312 by force. But such is the stone impelled by force: and in the stone its own innate and intrinsic heaviness is lost in the same manner as it is also lost when it is placed in media heavier than itself. For a stone which floats on mercury, for example, and does not go down in it, loses all its own heaviness; and it even loses its heaviness so much and dons lightness, that it also resists energetically a lot of heaviness coming to it from outside (as if someone exerts pressure on it): and wood also becomes so light in water, that it cannot be kept down unless by force. And yet neither the stone nor the wood lose their own natural heaviness, but, taken out of those heavier media, they resume their proper heaviness: in the same way a projectile, freed from the force projecting it, displays, by going down, its own real, intrinsic heaviness. {1}
Furthermore, those who defend points of view opposed to ours ask themselves in which part of the mobile that force is received: on the surface, or at the center, or in some other part. I answer briefly this, that they should first make clear to me in what part of iron heat is received; and I will then tell them where the motive force is received and I will place it where they place heat: and if the heat is received only on the surface, I will say that the force is received only on the surface; and if it is at the center, at the center; and if they should say that the heat is received where the cold was before, I will say that the extrinsic lightness has entered those parts in which formerly the native heaviness resided.
Finally, my adversaries are astonished at how the same hand has the ability to impress, sometimes lightness, sometimes heaviness, but also sometimes what seems to be neither heavy nor light. But then why are they not instead astonished at how they now want something, and a little latter they do not want that same thing; and how they believe something, and then on the same subject they hesitate and have doubts about it, and sometimes even disbelieve it? But if, as in these cases, it depends on one's will that he can now raise his arm, then lower it, then move it in different directions, and if the arm, governed by the will, has the ability now to exert weight, now to lift; why should we be astonished that what is weighed down by the arm receives heaviness, but what is lifted is clothed in lightness?
But, because this does not diverge from our subject, let me not pass over in silence a certain quite common error: namely, that of those who believe that air and water because they are fluids, can be moved more easily and more swiftly, especially air; led by this, they have believed that the thrower moves the air more than he does the projectile, and that the air carries projectiles along.But the matter is quite different: as even they themselves sometimes admit, those who, following their master Aristotle, sometimes say that the air, in order that it may carry along projectiles, is moved very swiftly because of its lightness, since it has practically no resistance; but sometimes they say that things which have neither heaviness nor lightness cannot be moved, since it is necessary that what is moved resists: and, by talking this way, they sometimes believe and sometimes deny the same things, according to what suits their intentions better. However the fact is that the lighter a mobile is, the more easily it is moved while it is linked to the mover, but, once released by it, it retains the received impetus for only a short time: as is evident if one throws a feather, applying as much force as if one had to throw a pound of lead; for surely he will move the feather more easily than the lead; but the impressed force will be conserved for a longer time in the lead than in the feather, and it <the lead> will be thrown much farther.Now if it were the air that carried the projectile along, who would ever believe that the air would carry the lead more easily than the feather? We see, then, that the ligher a thing is, the more easily it is moved, but the less it retains the impetus it has received: hence, since the air, as has been demonstrated above {1}, has no heaviness in its own proper place, it will indeed be moved easily, but it will however conserve only minimally the impetus it has received. Now we will demonstrate below why light things do not retain the impetus. {1}
Nor is there any impact in the example they hand down of a pebble projected in a lake; they say that by it, water is moved in a circle over a very great distance. {1}For, first, it is false that the water is moved: as is evident if pieces of wood or straw are floating on the water: these will not at all be moved from their place by the eddies of water, but will only be raised a little by the wavelets and will not follow the circumferences of the circles. Second: the comparison does not hold good in the case of air, whose surface is not moved by the thrower, as it is only the surface of the water which is moved by the pebble; and this topmost surface of the water is raised and lowered only because it offers resistance to being raised and carried into the place of air: but in the middle of the air the motive force cannot be impressed, for then the air offers no resistance, since it is not driven away from its place towards the place of another medium. And this would also happen in the middle of the water which would not conserve the impetus received, since its motion would have no inclination; for it would not have any natural one, because it would not be moved towards its own place,