281 undoubtably similarly not be absurd, that swiftness to swiftness can be arithmetically just as rareness to nothing. Thus, in a void also a mobile will be moved in the same way as in a plenum. For in a plenum a mobile is moved swiftly according to the excess of its own heaviness over the heaviness of the medium through which it is moved; and thus in a void it will be moved according to the excess of its heaviness over the heaviness of the void: since this is null, the excess of the heaviness of the mobile over the heaviness of the void will be the total heaviness of this same mobile; thus it will be moved swiftly according to its own total heaviness. But in no plenum will it be able to be moved as swiftly, since the excess of the heaviness of the mobile over the heaviness of the medium is less than the total heaviness of the mobile: thus the swiftness will also be less, than if it were moved according to its own total heaviness.
From this it can manifestly be concluded, how in a plenum, as is the case around us, in no way do things weigh according to their proper and natural heaviness; but always they will be lighter to the extent that they are in a heavier medium, and they will be lighter by just as much as the heaviness, in the void, of a size of such a medium equal to the size of that thing: so that a lead sphere in water will be lighter than in a void by just as much as the heaviness of a sphere of water, equal to the lead sphere, in a void; and likewise a lead sphere in air is lighter than in a void by just as much as the heaviness of an airy sphere, equal in size to the lead sphere, in the void; and likewise in fire, and in other media. And since the swiftness of motion follows from the heaviness that the mobile has in the medium in which it is moved, its motion will be swifter, the heavier the mobile is in accordance with the variety of the media. However, the following argument is invalid: The void is a medium infinitely lighter than any plenum; hence in it motion will happen infinitely more swiftly than in a plenum; therefore it will happen in an instant. For it is true that the void is infinitely lighter than any medium whatever; one must not, however, say that such a medium is of infinite heaviness; instead what must be understood is that between the lightness of, say, air and the void there can exist an infinity of media, lighter than air, but heavier than a void. And if the matter is understood this way, between swiftness in air and swiftness in the void there can also exist an infinity of swiftnesses, greater than that which is found in air, but smaller than the swiftness in the void: so also between the heaviness of a mobile in air and its heaviness in the void there can exist an infinity of intermediate heavinesses, greater to be sure than the heaviness in air, but smaller