apstan motion between the engine and paddle wheels?" Others I have heard
say: "By the hydraulic press you can obtain unlimited power; why not then
interpose a hydraulic press between the engines and the paddles?" To these
questions the reply is sufficiently obvious. Whatever you gain in force you
lose in velocity; and it would benefit you little to make the paddles
revolve with ten times the force, if you at the same time caused them to
make only a tenth of the number of revolutions. You cannot, by any
combination of mechanism, get increased force and increased speed at the
same time, or increased force without diminished speed; and it is from the
ignorance of this inexorable condition, that such myriads of schemes for
the realization of perpetual motion, by combinations of levers, weights,
wheels, quicksilver, cranks, and other mere pieces of inert matter, have
been propounded.

49. _Q._--Then a force once called into existence cannot be destroyed?

_A._--No; force is eternal, if by force you mean power, or in other words
pressure acting though space. But if by force you mean mere pressure, then
it furnishes no measure of power. Power is not measurable by force but by
force and velocity combined.

50. _Q._--Is not power lost when two moving bodies strike one other and
come to a state of rest?

_A._--No, not even then. The bodies if elastic will rebound from one
another with their original velocity; if not elastic they will sustain an
alteration of form, and heat or electricity will be generated of equivalent
value to the power which has disappeared.

51. _Q._--Then if mechanical power cannot be lost, and is being daily
called into existence, must not there be a daily increase in the power
existing in the world?

_A._--That appears probable unless it flows back in the shape of heat or
electricity to the celestial spaces. The source of mechanical power is the
sun which exhales vapors that descend in rain, to turn mills, or which
causes winds to blow by the unequal rarefaction of the atmosphere. It is
from the sun too that the power comes which is liberated in a steam engine.
The solar rays enable plants to decompose carbonic acid gas, the product of
combustion, and the vegetation thus rendered possible is the source of coal
and other combustible bodies. The combustion of coal under a steam boiler
therefore merely liberates the power which the sun gave out thousands of
years before.

FRICTION.

52. _Q._--What