259 namely that one weight is then carried downward and raises the other, when it is heavier than the other. Besides, in the case of natural mobiles, just as in the case of the weights of a balance, the cause of all motions, upward as well as downward, can be reduced to heaviness alone. For when a thing is carried upward, it is at that time raised by the heaviness of the medium; just as, if a beam lighter than water were being kept by force under water, then, because the submerged beam has lifted a quantity of water equal to its own size, and an amount of water as great in size as the size of the beam is heavier than the beam, then, doubtless, the beam will be raised by the heaviness of that water, and it will be impelled upward: and in this way upward motion will be brought about by the heaviness of the medium and the lightness of the mobile{1}; and downward motion by the heaviness of the mobile and the lightness of the medium. {2}And from this, contrary to Aristotle in text #89 of Book I of the <i>De Caelo</i>, someone will easily be able to conclude, in what way things that are moved, are moved, somehow, by force and through the extrusion of the medium: for water extrudes violently the beam that has been submerged by force, when, in going down, it returns towards its proper region, and does not want to suffer that, that which is lighter than it, should remain under it; and, in the same way, the stone is extruded {1} and impelled downward because it is heavier than the medium.
It is consequently evident that such a motion can be called forced; although wood, in water, is commonly said to be carried upward naturally, and stone downward.
Nor is Aristotle's argument valid, when he says {1}, If it were forced it would be weakened at the end, and not be increased, as it is: for forced motion is weakened only when the mobile will have been outside the hand of the mover, and not while it is linked to the mover. {2}It is consequently evident, in what way the motion of natural mobiles may be suitably reduced to the motion of weights in a balance: namely in such a way that the natural mobile plays the role of one weight in the balance; and an amount of the medium as great in size as the size of the mobile represents the other weight in the balance. So that, if an amount of the medium as great in size as the size of the mobile will be heavier than the mobile, and the mobile will be lighter, then the mobile, being the lighter weight, will be carried upward: but if the mobile will be heavier than the same amount of the medium then, being the heavier weight, it will go down: and if, finally, the said amount of the medium will be equal in heaviness to the mobile, the mobile will be carried neither upward nor downward: just as in the balance weights that are equal to one other are neither pressed down nor raised. And because natural mobiles are very appropriately compared