The Mathematical Principles of Natural Philosophy, Book III: Of the System of the World
Proposition IX. Theorem IX
That the force of gravity, considered downward from the surface of the planets, decreases nearly in the proportion of the distances from their centres.
If the matter of the planet were of an uniform density, this Proposition would be accurately true (by Prop. LXXIII. Book I). The error, therefore, can be no greater than what may arise from the inequality of the density.