of the ratio of the same mobile in different media: and first let us examine whether or not Aristotle's way of thinking concerning this is more truthful than the one set forth above. </s>
</p>
<p>
<s id="id.1.1.8.08.00.fig"/>
<s id="id.1.1.8.08.01">Now Aristotle believed that the motions of the same mobile in different media observe the same ratio with one another in swiftness, as the ratio that the subtleties of the media have with one another; and this he has openly written in Book IV of the Physics, text #71 [215b 1-11], when he has said: A medium hinders more, because it is thicker; thus a will be moved through a space b in time c, but through a space d, when it is more subtle, in time e, according to the ratio of the hindrance, if the length is equal; so that, if b is water, but d air, then to the extent that air is more subtle than water, to that extent a will be moved more swiftly through d than through b. </s>
<s id="id.1.1.8.08.02">Therefore speed has to speed the ratio that air has to water: and so, if air is twice as subtle as water, a will travel along line b in twice the time as line d; and time c will be twice time e. </s>
<s id="id.1.1.8.08.03">These are Aristotle's words, which surely contain a false way of thinking: now in order that this may appear clearer than daylight, I will give form to the following demonstration. </s>
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<p>
<s id="id.1.1.8.09.00.fig"/>
<s id="id.1.1.8.09.01">If speed has the same ratio to speed as subtlety of medium to subtlety, let there be a mobile o, and medium a, whose subtlety is of 4, and let that be, for example, water; but let the subtlety of medium b be 16, namely greater than the subtlety of a, and let b, for example, be air; and let mobile o be such that it does not go down in water; but let the swiftness of this same mobile in medium b be 8. {1}</s>
<s id="id.1.1.8.09.02"> Therefore, since the swiftness of mobile o in medium b is 8, but it is zero in medium a, surely some medium can be found in which the swiftness of mobile o is 1. </s>
<s id="id.1.1.8.09.03">Let such a medium be c. </s>
<s id="id.1.1.8.09.04">Thus since o is moved more swiftly in medium b than in medium c, it is necessary that the subtlety of this medium c be less than the subtlety of b, and, according to our adversaries, that it be </s>
