_B._ Why not? What else can you think makes the diurnal motion of the earth but the sun? And for the moon, if it did not turn upon its own centre, we should see sometimes one, sometimes another face of the moon, which we do not. ========== CHAPTER III. PROBLEMS OF VACUUM. _A._ What convincing argument is there to prove, that in all the world there is no empty place? _B._ Many; but I will name but one; and that is, the difficulty of separating two bodies hard and flat laid one upon another. I say the difficulty, not the impossibility. It is possible, without introducing vacuum, to pull asunder any two bodies, how hard and flat soever they be, if the force used be greater than the resistance of the hardness. And in case there be any greater difficulty to part them, besides what proceeds from their hardness, than there is to pull them further asunder when they are parted, that difficulty is argument enough to prove there is no vacuum. _A._ These assertions need demonstration. And first, how does the difficulty of separation argue the plenitude of all the rest of the world? _B._ If two flat polished marbles lie one upon another, you see they are hardly separated in all points at one and the same instant; and yet the weight of either of them is enough to make them slide off one from the other. Is not the cause of this, that the air succeeds the marble that so slides, and fills up the place it leaves? _A._ Yes, certainly. What then? _B._ But when you pull the whole superficies asunder, not without great difficulty, what is the cause of that difficulty? _A._ I think, as most men do, that the air cannot fill up the space between in an instant; for the parting is in an instant. _B._ Suppose there be vacuum in that air into which the marble you pull off is to succeed, shall there be no vacuum in the air that was round about the two marbles when they touched? Why cannot that vacuum come into the place between? Air cannot succeed in an instant, because a body, and consequently cannot be moved through the least space in an instant. But emptiness is not a body, nor is moved, but is made by the act itself of separation. There is therefore, if you admit vacuum, no necessity at all for the air to fill the space left in an instant. And therefore, with what ease the marble coming off presseth out the vacuum of the air behind it, with the same ease will the marbles be pulled asunder. Seeing then, if there were vacuum, there would be no difficulty of separation, it follows, because there is difficulty of separation, that there is no vacuum. _A._ Well, now, supposing the world full, how do you prove it possible to pull those marbles asunder?