the cause was sought why natural motion is faster at the end than at the middle, and faster in the middle than at the beginning. </s>
<s id="id.4.0.0.102.02">Thus we must recall what was made clear above; namely that a mobile, when it is moved with a violent motion, is moved up to the point where the force impressed by the motor is greater than the resisting heaviness: from this it follows that when a heavy thing ceases to go up, the force impressed in it is equal to its heaviness; from this it clearly follows that, when a heavy thing begins to be moved downward, it is not moved with a motion that is purely and simply natural. </s>
<s id="id.4.0.0.102.03">For at the beginning of such a motion there is still in this mobile some of the impressed force which was impelling it upward: this force, because it is smaller than the heaviness of the mobile, does not impel it any farther upward; however it still resists the heavy thing that is heading downward, because it has not yet been annihilated.</s>
<s id="id.4.0.0.102.04">For it has been demonstrated that it is successively weakened: and thus it happens that the mobile at the beginning of its own natural motion is moved slowly: but afterwards, since the contrary force is weakened and diminished, the moving thing, encountering lesser resistance, is moved more swiftly. </s>
<s id="id.4.0.0.102.05">As if, for example, we imagine a mobile which is moved with a violent motion from a to b, whose heaviness is 4, it is manifest that the force which impels it will be greater than 4 at any point of the line ab described by the forced motion: but at b itself it will not be greater than 4 (for if it were greater, the same mobile would be impelled by it beyond b); also it will not be smaller (for it would have been equal to it before [the body was at] b; but it has been demonstrated that it has always been greater); hence the force at b will be equal to the heaviness of the mobile, namely it will be 4.</s>
<s id="id.4.0.0.102.06">Therefore, when the mobile recedes from b, the force which was as 4 begins to weaken, and, on account of this, the mobile begins to have a lesser resistance to its own heaviness; because this resistance is continuously weakened, it results that the natural motion is continuously intensified. </s>
</p>
<p>
<s id="id.4.0.0.103.00"/>
<s id="id.4.0.0.103.01"> DO. This solution more than pleases me: however it seems to have its place only in the case of a natural motion preceded by a violent one. </s>
<s id="id.4.0.0.103.02">But when someone having a stone in hand does not impel it upward, but only lets go of it, in this motion, which is not preceded by a violent [motion], what will be the cause of the intensification? {1}</s>
</p>
<p>
<s id="id.4.0.0.104.00.fig"/>
<s id="id.4.0.0.104.01">AL. This very doubt had also come to my mind when I was working out an explanation to this problem; but when I examined the matter more carefully, I discovered that it was of little importance. </s>
<s id="id.4.0.0.104.02">Thus the intensification occurs by the same cause in each of the two motions [downward], as much in that which is preceded by a state of rest, than in that [of a violent motion {1}] </s>
