is really wonderful to observe how extraordinarily productive

matter; that these central forces alone keep the planets and comets

in their orbits, and cause them to move in the proportion before set

down. I demonstrate to you that it is impossible there should be

any other cause which keeps the planets in their orbits than that

general phenomenon of gravity. For heavy bodies fall on the earth

according to the proportion demonstrated of central forces; and the

planets finishing their course according to these same proportions,

in case there were another power that acted upon all those bodies,

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