That by which is caused the swiftness and slowness of natural motion. {1}</s>
</p>
<p>
<s id="id.1.1.7.01.00"/>
<s id="id.1.1.7.01.01">Since it has been quite abundently explained in the preceding [chapters], how natural motions come from heaviness and lightness, now it must be considered whence the greater or lesser swiftness of this motion comes about. </s>
<s id="id.1.1.7.01.02">In order that we be able to accomplish this more easily, the following distinction must be made: namely that inequalities in the slowness and swiftness of motion occur in two ways: for either the same mobile is moved in different media; or the medium is the same, but the mobiles are different. </s>
<s id="id.1.1.7.01.03">We will demonstrate shortly {1} that in both cases of motion the slowness and swiftness depend on the same cause, namely, the greater or lesser heaviness of the media and of the mobiles; {2} but first we will show that the cause of such an effect which has been conveyed by Aristotle is insufficient. </s>
</p>
<p>
<s id="id.1.1.7.02.00"/>
<s id="id.1.1.7.02.01">Thus Aristotle in Book IV of the <i>Physics</i>, text #71, has written{1}, that the same mobile is moved more swiftly in a more subtle medium than in a thicker one, and, therefore, that the cause of the slowness of motion is the thickness of the medium, and that of the speed, its subtlety; and he has confirmed this by appeal to no other reason than experience, namely, because we see that a mobile is moved faster in air than in water. </s>
<s id="id.1.1.7.02.02">But it will be easy to demonstrate that this cause is not sufficient. </s>
<s id="id.1.1.7.02.03">For if the speed of motion comes from the subtlety of the medium, the same mobile will always be moved more swiftly </s>
