doctrine of attractive and repulsive forces, without troubling
themselves about the philosophical difficulties which it involves. But
this has not always been the case; and the attempt of Le Sage, in the
last century, to show that the phenomena of attraction and repulsion
are susceptible of explanation by his hypothesis of bombardment by
ultra-mundane particles, whether tenable or not, has the great merit
of being an attempt to get rid of the dual conception of the causes
of motion which has hitherto prevailed. On this hypothesis, the
hammering of the ultra-mundane corpuscles on the bob confers its
kinetic energy, on the one hand, and takes it away on the other; and
the state of potential energy means the condition of the bob during
the instant at which the energy, conferred by the hammering during the
one half-arc, has just been exhausted by the hammering during the
other half-arc. It seems safe to look forward to the time when the
conception of attractive and repulsive forces, having served its
purpose as a useful piece of scientific scaffolding, will be replaced
by the deduction of the phenomena known as attraction and repulsion,
from the general laws of motion.

The doctrine of the conservation of energy which I have endeavored to
illustrate is thus defined by the late Clerk Maxwell:

'The total energy of any body or system of bodies is a quantity which
can neither be increased nor diminished by any mutual action of such
bodies, though it may be transformed into any one of the forms of
which energy is susceptible.' It follows that energy, like matter, is
indestructible and ingenerable in nature. The phenomenal world, so far
as it is material, expresses the evolution and involution of energy,
its passage from the kinetic to the potential condition and back
again. Wherever motion of matter takes place, that motion is effected