But in percussion (which is the motion of the bullet against a wall,) the bullet no sooner goeth off than it loseth of its swiftness, and inclineth to the earth by its weight. So that the angles made in falling on and going off, cannot be equal, unless they be measured close to the point where the stroke is made. _A._ If a man set a board upright upon its edge, though it may very easily be cast down with a little pressure of one’s finger, yet a bullet from a musket shall not throw it down, but go through it. What is the cause of that? _B._ In pressing with your finger you spend time to throw it down. For the motion you give to the part you touch is communicated to every other part before it fall. For the whole cannot fall till every part be moved. But the stroke of a bullet is so swift, as it breaks through, before the motion of the part it hits can be communicated to all the other parts that must fall with it. _A._ The stroke of a hammer will drive a nail a great way into a piece of wood on a sudden. What weight laid upon the head of a nail, and in how much time will do the same? It is a question I have heard propounded amongst naturalists. _B._ The different manner of the operation of weight from the operation of a stroke, makes it incalculable. The suddenness of the stroke upon one point of the wood takes away the time of resistance from the rest. Therefore the nail enters so far as it does. But the weight not only gives them time, but also augments the resistance; but how much, and in how much time, is, I think, impossible to determine. _A._ What is the difference between reflection and recoiling? _B._ Any reflection may, and not unproperly, be called recoiling; but not contrariwise every recoiling reflection. Reflection is always made by the reaction of a body pressed or stricken; but recoiling not always. The recoiling of a gun is not caused by its own pressing upon the gunpowder, but by the force of the powder itself, inflamed and moved every way alike. _A._ I had thought it had been by the sudden re-entering of the air after the flame and bullet were gone out. For it is impossible that so much room as is left empty by the discharging of the gun, should be so suddenly filled with the air that entereth at the touchhole. _B._ The flame is nothing but the powder itself, which scattered into its smallest parts, seems of greater bulk by much, than in truth it is, because they shine. And as the parts scatter more and more, so still more air gets between them, entering not only at the touchhole, but also at the mouth of the gun, which two ways being opposite, it will be much too weak to make the gun recoil. _A._ I have heard that a great gun charged too much or too little, will shoot, not above, nor below, but beside the mark; and charged with one certain charge between both, will hit it.