_A._ There is a sphere of glass, which they call a recipient, of the capacity of three or four gallons. And there is inserted into it the end of a hollow cylinder of brass above a foot long; so that the whole is one vessel, and the bore of the cylinder three inches diameter. Into which is thrust by force a solid cylinder of wood, covered with leather so just, as it may in every point exactly touch the concave superficies of the brass. There is also, to let out the air which the wooden cylinder as it enters (called the sucker) drives before it, a flap to keep out the external air while they are pulling the sucker. Besides, at the top of the recipient there is a hole to put into it anything for experiment. The sucker being now forced up into the cylinder, what do you think must follow? _B._ I think it will require as much strength to pull it back, as it did to force it in. _A._ That is not it I ask, but what would happen to the recipient? _B._ I think so much air as would fill the place the sucker leaves, would descend into it out of the recipient; and also that just so much from the external air would enter into the recipient, between the brass and the wood, at first very swiftly, but, as the place increased, more leisurely. _A._ Why may not so much air rather descend into the place forsaken, and leave as much vacuum as that comes to in the recipient? For otherwise no air will be pumped out, nor can that wooden pestle be called a sucker. _B._ That is it I say. There is no air either pumped or sucked out. _A._ How can the air pass between the leather and the brass, or between the leather and the wood, being so exactly contiguous, or through the leather itself? _B._ I conceive no such exact contiguity, nor such fastness of the leather: for I never yet had any that in a storm would keep out either air or water. _A._ But how then could there be made in the recipient such strange alteration both on animate and inanimate bodies? _B._ I will tell you how. The air descends out of the recipient, because the air which the sucker removeth from behind itself, as it is pulling out, has no place to retire into without, and therefore is driven into the engine between the wood of the sucker and the brass of the cylinder, and causes as much air to come into the place forsaken by the retiring sucker; which causeth, by oft repetition of the force, a violent circulation of the air within the recipient, which is able quickly to kill anything that lives by respiration, and make all the alterations that have appeared in the engine. ========== CHAPTER IV. OF THE SYSTEM OF THE WORLD. _B._ You are come in good time; let us therefore sit down. There is ink, paper, ruler, and compass. Draw a little circle to represent the body of the sun. _A._ It is done. The centre is A, the circumference is L M.