417 Aristotle, in Book I, text #44 of <i>De Caelo</i> [273a9-13], says that if one of two contraries is delimited it is thus necessary that the other be delimited as well; and because the center, which is the limit of downward motion, is limited, it is necessary that upward [motion] itself be delimited . And the same thing is concluded from text #58 [274b9-15]: {1} read {2} the text.
Aristotle, in Book I, text #51 of <i>De Caelo</i> [273b32-274a2] {1} says, Speed is to speed as heaviness is to heaviness.
Aristotle, by text #89 of Book I, of <i>De Caelo</i> [277a33-277b2], shows that things that are moved naturally are not moved by force and through extrusion. {1}
Aristotle, in Book I, text #96 of <i>De Caelo</i> [278b14-15], writes: For as a rule we have called the outermost thing and what is uppermost the heaven. And in text #21 [270b6-8] he says: And the place which is above that is assigned to God. {1}
Things that are moved upward, go up more violently than naturally: for ascent has an external cause, but descent an internal one.
Aristotle, in Book I, text #5 of <i>De Caelo</i> [268b21-22], says: Now I call upward what is away from the center, but downward what is toward the center. {1}
It may be asked whether heavy things are truely moved toward the center; on this, Ptolemy, <i>Almagest</i>, Book I, chapter 7.
Whether the slowness and the swiftness of natural motion is due to the rareness or the de....of the me.... .
Concerning the ratio of the heavinesses of the same heavy thing in different media, on which depends the question of the ratio of its motions.
Given the heaviness of the medium and the speed of the mobile, is also given the heaviness of the mo.... .
One must consider the ratio of motions on different inclined planes, and whether perchance lighter things go down more swiftly at the beginning; just as in the balance, the smaller the weights, the more easily the motion takes place.
The medium retards natural motion in the following manner: as, when a bell goes down, it is so to speak a solid body consisting of air enclosed .... by the metal, and because of this it is lighter than if air were not present.