The theory resembles that about the stone being worn away by the drop of water or split by plants growing out of it: if so much has been extruded or removed by the drop, it does not follow that half the amount has previously been extruded or removed in half the time; but, as in the case of the hauled ship, so many drops set so much in motion, but a part of them will not set as much in motion in any period of time.