The motion which continues after the body has ceased to be in contact
with the _sensible_ impelling cause, is named continuous motion. The
body impelled receives neither force nor motion from the impelling
cause: neither force nor motion is anything transferrable or anything
communicable; forcible velocity and change of place are but accidents
of matter, and but local, casual circumstances of bodies. Being
_inert_, the body cannot move itself. Motion, therefore, is but a
physical effect, and must have a cause equal to the duration of the
effect: motion after impulse has ceased, would be effect without
cause--which is an absurdity and impossibility; therefore impulse
is constant as motion, however insensible the impelling cause.
These dynamic principles cannot be too frequently brought to mind,
considering the general erroneous opinion on the subject which
maintains, that "a body continues in motion because it cannot stop
itself;" which is effect without its equal of cause.
A body in motion is under unequal pressure on opposite sides, greater
on the rear than front. The air in front resists, that in the rear
may be said to recede from the body; therefore neither impels the
projectile. Under such circumstances there remains but the alternative,
that of the electric constitution of the body being changed by the
previous impulse, by which medium of space accumulates on one side,
or decreases on the opposite. The phenomenon admits of being thus
illustrated:
The first, previous or sensible impulse, effects de-electrisation of
the body on the rear or side of impulse; influent medium of space
immediately occupies the vacated rear, and by its pressure impels the
body through the air. The velocity of the previous impulse, gives
momentum to the body greater than the included freely-removable
elementary matter can obtain; of consequence the latter is left behind
in the air, and the pressure of the acquired medium of space in the
rear, is the continuous impelling cause. Thus is the mistake of Dugald
Stuart made evident, that "motion is the immediate and only effect of
impulse."
It is not the air's resistance which makes the motion of a projectile
decline and end. Taking impulse as ten, resistance four, there remains
six degrees of unresisted impulse, which should impel the body for
ever through the atmosphere. The decline and cessation of impulse is
that which brings the projectile to rest.
From the instant the body has ceased to be in contact with the sensible
impelling cause, electric matter is re-entering the rear, which
displaces gradually the impelling medium; and as are the increments of
the former, so are the decrements of the latter, and so is the decline
of motion.
_Ascending and Descending Motion._--The rear of the vertically-impelled
body becomes vacated of minus-pressure matter, and replaced with medium
of space; by the latter, and general pressure, the body is forced
upwards as a cork by water. While ascending, the rear is acquiring
electric matter and losing the impelling medium,--the velocity of
course declines; and when at the highest, the body is at rest in the
air for an instant, then is precipitated to the ground. During the
entire of the descent, electric matter is vacating the rear and medium
of space entering, consequently the fall is accelerated. Now as the
body cannot fall of itself; as descending motion is of increasing
velocity, while motion in every other direction is retarded; and,
because all descent has the same _centripetal_ direction, so should
there be some distinct cause to produce these conspicuous effects,
which, to trace, suggest the following hypothesis:
_Centripetal Flow._--The different motions of the globe affect all
bodies on its surface, so as to appear to the inhabitants as if the
whole were at rest; supposing thence, that the centre of the earth
is the centre of motion, the following may be considered probable
consequences:--The general pressure being less at the centre and axis
than on the surface of the earth, obliges the medium of space to
flow through the atmosphere and entire surface, _centripetally_, to
the centre, thence along the axis, carrying with it electric matter,
and has exit at the poles, which polarizes the globe and produces
the boreales. The centripetal flow retains the atmosphere to the
earth; precipitates bodies from the air in a centripetal direction;
accelerates the descent; and retards all motion not in its own
direction: it prevents vertical ascent being equal to impulse, the
difference being employed in bearing against the flow. The flow makes
bodies ponderate or have weight, causes the dip and direction of the
compass-needle.