_B._ Take a piece of soft wax; do not you think the one half touches the other half as close as the smoothest marbles? Yet you can pull them asunder. But how? Still as you pull, the wax grows continually more and more slender; there being a perpetual parting or discession of the outermost part of the wax one from another, which the air presently fills; and so there is a continual lessening of the wax, till it be no bigger than a hair, and at last separation. If you can do the same to a pillar of marble, till the outside give way, the effect will be the same, but much quicker, after it once begins to break in the superficies; because the force that can master the first resistance of the hardness, will quickly dispatch the rest. _A._ It seems so by the brittleness of some hard bodies. But I shall afterward put some questions to you, touching the nature of hardness. But now to return to our subject. What reason can you render (without supposing vacuum) of the effects produced in the engine they use at Gresham college? _B._ That engine produceth the same effects that a strong wind would produce in a narrow room. _A._ How comes the wind in? You know the engine is a hollow round pipe of brass, into which is thrust a cylinder of wood covered with leather, and fitted to the cylinder so exactly as no air can possibly pass between the leather and the brass? _B._ I know it; and that they may thrust it up, there is a hole left in the cylinder to let the air out before it, which they can stop when they please. There is also in the bottom of the cylinder a passage into a hollow globe of glass, which passage they can also open and shut at pleasure. And at the top of that globe there is a wide mouth to put in what they please to try conclusions on, and that also to be opened and shut as shall be needful. It is of the nature of a pop-gun which children use, but great, costly, and more ingenious. They thrust forward and pull back the wooden cylinder (because it requires much strength) with an iron screw. What is there in all this to prove the possibility of vacuum. _A._ When this wooden cylinder covered with leather, fit and close, is thrust home to the bottom, and the holes in the hollow cylinder of brass close stopped, how can it be drawn back, as with the screw they draw it, but that the space it leaves must needs be empty: for it is impossible that any air can pass into the place to fill it?