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: