for, if someone wants to impel someone else running in the same direction as himself, it is necessary for him to run faster than the other and to be rapidly flung in the direction in which he runs. </s>
</p>
<p>
<s id="id.1.2.6.03.00.fig"/>
<s id="id.1.2.6.03.01">But this does not happen in the case of a natural mobile; on the contrary, the air is moved in the opposite direction: as, if a sphere abc goes down, the surrounding air, rushing back from parts b, c towards a, the back of the mobile, will be moved upward with respect to the downward motion of the sphere. </s>
<s id="id.1.2.6.03.02">This is also admitted by them, when they say that the medium resists motion, since it must be split: hence if the medium must be split, surely it will not be moved in the same direction as the mobile. </s>
<s id="id.1.2.6.03.03">Therefore, either it will be at rest, or it will be moved in the opposite direction to the mobile, or, if we want it to be moved in the same direction, it will at least be moved more slowly: but since this is so, in what way will it help motion? </s>
<s id="id.1.2.6.03.04">In the fourth place, they do not seek a cause per se of the acceleration of motion, but they only bring up an accidental cause; for it is by accident that a mobile is moved in a plenum, and that its speed is either hindered or helped by the medium: but we are asking why a natural mobile, when it is moved naturally by its proper heaviness, without any consideration of the medium, is moved faster at the end than in the middle, and faster in the middle that at the beginning; and how from a consideration of motion, it is necessary that it be weaker at the beginning. </s>
<s id="id.1.2.6.03.05">So much for those who hang on to this opinion. </s>
</p>
<p>
<s id="id.1.2.6.04.00"/>
<s id="id.1.2.6.04.01">Others have said that the mobile is moved faster at the end because the parts of the medium which must be split by it are less numerous; and for this reason, since it has lesser resistance, they have believed that it is carried faster. </s>
<s id="id.1.2.6.04.02">But this way of thinking is not only false, but ridiculous: for, if it were true, it would follow that a stone going down from a very high tower would be moved more slowly at mid-tower, than if the same stone were falling to the ground from a very low place, and for this reason the mobile [falling from a greater height] would also make a lesser impact. </s>
</p>
<p>
<s id="id.1.2.6.05.00.fig"/>
<s id="id.1.2.6.05.01">In order that this may be more clearly understood, let there be a line abc, and let ac be much longer than cb: I say, then, that if the stone were going down from a, it would be moved more slowly when it was around c, than if the same stone were released from c, near b; because, of course, there would be fewer parts of the air left to be split by the mobile when it was around b, having been released from c, than when it was around c, coming from a. </s>
<s id="id.1.2.6.05.02">It can also be added that the stone in going down from a would precipitate itself to the ground with the same impetus, as if it were going down from c: and the reason is that, in going down from a, when it was a bit under c, it was not moved faster than when it was a bit under c, in going down from c, since in the former case no fewer parts of the air remained to be split than in the latter case; and they say that it is on such </s>
