_B._ The same motion which we have supposed from the beginning for the cause of so many other effects. Which motion not being upon the centre of the part moved, but the part itself going in another circle to and again, it is not necessary that the motion be perfectly circular. For it is not circulation, but the reciprocation, I mean the to and again, that does cast off, and lacerate the air, and consequently produce the fore-mentioned effects. For the cause therefore of hardness, I suppose the reciprocation of motion in those things which are hard, to be very swift, and in very small circles. _A._ This is somewhat hard to believe. I would you could supply it with some visible experience. _B._ When you see, for example, a cross-bow bent, do you think the parts of it stir? _A._ No. I am sure they do not. _B._ How are you sure? You have no argument for it, but that you do not see the motion. When I see you sitting still, must I believe there is no motion in your parts within, when there are so many arguments to convince me there is. _A._ What argument have you to convince me that there is motion in a cross-bow when it stands bent? _B._ If you cut the string, or any way set the bow at liberty, it will have then a very visible motion. What can be the cause of that? _A._ Why the setting of the bow at liberty. _B._ If the bow had been crooked before it was bent, and the string tied to both ends, and then cut asunder, the bow would not have stirred. Where lies the difference? _A._ The bow bent has a spring; unbent it has none, how crooked soever. _B._ What mean you by spring? _A._ An endeavour of restitution to its former posture. _B._ I understand spring as well as I do endeavour. _A._ I mean a principle or beginning of motion in a contrary way to that of the force which bent it. _B._ But the beginning of motion is also motion, how insensible soever it be. And you know that nothing can give a beginning of motion to itself. What is it therefore that gives the bow (which you say you are sure was at rest when it stood bent) its first endeavour to return to its former posture? _A._ It was he that bent it. _B._ That cannot be. For he gave it an endeavour to come forward, and the bow endeavours to go backward. _A._ Well, grant that endeavour be motion, and motion in the bow unbent, how do you derive from thence, that being set at liberty it must return to its former posture? _B._ Thus there being within the bow a swift (though invisible) motion of all the parts, and consequently of the whole; the bending causeth that motion, which was along the bow (that was beaten out when it was hot into that length) to operate across the length in every part of it, and the more by how much it is more bent; and consequently endeavours to unbend it all the while it stands bent. And therefore when the force which kept it bent is removed, it must of necessity return to the posture it had before.