is about four feet from the ground;
let the person pull this rope from 1 towards 2, turning his back to
the machine, and pulling the rope over his shoulder--Pl. 2. Fig. 6. As
the pulley may be either too high or too low to permit the rope to be
horizontal, the person who pulls it should be placed ten or fifteen
feet from the machine, which will lessen the angular direction of the
cord, and the inaccuracy of the experiment. Hang weights to the other
end of the scale-beam, until the person who pulls can but just walk
forward, pulling fairly without propping his feet against any thing.
This weight will estimate the force with which he can draw
horizontally by a rope over his shoulder.[22] Let a child who tries
this, walk on the board with dry shoes; let him afterwards chalk his
shoes, and afterwards try it with his shoes soaped: he will find that
he can pull with different degrees of force in these different
circumstances; but when he tries the following experiments, let his
shoes be always dry, that his force may be always the same.

_To show the power of the three different sorts of levers._

EXPERIMENT II.

Instead of putting the cord that comes from the scale-beam, as in the
last experiment, over the shoulder of the boy, hook it to the end 1 of
the lever L, Fig. 2. Plate 2. This lever is passed through a
socket--Plate 2. Fig. 3.--in which it can be shifted from one of its
ends towards the other, and can be fastened at any place by the screw
of the socket. This socket has two gudgeons, upon which it, and the
lever which it contains, can turn. This socket and its gudgeons can be
lifted out of the holes in which it plays, between the rail R R, Plate
2. Fig. 2. and may be put into other holes at R R, Fig. 5. Loop
another rope to the other end of this lever, and let the boy pull as
before. Perhaps it should be pointed out, that the boy must walk in a
direction contrary to that in which he walked before, viz. from 1
towards 3. The height to which the weight ascends, and the distance to
which the boy advances, should be carefully marked and measured; and
it will be found, that he can raise the weight to the same height,
advancing through the same space as in the former experiment. In this
case, as both ends of the lever moved through equal spaces, the lever
only changed the direction of the motion, and added no mechanical
power to the direct strength of the boy.

EXPERIMENT III.

Shift the lever to its extremity in