  In other words, the measure of the change in motion is the distance between the place where the body would have been after a given time had it not been acted on by the force and the place it is after that time. This is in keeping with the measure universally used at the time for the strength of the acceleration of surface gravity, namely the distance a body starting from rest falls vertically in the first second. The only special provision that Newton has to make is for non-uniform continuously acting forces, for which, in accord with Lemma 10, he takes the distance AB to vary “at the very beginning of the motion in the squared ratio of the  times.”[21]