'This proves nothing,' Hooke said as they rode home through the dark. 'The scale is not precise enough. But if one were to construct a clock, driven by a pendulum, in a sealed glass vessel, so that changes in moisture and baroscopic pressure would not affect its speed . . . and if one were to run that clock in the bottom of a well for a long period of time . . . any difference in the pendulum's weight would be manifest as a slowing, or quickening, of the clock.'