cannot admit of such a propagation of motion, and I am about to show
here the way in which I conceive it must occur. For this, it is
needful to explain the property which hard bodies must possess to
transmit movement from one to another.

When one takes a number of spheres of equal size, made of some very
hard substance, and arranges them in a straight line, so that they
touch one another, one finds, on striking with a similar sphere
against the first of these spheres, that the motion passes as in an
instant to the last of them, which separates itself from the row,
without one's being able to perceive that the others have been
stirred. And even that one which was used to strike remains motionless
with them. Whence one sees that the movement passes with an extreme
velocity which is the greater, the greater the hardness of the
substance of the spheres.

But it is still certain that this progression of motion is not
instantaneous, but successive, and therefore must take time. For if
the movement, or the disposition to movement, if you will have it so,
did not pass successively through all these spheres, they would all
acquire the movement at the same time, and hence would all advance
together; which does not happen. For the last one leaves the whole row
and acquires the speed of the one which was pushed. Moreover there are
experiments which demonstrate that all the bodies which we reckon of
the hardest kind, such as quenched steel, glass, and agate, act as
springs and bend somehow, not only when extended as rods but also when
they are in the form of spheres or of other shapes. That is to say
they yield a little in themselves at the place where they are struck,
and immediately regain their former figure. For I have found that on
striking with a ball of glass or of agate against a large and quite