Newton discovered that bodies which have equal heaviness have equal inertia, i.e. that if the property of inertia is possed to an equal degree by two different substances, the have equal heaviness. One of his proofs was founded on the celebrated guinea and feather experiment, showing that the guinea and feather fall at the same rate when the resistance of the air is removed. Another was founded upon making pendulums of different substances—lead, iron, and wood to vibrate, and observing their times of vibration. This discovery is the very essence of the Newton's 'first great point' pertaining to the law of gravitation. The other is that the force between any two bodies diminishes as the distance increases, according to the law of inverse square of the distance.
- W. Thomson (Lord Kelvin) (August 25, 1882). August 25, 1882. The British Association, The Glasgow Science Lectures Association.