The Essential Galileo (38 page)

S
AGR.
    What do you say, Simplicio? Does it seem that Salviati knows and can explain the Ptolemaic and Aristotelian reasons? Do you think that any Peripatetic is equally knowledgeable of the Copernican demonstrations?

S
IMP.
    If the discussions so far had not produced in me such a high opinion of Salviati's well-founded understanding and of Sagredo's sharp intelligence, I (with their permission) would be ready to leave without listening to anything else. For it seems to me impossible that one can contradict such palpable observations; moreover, I would like to keep my old opinion without having to hear anything else, because it seems to me that even if it were false, the fact that it is supported by such likely reasons would render it excusable. If these are fallacies, what true demonstrations were ever so beautiful?

S
AGR.
    Still, it will be good to hear Salviati's answers. If these should be true, they must be even more beautiful and infinitely more beautiful, and those others must be ugly, indeed very ugly; this would follow if there is truth in the metaphysical proposition that truth and beauty are the same thing, as falsehood and ugliness also are. However, Salviati, let us not lose any more time.

[§8.5 Day II:Vertical Fall, Conservation of Motion,
and the Role of Experiments]
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[164] S
ALV.
    So we can now go on to the fourth argument, which should be discussed at great length since it is based on an observation from which most of the remaining arguments then derive their strength. Aristotle says
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that a most certain argument for the earth's immobility is based on the fact that we see bodies which have been cast upwards return perpendicularly by the same line to the same place from which they were thrown, and that this happens even when the motion reaches a great height; this could not happen if the [165] earth were moving because, while the projectile moves up and down separated from the earth, the place of ejection would advance a long way toward the east due to the earth's turning, and in falling the projectile would strike the ground that much distance away from the said place. Here we may also include the argument from the cannon ball shot upwards, as well as another one used by Aristotle and Ptolemy, namely, that one sees bodies falling from great heights move in a straight line perpendicular to the earth's surface. Now, to begin to untie these knots, I ask Simplicio how Aristotle and Ptolemy would prove, if someone denied it, that bodies falling freely from on high move in a straight and perpendicular line, namely, in the direction of the center.

S
IMP.
    By means of the senses: they assure us that the tower is straight and perpendicular; they show us that the falling rock grazes it without inclining so much as a hairbreadth to one side or the other; and they show that the rock lands at the foot of the tower exactly under the place from which it was dropped.

S
ALV.
    But if by chance the terrestrial globe were rotating and consequently were also carrying the tower along with it, and if the falling rock were still seen to graze the edge of the tower, what would its motion have to be?

S
IMP.
    In that case one would rather have to speak of “its motions”; for there would be one that would take it from above downwards, and it would have to have another in order to follow the course of the tower.

S
ALV.
    Therefore, its motion would be a compound of two, namely, one with which it grazes the edge of the tower, and another one with which it follows the tower; the result of this compound would be that the rock would no longer describe a simple straight and perpendicular line, but rather an inclined, and perhaps not straight, one.
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S
IMP.
    I am not sure about its not being straight; but I understand well that it would have to be inclined and different from the straight perpendicular one it would describe on a motionless earth.

S
ALV.
    Therefore, from just seeing the falling rock graze the tower, you cannot affirm with certainty that it describes a straight and perpendicular line unless you first assume the earth to be standing still.

S
IMP.
    That is correct; for if the earth were moving, the rock's motion would be inclined and not perpendicular.

[166] S
ALV.
    Here, then, is the paralogism of Aristotle and Ptolemy made clear and evident, and discovered by yourself; the argument is assuming as known what it is trying to prove.

S
IMP.
    In what way? To me it seems to be a syllogism in proper form and not a fallacy of question begging.

S
ALV.
    Here is how. Tell me: does not the demonstration regard the conclusion as unknown?

S
IMP.
    Yes, unknown, for otherwise it would be superfluous to demonstrate it.

S
ALV.
    But, should not the middle term be known?

S
IMP.
    That is necessary, for otherwise it would be an attempt to prove the unknown by means of what is equally unknown.

S
ALV.
    Is not the conclusion to be proved, and which is unknown, the proposition that the earth stands still?

S
IMP.
    It is.

S
ALV.
    Is not the middle term, which must be already known, the straight and perpendicular fall of the rock?

S
IMP.
    That is the middle term.

S
ALV.
    But, did we not just conclude that we can have no knowledge that this fall is straight and perpendicular unless we first know that the earth is standing still? Therefore, in your syllogism the certainty of the middle term is inferred from the uncertain conclusion. So you see the type and the seriousness of the paralogism.

S
AGR.
    On behalf of Simplicio, I should like to defend Aristotle, if possible, or at least to understand better the strength of your inference. You say: seeing the rock graze the tower is not enough to become certain that its motion is perpendicular (which is the middle term of the syllogism) unless one assumes that the earth stands still (which is the conclusion to be proved); for, if the tower were moving together with the earth and the rock grazed it, the rock's motion would be inclined and not perpendicular. However, I will answer that, if the tower were moving, it would be impossible for the falling rock to graze it; hence, from seeing the falling rock graze it one infers that the earth is motionless.

S
IMP.
    That is correct. For, if the falling rock should graze the tower while the latter was carried along by the earth, the rock would have to have two natural motions (namely, straight toward the center and circular around the center); and this is impossible.

S
ALV.
    Therefore, Aristotle's defense consists in its being impossible, [167] or at least in his having regarded it as impossible, that the rock could move with a motion mixed of straight and circular; for, if he had not regarded it as impossible that the rock could move simultaneously toward the center and around the center, he would have understood that it could happen that the falling rock could graze the tower when it is moving as well as when it is standing still; consequently, he would have realized that from this grazing nothing could be inferred regarding the motion or the rest of the earth. However, this does not in any way excuse Aristotle, because he should have said so if he had had this thought in mind, it being such a key point in his argument; moreover, one cannot say either that this effect is impossible or that Aristotle regarded it as impossible. The first cannot be said, because I will soon show that it is not only possible but necessary. Nor can one say the second, for Aristotle himself grants
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that fire goes naturally upward in a straight line and turns by participation with the diurnal motion, which is transferred by the heavens to all of the element fire and to most of the air; if, then, he did not regard it as impossible to mix straight upward motion with the circular one communicated to fire and air by the inside of the lunar orb, much less should he regard it as impossible to mix the rock's straight downward with the circular one that would be natural for the whole terrestrial globe of which the rock is a part.

S
IMP.
    It does not look that way to me; for, if the element fire turns together with the air, it is very easy and indeed necessary that a particle of fire rising from the earth and going through the rotating air should receive the same motion, since it is such a rarefied and light body and most ready to move; but it is completely incredible that a very heavy rock or cannon ball falling through the air should let itself be carried along by it or anything else. Furthermore, there is the very appropriate experiment of the rock dropped from the top of a ship's mast; that is, when the ship is standing still it falls at the foot of the mast, but when the ship is going forward it falls away from the same place at a distance equal to that traversed by the ship during the rock's fall (which amounts to many cubits when the ship's course is fast).

S
ALV.
    There is a great disparity between the case of the ship and that of the earth, if the diurnal motion should belong to the terrestrial globe. For it is most evident that the ship's motion does not belong to it naturally, [168] just as it is an accidental property of all things in it; so it is not surprising that, when the rock is let go after being held at the top of the mast, it should fall without any obligation to follow the ship's motion. However, the diurnal rotation would be attributed to the terrestrial globe (and consequently to all its parts) as their own natural motion, and it would be regarded as indelibly impressed in them by nature; hence, a primary instinct of the rock at the top of the tower would be to go around the center of the whole of which it is a part every twenty-four hours, and it would eternally exercise this natural inclination regardless of the conditions in which it might be placed. To be persuaded of this, you have only to change an old impression and say to yourself: “Up to now, I have thought it is a property of the terrestrial globe to stay motionless at the center, and so I have never felt any difficulty or repugnance in understanding that every one of its particles is also naturally in the same state of rest; similarly, if the terrestrial globe had the natural instinct to rotate in twenty-four hours, then one would have to say that every one of its parts has the intrinsic and natural inclination to follow the same course and not to stand still.” Thus, without encountering any inconvenience, one may conclude that when the rock is separated from the ship, it must regain its natural state and return to exercise its pure and simple natural instinct, for the motion transmitted from the power of the oars to the ship and from the ship to all the things it contains is not natural but foreign to them.

It should be added that it is necessary that the lower part of the air below the higher mountains would be captured and carried around by the roughness of the earth's surface, or that it would naturally follow the diurnal motion insofar as it is mixed with many earthly vapors and emanations; this does not happen to the air around the ship, which is propelled by the oars. Therefore, to argue from the case of the ship to the case of the earth has no inferential force. For the rock falling from the top of the mast enters a medium that does not share the ship's motion; but the one released from the top of the tower finds itself in a medium that shares the same motion as the terrestrial globe, and so it can follow the general course of the earth without being hindered by the air but rather being favored by its motion.

S
IMP.
    I do not understand how the air can impart its own motion to a very large rock or a large iron or lead ball, which, [169] for example, might exceed two hundred pounds. Perhaps it transmits its motion to feathers, snow, and other very light objects; but I see that a weight of that kind is not displaced by a single inch even when exposed to the fiercest wind. Now, think whether the air can carry it along.

S
ALV.
    There is a great disparity between your experiment and our case. You have the wind come upon the rock lying at rest, whereas we expose to the already moving air a rock which is itself moving at the same speed; thus, the air does not have to impart to it some new motion, but rather must keep it in motion, or (to be more exact) not hinder the motion already acquired. You want to push the rock into a motion foreign to it and against its nature; we want to conserve it in its natural motion. If you want to present a more appropriate experiment, you could say that one should observe (with the mind's eye, if not with the real one) what would happen when an eagle carried by the wind releases a rock from its claws; because the rock is moving like the wind at the moment of separation from the claws, and thereafter it enters a medium which is moving at the same speed, I am strongly inclined to think that we would not see it fall perpendicularly, but that it would follow the course of the wind and add to this the motion due to its own gravity, and so it would move with an inclined motion.

S
IMP.
    One would have to be able to make such an experiment and then form a judgment depending on the result; however, so far the ship experiment seems to favor our opinion.

S
ALV.
    Well said, “so far”; for perhaps before long, appearances may change. In order not to keep you in suspense any longer, tell me, Simplicio, do you think the ship experiment fits our purpose so well that it is reasonable to believe that what is seen to happen on the ship should likewise happen on the terrestrial globe?

S
IMP.
    Up to now I think so; although you have advanced some small differences, they do not seem to me to be of such import as to make me change my mind.