is in the power,[23] or in
the resistance, is always compensated by the difference of space
through which the inferiour passes.

The experiments which we have mentioned, are sufficiently satisfactory
to a pupil