st as the sledge, and the
top of each radius successively (and the boy's hand which unfolds the
web) will move twice as fast as the centre of the pulley and the
sledge.

If a person, holding a stick in his hand, observes the relative
motions of the top, and the middle, and the bottom of the stick,
whilst he inclines it, he will see that the bottom of the stick has no
motion on the ground, and that the middle has only half the motion of
the top. This property of the pulley has been dwelt upon, because it
elucidates the motion of a wheel rolling upon the ground; and it
explains a common paradox, which appears at first inexplicable. "The
bottom of a rolling wheel never moves _upon_ the road." This is
asserted only of a wheel moving over hard ground, which, in fact, may
be considered rather as laying down its circumference upon the road,
than as moving upon it.

_The inclined Plane and the Wedge._

The _inclined plane_ is to be next considered. When a heavy body is to
be raised, it is often convenient to lay a sloping artificial road of
planks, up which it may be pushed or drawn. This mechanical power,
however, is but of little service without the assistance of wheels or
rollers; we shall, therefore, speak of it as it is applied in another
manner, under the name of _the wedge_, which is, in fact, a moving
inclined plane; but if it is required to explain the properties of the
inclined plane by the panorganon, the wooden road may be raised and
set to any inclination that is required, and the sledge may be drawn
upon it as in the former experiments.

Let one end of a lever, N. Plate 2. Fig. 7. with a wheel at one end of
it, be hinged to the post of the frame, by means of a gudgeon driven
or screwed into the post. To prevent this lever from deviating
sideways, let a slip of wood be connected with it by a nail, which
shall be fast in the lever, but which moves freely in a hole in the
rail. The other end of this slip must be fastened to a stake driven
into the ground at three or four feet from the lever, at one side of
it, and towards the end in which the wheel is fixed (Plate 2. Fig 10.
which is a _vue d'oiseau_) in the same manner as the treadle of a
common lathe is managed, and as the treadle of a loom is sometimes
guided.[25]

EXPERIMENT XI.

Under the wheel of this lever place an inclined plane or half-wedge
(Plate 2. Fig. 7.) on the wooden road, with rollers under it, to
prevent friction;[26] fasten a rope to t