'And suppose,' Newton continued, 'that we had a rope with regularly spaced knots, such as mariners use to log their speed, and we anchored one end of it on the Bridge – for the Bridge is a fixed point in absolute space. If that rope were stretched tight it would be akin to one of the numbered lines employed by Monsieur Descartes in his Geometry. By stretching it between the Bridge and the punt, we could measure how far the punt had drifted down-river, and in which direction.'