CONCERNING CERTAIN PROBLEMS. 397
in the beginning vigour adds alacrity, and near the end the hope of coming to what the animal aims afc. Cu. I cannot tell how it is with other animals ; but I have got a maid-servant who is weary before she begins, and tired before she ends. But return to what you begun.
Al. I say, heavy things are carried downward by a natural motion; and by how much the heavier anything is, by so much a swifter motion it is carried towards the earth, and by how much the lighter it is, by so much the swifter motion it is carried toward heaven. It is quite otherwise in a violent motion, which, being swift at first, grows slower by degrees; and contrary in a natural motion, as an arrow shot into the air, and a stone falling from on high. Cu. I used to think that men ran about upon the globe of the earth like little ants on a great ball; they stick upon it everywhere, and none fall off. Al. That is to be attributed to the ruggedness of the globe, and a certain roughness in the feet of the ants, which, indeed, is common to all insects in a manner ; and lastly, to the lightness of their bodies. If you do not believe me, make a glass globe very smooth and sleek; you will see that only those ants do not fall that are at the upper part of it. Cu. If any god should bore through the centre of the earth, quite down to the antipodes, in a perpendicular line, and as cosmographers use to represent the situation of the globe of the earth, and a stone were let fall into it, whither would it go? Al. To the centre of the earth ; there all heavy bodies rest. Cu. What if the antipodes should let fall a stone on their side 1 Al. Then one stone would meet the other about the centre, and stop there.
Cu. But hark you, if what you said just now be true, that a natural motion by its progress grew more and more strong, if nothing hindered a stone or lead cast into the hole, by reason of the vehemence of its motion it would pass beyond the centre, and having got beyond the centre, the motion would grow more violent. Al. Lead would never come to the centre unless it were melted ; but a stone, if it did pass the centre with so violent a motion, would go at first more heavily and return to the centre again, just as a stone thrown up into the air returns again to the earth. Cu. But returning back again by its natural motion, and again recovering force, it would go beyond the centre, and so the stone would never rest. Al. It would lie still at last by running beyond, and then running back again till it came to an equilibrium. Cu. But if there be no vaciium in nature, then that hole must be full of air. Al. Suppose it to be so. 'Cu. Then a body that is by nature heavy will hang in the air. Al. "Why not ? as steel does, being borne up by the loadstone. What wonder is it that one stone hang in the very middle of the air, when the whole earth laden with so many rocks hangs after the same manner ?
Cu. But where is the centre of the earth? AL Where is the centre of a circle ? Cu. That is a point that is indivisible ; if the centre of the earth be so small, whosoever bores through the centre takes it away, and then heavy bodies have nowhere to tend to. Al. Xow you talk idly enough. Cu. Pray, do not be angry ; what I say is for the sake of information. If any one should bore through the globe of the earth, and not through the centre itself, as suppose one hundred furlongs aside of it, where would a stone fall then? Al. It would not pass straight through the hole. It would, indeed, go
