  Unfortunately, pendulum-decay turned out not to be as well behaved a phenomenon as Newton anticipated it was going to be while he was working on the first edition. The problem, as he later realized, was that he had to let the pendulum swing many times in order to measure the rate of decay, and in the process it gave rise to a “to and fro” motion in the surrounding fluid, so that the relative velocity between the bob and the fluid, which is the velocity that matters in resistance, could not be determined or controlled. The General Scholium following Section  6[39] reports detailed decay-rate data for an impressive range of experiments, including different size bobs in air and bobs moving as well in water and mercury. The reader is also shown in detail how to proceed from the data in each case to a polynomial as above defining the resistance force. Any reader who worked through the data discovered what Newton knew, but was less than candid about: no polynomial fit the data. The experiments did clearly indicate that resistance forces involve no power of velocity greater than 2, and they provided good evidence that a velocity squared effect was dominant, even to the extent of masking any effect involving some other power. Newton also managed to extract some highly qualified evidence that the velocity squared effect varies as the density of the fluid and the frontal area (that is, the square of the diameter) of spheres.