How to Measure the Forces Between Two Things

If you know the mass of a mountain, then you can measure big Gs. You might think that you also need to know theMass of the earth because that's also pulling on your pendulum. But if you know the volume of the mountain and theVolume of the earth, which we do, then the angle of deflection of the pendulum depends on the relative densities of the earth and the mountain. Which one is denser? So really, the experiment measures the density of the earth - which wasn't known at the time. Of course, knowing thedensity of the earth lets you calculate the mass of the earth  and therefore let you get big G. I guess