Powers which intercede the Particles. The _Vis inerti _ is a passive
Principle by which Bodies persist in their Motion or Rest, receive
Motion in proportion to the Force impressing it, and resist as much as
they are resisted. By this Principle alone there never could have been
any Motion in the World. Some other Principle was necessary for putting
Bodies into Motion; and now they are in Motion, some other Principle is
necessary for conserving the Motion. For from the various Composition of
two Motions, 'tis very certain that there is not always the same
quantity of Motion in the World. For if two Globes joined by a slender
Rod, revolve about their common Center of Gravity with an uniform
Motion, while that Center moves on uniformly in a right Line drawn in
the Plane of their circular Motion; the Sum of the Motions of the two
Globes, as often as the Globes are in the right Line described by their
common Center of Gravity, will be bigger than the Sum of their Motions,
when they are in a Line perpendicular to that right Line. By this
Instance it appears that Motion may be got or lost. But by reason of the
Tenacity of Fluids, and Attrition of their Parts, and the Weakness of
Elasticity in Solids, Motion is much more apt to be lost than got, and
is always upon the Decay. For Bodies which are either absolutely hard,
or so soft as to be void of Elasticity, will not rebound from one
another. Impenetrability makes them only stop. If two equal Bodies meet
directly _in vacuo_, they will by the Laws of Motion stop where they
meet, and lose all their Motion, and remain in rest, unless they be
elastick, and receive new Motion from their Spring. If they have so much
Elasticity as suffices to make them re-bound with a quarter, or half, or
three quarters of the Force with which they come together, they will
lose three quarters, or half, or a quarter of their Motion. And this may
be try'd, by letting two equal Pendulums fall against one another from
equal heights. If the Pendulums be of Lead or soft Clay, they will lose
all or almost all their Motions: If of elastick Bodies they will lose
all but what they recover from their Elasticity. If it be said, that
they can lose no Motion but what they communicate to other Bodies, the
consequence is, that _in vacuo_ they can lose no Motion, but when they
meet they must go on and penetrate one another's Dimensions. If three
equal round Vessels be filled, the one with Water, the other with Oil,
the third with molten Pitch, and the Liquors be stirred about alike to
give them a vortical Motion; the Pitch by its Tenacity will lose its
Motion quickly, the Oil being less tenacious will keep it longer, and
the Water being less tenacious will keep it longest, but yet will lose
it in a short time. Whence it is easy to understand, that if many
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