therefore in L the waight D has acquired a velocity equal to the velocity of the waight of the cylinder. In the same letter you desire to knowe, how any mediū, as water, retardeth the motion of a stone that falls into it. To w^{ch} I answer out of that you say afterwards, that nothing can hinder motion but contrary motion: that the motion of the water, when a stone falls into it, is point blanke contrary to the motion of the stone. For the stone by descent causeth so much water to ascend as the bignesse of the stone comes to. For imagine so much water taken out of the place w^{ch} the stone occupies, and layd upon the superficies of the water: it presseth downeward as the stone does, and maketh the water that is below to rise upwards, and this rising upwards is contrary to the descent, and is no other operation than we see in scales, when of two equal bullets in magnitude that w^{ch} is of heavier metal maketh the other to rise. And thus farre goes your letter of Dec. 30. For the first quære in your second letter, concerning how we see in the time the lucide body contracts itselfe, I have no other solution but that w^{ch} your selfe hath given: w^{ch} is, that the reciprocation is so quicke, that the effect of the first motion lasteth till the next comes, and longer. For by experience we observe that the end of a firebrand swiftely moved about in circle, maketh a circle of fire: w^{ch} could not be, if the impression made at the beginning of the circulation did not last till the end of it. For if the same firebrand be moved slower, there will appear but a peece of a circle, bigger or lesser according to the swiftnesse or slownesse of y^e motion. For the cause of such reciprocation, it is hard to guess what it is. It may well be the reaction of the medium. For though the mediū yeld, yet it resisteth to: for there can be no passion w^{th}out reaction. And if a man could make an hypothesis to salve that contraction of y^e sun, yet such is the nature of naturall thinges, as a cause may be againe demanded of such hypothesis: and never should one come to an end w^{th}out assigning the immediate hand of God. Whereas in mathematicall sciences wee come at last to a definition, w^{ch} is a beginning or principle, made true by pact and consent amongst ourselves. Further, you conceave a difficulty how the medium can be continually driven on, if there be such an alternate contraction. To w^{ch}, first I answer, that the motion forward is propagated to the utmost distance in an instant, and the first push is therefore enough, and in another instant is made the returne back in y^e like manner. And though it were not done in an instant, yet we see by experience in rivers, as in y^e Thames, that the tide goes upward towards London pushed by the water below, and yet at the same instant the water below is going backe to the sea. For seeing it is high-water at Blackwall before ’tis so at Greenwich, the water goes backe from Blackwall when it goes on at Greenwich.