335 for which less heavy things must be moved more swiftly is not yet apparent. This objection is surely of great importance; but nevertheless it is not so powerful that it can obscure the splendor of the truth. Now in order that we may do away with it, it must be noted that the contrary quality in a mobile is not weakened because it is assailed by the heaviness of the mobile; for the heaviness cannot accomplish this, since in a mobile filled with the contrary quality it is entirely nullified; but it is by itself that that quality is weakened and leaves the mobile: as is also the case when white-hot iron grows cold: the heat in it weakens not because contrary coldness fights against it; for there is no cold in the iron at that time; but by its own nature it recedes from the iron bit by bit. Secondly, it must be noted that the contrary quality recedes more easily and the more swiftly, the lighter the mobile in which it has been impressed: this can be confirmed by many examples. Thus, if from the same cannon at the same time two acorn-sized balls, one of lead, the other of wood, are shot, then, beyond any doubt, the same force will be impressed in both; yet it will be kept more penetratingly and for a longer time in the lead than in the wood: an indication of this is the fact that the lead will be moved in violent motion farther and for a longer time. And the same thing is again evident, if someone projects, with the same hand, at the same moment, together, 2 pieces, one of wood, the other of iron; the iron or lead will be moved on a longer distance: which presumably indicates that, the motive force hangs on more penetratingly in the iron and is kept for a longer time in it than in the wood. {1}The same thing is evident if two weights, one of wood, the other of lead, are suspended from two equal threads and, when they have received an impetus from an equal distance from the perpendicular, they are released; of the two, the lead will certainly be moved back and forth for a longer interval of time. {1}And finally, it is manifest in all cases that all contrary qualities are kept longer, the heavier and the denser and the more contrary to them is the material in which they have been impressed. For if wood and lead are heated, in such a way that both of them are equally hot at the beginning, nevertheless it will be kept longer in the lead, even though the greater coldness of the lead is more opposed to the heat than the modest coldness of the wood. And this is manifestly evident in air, which, when it has been warmed directly and strongly by fire, if the fire is removed or covered with ashes, immediatly becomes cold: if on the other hand water is warmed by fire, I would not say to the point of boiling, but only so that it is as warm
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