e divided by the vertical height, will give the amount
of centripetal force, and the velocity of revolution requisite to produce
an equivalent centrifugal force may be found by multiplying the centripetal
force of the ball in terms of its own weight by 70,440, and dividing the
product by the diameter of the circle made by the centre of the ball in
inches; the square root of the quotient is the number of revolutions per
minute. By this rule you fix the length of the arms, and the diameter of
the base of the cone, or, what is the same thing, the angle at which it is
desired the arms shall revolve, and you then make the speed or number of
revolutions such, that the centrifugal force will keep the balls in the
desired position.

44. _Q._--Does not the weight of the balls affect the question?

_A._--Not in the least; each ball may be supposed to be made up of a number
of small balls or particles, and each particle of matter will act for
itself. Heavy balls attached to a governor are only requisite to overcome
the friction of the throttle valve which shuts off the steam, and of the
connections leading thereto. Though the weight of a ball increases its
centripetal force, it increases its centrifugal force in the same
proportion.

THE MECHANICAL POWERS.

45. _Q._--What do you understand by the mechanical powers?

_A._--The mechanical powers are certain contrivances, such as the wedge,
the screw, the inclined plane, and other elementary machines, which convert
a small force acting through a great space into a great force acting
through a small space. In the school treatises on mechanics, a certain
number of these devices are set forth as the mechanical powers, and each
separate device is treated as if it involved a separate principle; but not
a tithe of the contrivances which accomplish the stipulated end are
represented in these learned works, and there is no very obvious necessity
for considering the principle of each contrivance separately when the
principles of all are one and the same. Every pressure acting with a
certain velocity, or through a certain space, is convertible into a greater
pressure acting with a less velocity, or through a smaller space; but the
quantity of mechanical force remains unchanged by its transformation, and
all that the implements called mechanical powers accomplish is to effect
this transformation.

46. _Q._--Is there no power gained by the lever?

_A._--Not any: the power is merely pu