411 Alexander thought he had well refuted Hipparchus' opinion concerning the acceleration at the end of natural motion, after he adduced in opposition a natural motion that was not preceded by a violent motion: but surely even Hipparchus did not notice that a violent motion precedes every case of natural motion, as we have made clear. {1}
One must crticize the false opinion of those who say that when a pebble has been projected into water, the water is later, by itself, moved in circle.
Burley, in his <i>Commentary on the Physics of Aristotle</i>, Book VIII, text #76, and Contarini, <i>De Elementis</i>, Book I, assign the cause of acceleration of natural motion at the end to the parts of the air, both those that precede and those that follow.
See Aristotle, <i>De Caelo</i> Book I, text #88 [277a27-33], where he says that the speed of natural motion is always increased, and, if [a body] were moved without end, speed would also be increased without end.
And see text #89 [277b4-8], where he says that a greater portion of earth is carried more swiftly than a smaller one, and that natural motion is not accelerated because of the extrusion of the air: for in this way a greater portion of earth would be moved more slowly than a small one, because it would be extruded with more difficulty; and it would not be accelerated in its motion, because it would be a violent motion, which is weakened.
Benedict Pereira, toward the end of chapter 3 of Book 14 {1}, writes thus: I would say without doubt that if the space of the air through which the rock is carried downward were infinite, the rock's motion would always be swifter and more vehement, and yet no addition would be made to its weight.Now notice the error of Pereira in the following words.For he says {1}: Aristotle does not conclude correctly, Because speed grows in natural motion, heaviness must also grow in the mobile: for if a stone is moved through a certain distance, whose first part is denser and thicker, while the last part is more tenuous and more rare, without doubt the motion will be faster at the end, and that will not happen on account of an increment in heaviness.
Light things are not moved faster by a greater force. Just as straw or tow is not made hotter by the greatest and most burning fire, because it does not await so much heat, but is previously burned up by a lesser one, so light things do not resist [motion] up until a great force is impressed in them, but are moved before that.