402 on the heaviness or the lightness of the medium and the mobile.{1}Thus I say that, in a void heavier things would go down more swiftly than lighter ones; since the excess [of heaviness] of the heavier things over [the heaviness] of the medium would be greater than the excess [of heaviness] of lighter things. {1}
And moreover what Aristotle has said concerning the ratio of the motions as compared with the heavinesses of the mobiles is not true: namely that, for example, the ratio of swiftness to swiftness [of two mobiles] in the same medium is the same as the ratio of the heaviness [of one] to the heaviness [of the other]; as if, for example, a were twice as heavy as b, the swiftness of a also would be twice the swiftness of b. For we shall demonstrate this in the same manner as above.For if the heaviness of a is 4, but the heaviness of b is 2; and a goes down in the medium water, and its swiftness is 2, but b does not go down; it is then clearly evident that the swiftness of a will not be twice the swiftness of b, since b is not moved. But here also an arithmetic ratio will be observed by the swiftnesses, that is in accordance with the excess of the heavinesses over the heaviness of the medium; thus if a is 4, but b 2, and the medium is 1 in heaviness, then the swiftness of a will be 3 as compared with the swiftness of b, which will be as 1. Therefore, to summarize I will say, that the slowness and the speed of all downward {1} motion comes, in the first place and per se, from the proper heaviness of the mobiles; motion comes to be weaker because this heaviness then comes to be less than the heaviness of media. But if the heaviness of the medium is equal to that of the mobiles, then, since the mobiles in such a medium have no heaviness, no motion takes place; but if, on the other hand, the heaviness of the medium is greater [than that of the mobiles], then the heaviness of the mobiles in comparison with the heaviness of the medium becomes lightness, and the mobile is carried upward; but if the heaviness of the medium is zero, then the mobiles will be moved according to their pure heaviness, and they will observe in their motion the ratio that their proper heavinesses have to each other. And from this another serious error is made evident; Aristotle's way of thinking on this matter is precisely the opposite it what it should have been. For he was saying that it is in a plenum that heavy things observed in their motions the ratio of the heavinesses; and in a void, not at all, but they are all moved in the same time. But, on the contrary, in a void they will observe the ratios of their heavinesses, since the excesses over [the heaviness of] the medium are the whole heavinesses of the mobiles; in the plenum, on the other hand, they will not observe this ratio, as has been demonstrated above. But, as has often been said above, it is necessary that you always understand and