no of the Newtonian system,

thing; as both the one and the other expressesthe force by which the body is impelled towardsthe earth, whether this force exists in the bodyitself, or out of it.

With this property of bodies, obvious as it is,the ancients were very imperfectly acquainted.They believed that there were substances, suchas vapours and smoke, that by their nature werelight, and would for that reason ascend. Thisnotion, however, as well as that of absolutelevity in general, is now known to be a mistake,and without the least foundation: for in an ex-hausted receiver, or a space void of air, allbodies whatever, smoke or a stone, a guinea ora feather, fall to the bottom in the same time.The distinction, therefore, between bodies lightand heavy, is merely relative, as they have all alike propensity to fall to the earth, and theirnature is the fame.

Neither can there be the least doubt but thatgravity is a sort of force: for whatever iscapable of putting a body in motion is properlyso called. But in all forces, there are two thingsparticularly to be considered ; the direction inwhich they act, and their intensity, or power.With respect to the direction of gravity, we aresufficiently assured, both by reason and expe-rience, that a body, in falling, moves towards thecenter of the earth; or, which is the famething, in a straight line that is perpendicular to

its