As effect and cause are necessarily equal, so is motion the measure of impulse, in time. Therefore as long as a body is in motion it is being impelled, however insensible the impelling cause. Motion must be in the direction of impulse; for, as a body cannot move itself, and is the passive patient of impulse, so must its direction be the same as that of impulse; therefore when the direction of motion is changed, it must be by a novel impulse in the novel direction. From all matter being in motion, and all effect consisting in motion, and because like effects everywhere are attributable to the like or same cause, so must there be a cause of motion as universal as matter; rather than that there should be a distinct impelling cause for every individual motion following after the body, to put and keep it in motion. In all philosophic research the golden rule of nature should be held in mind, which prescribes "the shortest mode and fewest materials:" _to mistake on the side of simplicity is more wise than censurable in the search after natural physical truths_. A universal cause of motion, it would seem, can be no other than a universal medium, a medium of pressure, one occupying the regions of planetary space, competent to keep the planets in interminable motion and effect all terrestrial minor motion: only by such means is it conceivable how the earth can be under endless, ever-varying impulse, productive of ever-changing direction. When impulsively pressed into motion by such a medium, the direction of a planet must be orbicular, on account of the pressure on the solar side being always less than on the opposite, by which the projectile direction is diverted from rectilinear to curvilinear.