LXIX. The motion with which heavy bodies descend, is the most frequent and common to our view. And what do we know of this? Of its properties very little; of its causes nothing. We know it acquires some degree of acceleration in its passage, because we see it; but what is the proportion of the increased acceleration, is matter of great debate, both among the philosophers, and mathematicians. We know that it is a motion of descent, but we do not know whether its course is directed to the centre, or the axis of the earth. The cause of this motion remains so hidden, that the philosophers to this day, have given no opinion concerning it, that I will not venture to pronounce an absurd one. The Aristotelites, by saying the cause of this motion proceeds from an innate disposition to move in every thing, say nothing, unless they point out the particular virtue or faculty, which excites motion in heavy bodies, for what we have just mentioned, is the reason they generally assign for all species of movements. They should not be allowed to dispute or controvert what they themselves have advanced; and if they should attempt to give a more rigorous definition of their opinion, they would fall into a still greater absurdity; which made the learned Father Saguens say, _Quis non palpat crassitiem hujus chimericæ opinionis?_ The Cartesians to account for this phenomenon, recur to the vertical motion of the subtil matter, which separating itself from the earth, and pursuing the angle or direction of tangents to the circle, impels heavy bodies to descend. But this reasoning has been confuted by most efficacious mathematical arguments. Gassendo invented an effluvia composed of terraqueous corpuscles, which mount into the air, and penetrate the pores of heavy bodies, which they generally encounter, and after turning their first course into a descending one, impel them downwards. Nothing has so much convinced me of the great difficulty of this question, as seeing a man of the subtil ingenuity of Gassendo, recur for the solution of it, to a fiction destitute of all probability; and which is exposed to invincible objections. Father Maignan, and his followers also, make use of the terraqueous effluvia for the solution of this difficulty; they do not allow that they act by impulse, but that by their sympathetic and magnetic virtue, when they come in contact with heavy bodies, they dispose them to descend.