Yup, that's exactly it: - get the gravitational constant with these two known masses - then can deduct the mass of the unknown Earth by its interaction with o

Yup, that's exactly it: - get the gravitational constant with these two known masses - then can deduct the mass of the unknown Earth by its interaction with other masses (say the "g" gravitational acceleration value) - then from the mass and the otherwise measured size of Earth the density pops out More details in good ol' Wikipedia: https://en.wikipedia.org/wiki/Cavendish_experiment#Derivation_of_G_and_the_Earth's_mass https://en.wikipedia.org/wiki/Cavendish_experiment#Derivatio...