On Natural Motions Depending on Gravity

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...it is perceived how from a natural operation, namely the downward descent, a certain contrary operation must be produced. This is the ascent of another part of the same fluid as it moves away from the center of the earth. It will therefore be worthwhile to clearly show the truth of this operation. We shall derive it from principles that are better known and more obvious.

PROPOSITION I.

When a heavy body is suspended from a point other than its center of gravity, one part of it ascends upward because the entire body descends downward.

Let there be an extended heavy body AB. Or let it be composed of two parts placed at the ends of the same horizontal balance AB. Let their common center of gravity be D. Let the whole balance then be supported and propped up at point C, which is distant from the center of gravity D. I state that the opposite part B will move upward through the arc BF for this reason alone:

A geometric diagram shows a horizontal balance beam AB. Point C serves as the pivot or fulcrum. Point A is a large circular weight on the left. Point B is a smaller circular weight on the right. Point D represents the center of gravity between weights A and B. A vertical line FG passes through the pivot C. Dotted circular arcs show the paths of movement. Point B moves upward toward point F. Point A moves downward toward point G. Point E is marked on the lower vertical axis between C and G.

because the whole heavy mass AB moves closer to the center of the earth than it was before. The two heavy parts A and B exert their gravity and compressive effort original: "conatum compressiuum." This refers to the force or pressure directed toward a center by the weight of a body. at their common center of gravity D. Since the aforementioned center D is distant from the stable support C, it will therefore form something like a pendulum original: "fune-pendulum." Borelli compares the swinging motion of the balance beam to a weight hanging from a cord. CD.