An enlarged edition of the Henry Cavendish volume has now gone to print. It includes an extended Appendix on Gravitation, as a homage to Cavendish’s extraordinary diligence and accuracy in determining the Mean Density of the Earth, which in modern terms fixes the Gravitational Constant (‘Big G’). The Appendix explains the significance of this, and provides a pictorial introduction to Einstein’s relativistic theory of gravitation. Despite the overthrow of previous concepts of space, time and matter, ‘Big G’ remains central. Of all the Fundamental Constants of Nature it is the one whose value has been least improved upon since the work of Henry Cavendish in the 18th century.