age of the
bob upwards from the centre of the arc to the summit of the left-hand
half-swing. Hence, at this point, the bob comes to a momentary rest.
The last fraction of kinetic energy is just neutralised by the action
of the attractive forces, and the bob has only potential energy equal
to that with which it started. So that the sum of the phenomena may be
stated thus: At the summit of either half-arc of its swing, the bob
has a certain amount of potential energy; as it descends it gradually
exchanges this for kinetic energy, until at the centre it possesses an
equivalent amount of kinetic energy; from this point onwards, it
gradually loses kinetic energy as it ascends, until, at the summit of
the other half-arc, it has acquired an exactly similar amount of
potential energy. Thus, on the whole transaction, nothing is either
lost or gained; the quantity of energy is always the same, but it
passes from one form into the other.

To all appearance, the phenomena exhibited by the pendulum are not to
be accounted for by impact: in fact, it is usually assumed that
corresponding phenomena would take place if the earth and the pendulum
were situated in an absolute vacuum, and at any conceivable distance
from, one another. If this be so, it follows that there must be two
totally different kinds of causes of motion: the one impact--a _vera
causa_, of which, to all appearance, we have constant experience; the
other, attractive or repulsive 'force'--a metaphysical entity which is
physically inconceivable. Newton expressly repudiated the notion of
the existence of attractive forces, in the sense in which that term is
ordinarily understood; and he refused to put forward any hypothesis as
to the physical cause of the so-called 'attraction of gravitation.' As
a general rule, his successors have been content to accept the
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