was the case with his First Law. "Change of motion," he states, "is
proportionate to the impressed Force, and takes place in the direction
of the straight line in which the Force acts."

Newton adds this explanation to his Second Law: "If a Force generates
any motion, a double Force will generate double motion, and a triple
Force triple motion, whether they are applied simultaneously or
gradually and successively. And this motion, if the body were already
moving, is either added to the previous motion, if it is in the same
direction, or subtracted from it if directly opposed to it, or is
compounded with the previous motion if the two are inclined at an
angle."

According to that, a force which presses or pushes with a four-pound
pressure per square inch, if doubled, would press with a force of eight
pounds per square inch, which fact agrees with experience. If the force
is applied gradually, then the change of motion would be gradual; if
applied suddenly, then the resultant motion would be sudden and violent.

The impressed force, therefore, always produces a definite and
corresponding effect on any moving body, however that force may be
originated, and however it may be applied. The effect so produced is
always a change of motion, or, in present scientific terms, a change of
momentum in the moving body. If the impressed force is halved, by an
alteration in the mass of the body which exerts the impressed force,
then the resultant momentum produced is halved also. If the impressed
force is doubled, through any alteration in the velocity of the body
which exerts the force, then the momentum produced in the moving body
will be doubled also. So that the impressed force is equal to the change
of momentum in the moving body upon which it is impressed.

When similar forces are impressed upon exactly similar bodies, the
velocities produced are exactly the same; but, if similar forces act on
dissimilar bodies, then the velocities produced in the different bodies
are not the same; yet the total motion produced on all bodies, according
to the Second Law of Motion, must always be proportionate to the
impressed force. So that when we compare the effect of similar forces on
different bodies, we find that there are two factors involved, viz., the
mass and velocity of the moving body. The product of these two
quantities is termed the momentum of the body.

When we apply the Second Law of Motion to the theory of aetherial
dynamics,