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[Salviati represents the Copernican point of view; the name is that of a friend of Galileo
Simplicio speaks for the Aristotelians; the name is taken from a 6th century follower of Aristotle
Sagredo is supposed to be a neutral voice in the debate—Ed.]
THE SECOND DAY
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SIMPLICIO:I must confess that I have been ruminating all night of what passed yesterday, and, to say the truth, I have met with many acute, new, and plausible notions; yet I remain convinced by the authority of so many great writers and in particular ... . I see you shaking your head, Sagredus, and grinning to yourself, as if I had uttered some great absurdity.
SAGREDO:I not only grin but actually am ready to burst with holding myself from laughing outright, for you have put me in mind of a very pretty episode that I witnessed not many years since, together with some others of my worthy friends which I could name to you.
SALVIATI:It would be well that you told us what it was, so Simplicius may not still think that he is the point of your laughter.
SAGREDO:Very well. One day at his home in Venice, I found a famous physician to whom some flocked for their studies, while others sometimes came thither out of curiosity to see certain bodies dissected by the hand of a no less learned than careful and experienced anatomist. It chanced upon that day, when I was there, that he was in search of the origin and stem of the nerves, about which there is a famous controversy between the Galenists and Peripatetics. The anatomist shewed how the great trunk of nerves, departing from the brain, their root, passed by the nape of the neck, extended themselves afterwards along the backbone, and branched out through all the body, while only a very small filament, as fine as a thread, went to the heart. Then he turned to a gentleman whom he knew to be a Peripatetic philosopher, and for whose sake he had uncovered and proved everything, and asked if he was satisfied and persuaded that the origin of the nerves was in the brain and not in the heart. The philosopher, after he had stood musing a while, answered: “You have made me see this business so plainly and sensibly that did not the text of Aristotle assert the contrary, which positively affirms the nerves to proceed from the heart, I should be constrained to confess your opinion to be true.”
SIMPLICIO:I would have you know, my Masters, that this controversy about the origin of the nerves is not yet so proved and decided, as some may perhaps persuade themselves.
SAGREDO:Nor doubtless shall it ever be if it finds such contradictors· but what you say does not at all lessen the extravagance of the answer of, that Peripatetic, who against such sensible experience did not produce other experiments or reasons of Aristotle but his bare authority and pure ipse dixit ... .
SIMPLICIO:But in case we should give up Aristotle, who is to be our guide in philosophy? Name you some author.
SALVIATI:We need a guide in unknown and uncouth parts, but in clear thoroughfares, and in open plains, only the blind stand in need of a leader; and, for such, it is better that they stay at home. But he who has eyes in his head and in his mind has to use these for his guide. Yet mistake me not, thinking that I speak this because I am against hearing Aristotle; for, on the contrary, I commend the reading and diligent study of him and only blame the servilely giving one’s self up a slave to him, so as blindly to subscribe to whatever he delivers, and receive it for an inviolable decree without search of any further reason. This is an abuse that carries with it the other extreme disorder that people will no longer take pains to understand the validity of his demonstrations. And what is more shameful in public disputes than, while someone is treating of demonstrable conclusions, to have someone else come up with a passage of Aristotle, quite often irrelevant, and with that stop the mouth of his opponent? But, if you will continue to study in this manner, I would have you lay aside the name of philosophers and call yourselves either historians or doctors of memory, for it is not fit that those who never philosophize should usurp the honourable title of philosophers. But it is best for us to return to shore and not launch out further into a boundless gulf, out of which we should not be able to get before night. Therefore, Simplicius, come with arguments and demonstrations of your own, or of Aristotle, but bring us no more texts and naked authorities, for our disputes are about the sensible world and not a paper one.
Now in our discourses of yesterday we retrieved the Earth from darkness and exposed it to the open sky, showing that the attempt to number it among those which we call celestial bodies was not a position so compromised and vanquished that it had no life left in it. It follows next that we proceed to examine what probability there is for holding its entire globe fixed and wholly immovable; what likelihood there is for making it movable with some motion; and of what kind that may be. And since I am doubtful in this question, while Simplicius is resolute, as Aristotle is, for its immobility, he shall one by one produce the arguments in favour of their opinion, and I will allege the answers and reasons on the contrary part; next, Sagredus shall tell us his thoughts and to which side he finds himself inclined.
SAGREDO:Content, provided always that I may reserve myself the liberty of alleging what straight natural reason shall sometimes dictate to me ... .
SALVIATI:Let our contemplation begin therefore with this view: that, whatever motion may be ascribed to the Earth as a whole, it is necessary that it be to us, as partakers of it, altogether imperceptible, so long as we have regard only to terrestrial things. On the other hand, it is equally necessary that the same motion should appear common to all other bodies and visible objects separated from the Earth and that therefore do not participate in it. So that the true method of finding it, and, once found, to know what it is, is to consider and observe if in bodies separated from the Earth one may discover any appearance of motion which equally suits all the rest of them. For a motion that is only seen, for example, in the Moon, and has nothing to do with Venus or Jupiter or any other stars, cannot in any way belong to the Earth or to any other save the Moon alone. Now there is a most general and grand motion above all others, and it is that by which the Sun, the Moon, the other planets, and the fixed stars, and, in a word the whole Universe, except only the Earth, appear to move from the east towards the west in the space of twenty four hours;and this, at first sight, might as well belong to the Earth alone, as, on the other hand, to all the rest of the world, except the Earth; for the same phenomena will appear in the one case as in the other. Hence it is that Aristotle and Ptolemy, having hit upon this consideration when they go about to prove the Earth to be immovable, argue only against this diurnal motion; except that Aristotle hints something in obscure terms against another motion ascribed to it by an Ancient, of whom we shall speak in its place.
SAGREDO:I very well perceive the necessity of your conclusion; but I meet with a doubt from which I do not know how to free myself. It is that, since Copernicus assigns another motion to the Earth beside the diurnal, which, according to the rule even now laid down, ought to be imperceptible to us but visible in all the rest of the world, I might necessarily infer either that he has manifestly erred in assigning the Earth a motion to which there does not appear a general correspondence in heaven or else that, if there be such a congruity therein, Ptolemy has failed in not confuting it, as he has done in the other.
SALVIATI:You have good cause for your doubts, and, when we come to treat of the other motion, you shall see how far Copernicus excelled Ptolemy in clearness and elevation of intellect in that he saw what the other did not; I mean the admirable correspondence whereby this motion reflected itself in all the other celestial bodies. But for the present we will suspend this particular and return to our first consideration. Beginning, then, with more general things, I will propose those reasons which seem to favour the mobility of the Earth and then await the answers which Simplicius shall make. First, if we consider only the immense magnitude of the starry sphere compared to the smallness of the terrestrial globe, and weigh the velocity of the motions which must in a day and night make an entire revolution, I cannot persuade myself that there is any man who believes it more reasonable and credible that it is the celestial sphere that turns round, while the terrestrial globe stands still.
SAGREDO:If in the totality of effects which may in Nature depend upon such like motions, there should follow in one hypothesis exactly all the same consequences as in the other. I would esteem at first inspection, that he who should hold it more rational to make the whole Universe move, in order to keep the Earth from moving, is less reasonable than he who being at the top of the dome of your Cathedral in Florence, in order to behold the city and the fields about it, should desire that the whole country might turn round, so that he might not be put to the trouble to turn his head. And surely the conveniences that could be drawn from this position would have to be many and great in order to equate in my mind, and to overcome, this absurdity in such manner as to make it more credible than the former. But perhaps Aristotle, Ptolemy, and Simplicius must find certain advantage therein, which they would do well to communicate to us also, if any such there be; or else they had better declare that there neither is nor can be any ... .
SALVIATI:... A third reason which makes the Ptolemaic hypothesis less probable is that it most unreasonably confounds the order that we assuredly see among those celestial bodies of which the circumgyration is not questionable but most certain. And that order is that, the greater a sphere, the longer is its revolution. Thus Saturn, describing a greater circle than all the other planets, completes it in thirty years; Jupiter finishes its, that is less, in twelve years; Mars in two; the Moon runs through its, so much less than the rest, in only a month. Nor do we see less sensibly that that one of the Medicean Stars which is nearest to Jupiter makes its revolution in a very short time, that is, in forty-two hours, or thereabouts, the next to that in three days and a half, the third in seven days, and the most remote in sixteen. And this rate holds well enough, nor will it alter at all when we assign the rotation of twenty-four hours to the terrestrial globe. But, if you would have the Earth immovable, it is necessary that, when you have passed from the short period of the Moon to the others successively bigger, until you come to that of Saturn, which is of thirty years, it is necessary, I say, that you pass to another sphere incomparably greater than that and make this accomplish an entire revolution in twenty-four hours. And this yet is the least disorder that can follow. For if anyone should pass from the sphere of Saturn to the starry orb, and make it so much bigger than that of Saturn, as proportion would require in respect of its own very slow motion of many thousands of years, then it must certainly be a jump much more absurd to skip from this to another still bigger one, the Primum Mobile, and to make it revolve in twenty-four hours.
But if the motion of the Earth is granted, the order of the periods will be exactly observed, and from the very slow sphere of Saturn we come to the fixed stars which are wholly immovable and so avoid a fourth difficulty we would have if the starry sphere be supposed movable. That is the immense disparity between the motions of those stars themselves, of which some would come to move most swiftly in most vast circles, others most slowly in circles very small, according as the former or the latter should be found nearer or more remote from the Poles. This is accompanied with still more inconveniences because we see that those of whose motion there is no question have been made to move all on a great circle; also, because it seems to be an act done with no good consideration to constitute bodies that are designed to move circularly at immense distances from the centre and afterwards to make them move in very small circles.
And not only the magnitudes of the circles and consequently the velocity of the motions of these stars shall be most different from the circles and motions of those others, but (which shall be the fifth inconvenience) the self-same star shall successively vary its circles and velocities. For those which two thousand years since were in the Equator, and consequently described very vast circles, being in our days many degrees distant, must of necessity become slower and be reduced to move in lesser circles. And it is not altogether impossible but that a time may come in which some of them which in aforetime had continually moved shall be reduced by uniting with the pole to a state of rest and then, after some time of cessation, shall return to their motion again; whereas the other stars whose motion none doubt all describe the great circle of their sphere and in that maintain themselves without any variation.
The absurdity is further increased (which is the sixth inconvenience) in that no thought can comprehend what ought to be the solidity of that immense sphere, whose depth so steadfastly holds fixed such a multitude of stars, which are with so much concord carried about without ever changing site among themselves with so great disparity of motions. Or else, supposing the heavens to be fluid, as we are with more reason to believe, so that every star wanders to and fro in it by ways of its own, what rules shall regulate their motions, and to what purpose, so that, seen from the Earth, they appear as though they were made by one single sphere? It is my opinion that they might so much more easily and more conveniently do that, by being constituted immovable, than by being made errant, by as much as it is easier to number the blocks in the pavement of a piazza than a rout of boys which run up and down upon them.
Lastly, which is the seventh instance, if we attribute the diurnal motion to the highest heaven, it must be constituted of such a force and efficacy as to carry along with it the innumerable multitude of fixed stars, bodies of cast magnitude far bigger than the Earth, and, moreover, all the spheres of the planets, notwithstanding that both the first and the last by their own nature move the contrary way. And, beside all this, it must be granted that also the element of fire and the greater part of the air are likewise forcibly hurried along with the rest and that only the little globe of the Earth contumaciously and pertinaciously stands unmoved against such an impulse. This in my thinking is very difficult; nor can I see how the Earth, a suspended body equilibrated upon its centre, exposed indifferently to either motion or rest, and environed with a liquid ambient, should not yield also like the rest and be carried about. But we find none of these obstacles in making the Earth move, a small body and insensible, compared to the Universe, and therefore unable to offer it any violence ... .
SIMPLICIO:For this purpose astronomers and philosophers have found another sphere, above all the rest, without stars, to which naturally belongs the diurnal motion; and this they call the Primum Mobile; this carries along with it all the inferior spheres, contributing and imparting its motion to them.
SAGREDO:But if, without introducing other spheres unknown and hugely vast, without other motions or communicated raptures, with each sphere having its sole and simple motion, without intermixing contrary motions, but making all turn one way, as they should, depending as they do all upon one principle, if we can find all things proceeding orderly and correspond with most perfect harmony, why do we reject this alternative and give our assent to those prodigious and laborious conditions?
SIMPLICIO:The difficulty lies in finding out this so natural and expeditious a way.
SAGREDO:In my judgment it is found. Make the Earth the Primum Mobile, that is, make it turn round its own axis in twenty-four hours and in the same direction as all the other spheres; and, without need of imparting this same motion to any other planet or star, all shall have their risings, settings, and all other appearances.
SIMPLICIO:The business is, to be able to make the Earth move without a thousand inconveniences.
SALVIATI:All the inconveniences shall be removed as fast as you propound them: The things said hitherto are only the primary and more general inducements which give us to believe that the diurnal revolution may not altogether without probability be applied to the Earth rather than to all the rest of the Universe. And these inducements I put forward, not as inviolable axioms, but as hints, which carry with them some likelihood. But I know very well that one sole experiment or conclusive demonstration produced to the contrary suffices to overthrow these and a thousand other probable arguments; hence it is fit not to stay here but to proceed forward and hear what Simplicius answers and what greater probabilities or stronger arguments he alleges on the contrary.
SIMPLICIO:I will first say something in general upon all these considerations together, and then I will descend to some particulars. It seems that you base all you say upon the greater simplicity and facility of producing the same effects. To this I reply that I am also of the same opinion if I think in terms of my own not only finite but feeble power; but, with respect to the strength of the Mover, which is infinite, it is no less easy to move the Universe than the Earth, yea, than a straw. And if his power be infinite, why should he not rather exercise a great part of it than a smaller? Therefore, I hold that your discourse in general is not convincing.
SALVIATI:If I had at any time said that the Universe did not move for want of power in the Mover, I would have erred, and your reproof would have been seasonable; and I grant you that to an infinite power it is as easy to move a hundred thousand as one. But what I did say does not concern the Mover; it refers only to the bodies, and, in them, not only to their resistance, which doubtless is less in the Earth than in the Universe, but to the many other particulars just considered. As to what you say in the next place, that of an infinite power it is better to exercise a great part than a small, I answer that one part of the infinite is not greater than another, if both are finite. Nor can it be said that, of the infinite number, a hundred thousand is a greater part than two, though the first be fifty thousand times greater than the second. If a finite power is necessary to move the Universe, though it is very great in comparison to that which suffices to move the Earth alone, yet it does not involve a greater part of the infinite power, nor is that part less infinite which remains unemployed. So that to apply to a particular effect a little more or a little less power means nothing; besides that, the operation of such virtue has not for its bound or end the diurnal motion only. But there are several other motions in the World that we know of, and there may be many others that are unknown to us. Therefore, with respect to the bodies, and granting it as out of question that it is a shorter and easier way to move the Earth than the Universe, and, moreover, having an eye to the many other abbreviations and aptnesses that are to be obtained only this way, a most true maxim of Aristotle teaches us that frustra fit per plura, quod potest fieri per pauciora, “that is done in vain by many means which may be done with fewer.” This renders it more probable that the diurnal motion belongs to the Earth alone than to the whole Universe, with the sole exception of the Earth.
SIMPLICIO:In quoting that axiom, you have omitted a small clause, which is as important as all the rest; that is, the words aeque bene, “equally well.” We ought therefore to examine whether this hypothesis satisfies equally well all particulars as the other ... . That disposition is not new but very old; and that you may see it is so, Aristotle refutes it; and his refutations are these: “First, if the Earth moves either in itself about its own centre or in a circle around the centre, it is necessary that that motion be violent, for it is not its natural motion; for, if it were, each of its parts would partake thereof; hut each of them moves in a straight line towards the centre. It being therefore violent and preternatural, it could never he perpetual. But the order of the world is perpetual. Therefore, etc. Secondly, all the other movables that move circularly seem to lag behind and to move with more than one motion, the Primum Mobile excepted; hence it would he necessary that the Earth also move with two motions. And, if that should he so, it would inevitably follow that mutations should take place in the fixed stars, which, however, are not perceived; nay, without any variation, the same stars always rise from the same places and set in the same places. Thirdly, the motion of the parts is the same with that of the whole and naturally tends towards the centre of the Universe; and for the same cause rests, being arrived thither.” He thereupon moves the question whether the motion of the parts has a tendency towards the centre of the Universe or to the centre of the Earth, and he concludes that it goes by proper instinct to the centre of the Universe and per accidens to that of the Earth; of this point, we largely discoursed yesterday. He lastly confirms the same with a fourth argument taken from the experiment of heavy bodies which, falling from on high, descend perpendicularly to the Earth’s surface; and in the same manner projectiles shot perpendicularly upwards return perpendicularly down again.
All these arguments prove their motion to he towards the centre of the Earth, which without moving at all waits for them and receives them. He intimates in the last place that the astronomers allege other reasons in confirmation of the Earth’s being immovable in the centre of the Universe, and instances only in one of them, to wit, that all the phenomena or appearances that are seen in the motions of the stars perfectly agree with the position of the Earth in the centre; this would not he so were the Earth located otherwise. I can give you now, if you please, the rest produced by Ptolemy and the other astronomers, or I can do so after you have spoken what you have to say in answer to Aristotle.
SALVIATI:The arguments brought up are of two kinds: some have respect to the terrestrial accidents without any relation to the stars, and others are taken from the phenomena and observations of things celestial. The arguments of Aristotle are for the most part taken from things near at hand, and he leaves the rest to astronomers; and therefore it is best to examine these taken from experiments touching the Earth and then proceed to those of the other kind. And as Ptolemy, Tycho, and the other philosophers produced certain arguments besides those of Aristotle which they adopted and confirmed, we will combine them, so we will not answer twice to the same objections. Therefore, Simplicius, choose whether you will recite them yourself or have me ease you of this task, for I am ready to serve you.
SIMPLICIO:It is better that you quote them, because, as having taken more pains in the study of them, you can produce them quicker and in greater number.
SALVIATI:For the strongest reason, all allege that heavy bodies falling downwards move by a straight line perpendicular to the surface of the Earth, an argument which is held to prove undeniably that the Earth is immovable. For otherwise a tower from the top of which a stone is let fall, being carried along by the rotation of the Earth in the time that the stone spends in falling, would be transported many hundred yards eastwards, and so far distant from the tower’s foot would the stone come to the ground. This effect they back with another experiment: letting a bullet of lead fall from the round top of a ship that lies at anchor and observing the mark it makes where it lights. This they find to be near the foot of the mast, but, if the same bullet be let fall from the same place when the ship is under sail, it will light as far from the former place as the ship has sailed in the time of the lead’s descent, because the natural motion of the ball being at liberty is by a straight line towards the centre of the Earth. They fortify this argument with the experiment of a projectile shot on high at a very great distance. For example, a ball shot out of a cannon erected perpendicular to the horizon spends so much time in ascending and falling that, in our parallel, both we and the cannon should be carried by the Earth many miles towards the East, so that that the ball in its return could never come near the piece but would fall as far west as the Earth had run east ... .
SALVIATI:Before I proceed any further, I must tell Sagredus that in these disputations I personate the Copernican and imitate him as if I were his mask. But what has been effected in my private thoughts by these arguments which I seem to allege in his favour I would not have you judge by what I say whilst I am in the heat of acting my part in the fable; after I have laid by my disguise, you may chance to find me different from what you see me upon the stage ... .
Therefore we may proceed to the fourth, upon which it is requisite that we stay some time, for the reason that it is founded upon that experiment from which the greater part of the remaining arguments derive all their strength. Aristotle says that it is a most convincing argument of the Earth’s immobility to see that projectiles thrown or shot upright return perpendicularly by the same line unto the same place from whence they were shot or thrown. And this holds true, although the motion be of a very great height. So that hither may be referred the argument taken from a shot fired directly upwards from a cannon, as also that other used by Aristotle and Ptolemy, of the heavy bodies that, falling from on high, are observed to descend by a direct and perpendicular line to the surface of the Earth. Now, that I may begin to untie these knots, I demand this of Simplicius: in case one should deny to Ptolemy and Aristotle that weights in falling freely from on high descend by a right and perpendicular line, that is, directly to the centre, what means would he use to prove it?
SIMPLICIO:The means of the senses, which assure us that that tower or other altitude is upright and perpendicular, and shew us that that stone slides along the wall, without inclining a hair’s breadth to one side or another, and lights on the ground just under the place from where it was let fall.
SALVIATI:But if it should happen that the terrestrial globe did move round, and consequently carry the tower also along with it, and that the stone did then also graze and slide along the side of the tower, what must its motion be then?
SIMPLICIO:In this case we may rather say its motions, for it would have one wherewith to descend from the top to the bottom and should then have another to follow the course of the said tower.
SALVIATI:So that its motion should be compounded of two; from this it would follow that the stone would no longer describe that simple straight and perpendicular line but one transverse and perhaps not straight.
SIMPLICIO:I can say nothing of its nonrectitude, but this I know very well: that it would of necessity be transverse.
SALVIATI:You see then that, merely observing the falling stone to glide along the tower, you cannot certainly affirm that it describes a line which is straight and perpendicular unless you first suppose that the Earth stands still.
SIMPLICIO:True; for, if the Earth should move, the stone’s motion would be transverse and not perpendicular.
...
SAGREDO:I would defend Aristotle in favour of Simplicius. Should the tower move, it would be impossible that the stone should fall gliding along the side of it; and therefore from its falling in that manner the stability of the Earth is inferred.
SIMPLICIO:It is so; for if you would have the stone in descending to graze along the tower, while being carried around by the Earth, you must allow the stone two natural motions, to wit, the straight motion towards the centre and the circular motion about the centre, which is impossible.
SALVIATI:Aristotle’s defense then consists in the impossibility, or at least in his esteeming it an impossibility, that the stone should move with a motion mixed of right and circular. For, if he did not hold it impossible that the stone could move at once to the centre and about the centre, he would have understood that it might come to pass that the falling stone might in its descent graze the tower as well when it moved as when it stood still. Consequently, he ought to have perceived that from this grazing nothing could be inferred touching the mobility or immobility of the Earth. But this does not any way excuse Aristotle; because he ought to have expressed it, if he had had such a notion, it being so material a part of his argument. Also because it cannot be said that such an effect is impossible or that Aristotle did esteem it so. The first cannot be affirmed, for by and by I shall shew that it is not only possible but necessary; nor can the second be averred, for Aristotle himself grants that fire moves naturally upwards in a right line, and moves about with the diurnal motion, imparted by the heavens to the whole element of fire and the greater part of the upper air. If therefore he held it possible to mix the straight motion upwards with the circular communicated to the fire and air from the concave of the sphere of the Moon, much less ought he to account impossible the mixture of the straight motion of the stone downwards with the circular which we presuppose natural to the whole terrestrial globe, of which the stone is a part.
SIMPLICIO:I see no such thing; for, if the element of fire revolves round together with the air, it is a very easy, even a necessary, thing that a spark of fire which mounts upwards from the Earth, in passing through the moving air, should receive the same motion, being a body so thin, light, and easy to be moved. But that a very heavy stone, or a cannon ball, that descends from on high, and that is at liberty to move whither it will, should suffer itself to be transported either by the air of any other thing is altogether incredible. Besides that, we have the experiment which is so proper to our purpose, of the stone let fall from the round top of the mast of a ship, which, when the ship lies still, falls at the foot of the mast, but, when the ship moves, falls as far distant from that place, as how far the ship in the time of the stone falling had run forward.
SALVIATI:There is a great disparity between the case of the ship and that of the Earth, if the terrestrial globe be supposed to have a diurnal motion. For it is manifest that, as the motion of the ship is not natural to it, the motion of all those things that are in it is accidental, whence it is no wonder that the stone which was retained in the round top, being left at liberty, descends downwardswithout any obligation to follow the motion of the ship. But the diurnal conversion is ascribed to the terrestrial globe for its proper and natural motion, and, consequently, it is so to all parts of the said globe; and, being impressed by Nature, is indelible in them. Therefore that stone that is on the top of the tower has an intrinsic inclination to revolve about the centre of its whole intwenty-four hours, and it exercises this same natural instinct eternally, be it placed in any state whatsoever. To be assured of the truth of this, you have to do no more than alter an antiquated impression made in your mind, and to say: as I, hitherto, held it to be the property of the terrestrial globe to rest immovable about its centre, and never doubted or questioned but that all particles doalso naturally remain in the same state of rest; it is reason, in case the terrestrial globe did move round by natural instinct in twenty-four hours, that the intrinsic and natural inclination of all its parts should also he to follow the same revolution. And thus, without running into any inconvenience, one may conclude that, since the motion conferred by the force of oars on the ship, and by it on all the things that are contained within her, is not natural but foreign, it is very reasonable that that stone, being separated from the ship, should bring itself back to its natural disposure and return to exercise its pure simple instinct give it by Nature. To this I add that it is necessary that at least that part of the air which is beneath the greater heights of mountains should be transported and carried round by the roughness of the Earth’s surface; or that, as being mixed with many vapours and terrene exhalations, it should naturally follow the diurnal motion, which does not occur in the air about the ship rowed by oars. Therefore your arguing from the ship to the tower has not the force of a conclusion, because the stone which falls from the round top of the mast enters into a medium which is unconcerned in the motion of the ship; but that which departs from the top of the tower finds a medium that has a motion in common with the whole terrestrial globe, so that, rather than being hindered, it is assisted by the motion of the air and may follow the universal course of the Earth.
SIMPLICIO:I cannot conceive that the air can imprint in a very great stone, or in a heavy globe of wood or ball of lead, as suppose of two hundredweight, the motion wherewith itself is moved, and which it perhaps communicates to feathers, snow, and other very light things. On the contrary, I see that a weight of that nature, being exposed to the most impetuous wind, is not thereby removed an inch from its place; now consider with yourself whether the air will carry it along therewith.
SALVIATI:There is a great difference between your experiment and our case. You introduce the wind blowing against that stone, supposedly in a state of rest, and we expose the stone to the air which already moves, with the same velocity as the stone, so that the air is not to confer a new motion upon it but only to maintain or, to say better, not to hinder the motion already acquired. You would drive the stone with a strange and preternatural motion, and we desire to conserve it in its natural one. If you would produce a more pertinent experiment, you should say that it is observed, if not with the eye of the forehead, yet with that of the mind, what would happen if an eagle that is carried by the course of the wind should let a stone fall from its talons. Now as the stone went along with the wind when it was let go, and after it started on its fall entered into a medium that moved with equal velocity, I am very confident that it would not be seen to descend in its fall perpendicularly but would move with a transverse motion.
SIMPLICIO:But it should first be known how such an experiment may be made; and then one might judge according to the event. In the meantime the effect of the ship hitherto inclines to favour our opinion.
SALVIATI:Well you said “hitherto,” for perhaps it may change countenance anon. And that I may no longer hold you in suspense, tell me, Simplicius, do you really believe that the experiment of the ship squares so very well with our purpose that it ought to be believed that that which we see happen in it ought also to take place in the terrestrial globe?
SIMPLICIO:As yet I am of that opinion; and, though you have alleged some small disparities, I do not think them of so great a moment that they should make me change my judgment.
SALVIATI:I rather desire that you would continue in that belief and hold for certain that the effect of the Earth would exactly answer that of the ship, provided that when it shall appear prejudicial to your cause, you will not be of humour to alter your thoughts. You say that, when the ship stands still, the stone falls at the foot of the mast and that, when she is under sail, it lights far from thence; therefore, by conversion, from the stone’s falling at the foot is inferred the ship’s standing still, and from its falling far from thence is inferred her moving. And because that which occurs to the ship ought likewise to befall the Earth, therefore from the falling of the stone at the foot of the tower is necessarily inferred the immobility of the terrestrial globe. Is this not your argumentation?
SIMPLICIO:It is; and reduced into such conciseness that it is become most easy to he apprehended.
SALVIATI:Now tell me; if the stone let fall from the round top when the ship is in swift course, should fall exactly in the same place of the ship in which it falls when the ship is at anchor, what service would these experiments do you, to the end of ascertaining whether the vessel does stand still or move?
SIMPLICIO:Exactly none. As, for example, from the beating of the pulse one cannot know whether a person is asleep or awake, seeing that the pulse heats in the same manner in sleeping as in waking.
SALVIATI:Very well. Have you ever tried the experiment of the ship?
SIMPLICIO:I have not; but yet I believe that those authors who allege the same have accurately observed it; besides that, the cause of the disparity is so manifestly known that it admits of no questions.
SALVIATI:It may be that those authors believed in it, without having made trial of it, and you yourself are a good witness to the point. For you, without having examined it, allege it as certain and in good faith rely on their authority; as it is now not only possible but obvious that they also relied on their predecessors, without ever arriving at one that had made the experiment. For, mark you, whosoever shall perform it shall find the event succeed quite contrary to what has been written of it. That is, he shall see the stone fall at all times in the same place of the ship, whether it stand still or move with any velocity whatsoever. So that, the same holding true in the Earth as in the ship, one cannot, from the stone’s falling perpendicularly at the foot of the tower, conclude anything touching the motion or rest of the Earth.
SIMPLICIO:If you were referring me to any other means than to direct experience, I verily believe our disputations would not come to an end in haste; for this seems to me a thing so remote from all human reason that it leaves not the least place for credulity or probability.
SALVIATI:And yet it has left place in me for both.
SIMPLICIO:How is this? You have not made a hundred tests, no, not even one test, and you so confidently affirm it for true? I for my part will return to my incredulity and to the confidence I had that the experiment has been tried by the principal authors who made use of it and that the event turned out as they affirm.
SALVIATI:I am assured that the effect will ensue as I tell you, without experiment, for so it is necessary that it should, and I further add that you yourself know that it cannot fall out otherwise, however you feign or seem to feign that you know it not. Yet I am so good a broker of minds that I will make you confess the same whether you will or no. But Sagredus stands very quiet, and yet, if I mistake not, I saw him make some move as if to speak.
SAGREDO:I had intended to speak a fleeting something; but my curiosity aroused by your promising that you would force Simplicius to uncover the knowledge which he conceals from us has made me depose all other thoughts. Therefore I pray you to make good your vaunt.
SALVIATI:Provided that Simplicius consents to reply to what I shall ask him, I will not fail to do it.
SIMPLICIO:I will answer what I know, assured that I shall not be much put to it, for, of those things which I hold to be false, I think nothing can he known, since Science concerns truths, not falsehoods.
SALVIATI:I do not desire that you should say that you know anything, save that which you most assuredly know. Therefore, tell me; if you had here a flat surface as polished as a mirror and of a substance as hard as steel that was not horizontal but somewhat inclining, and you put upon it a perfectly spherical ball, say, of bronze, what do you think it would do when released? Do you not believe (as for my part I do) that it would lie still?
SIMPLICIO:If the surface were inclining?
SALVIATI:Yes, as I have already stated.
SIMPLICIO:I cannot conceive how it should lie still. I am confident that it would move towards the declivity with much propenseness.
SALVIATI:Take good heed what you say, Simplicius, for I am confident that it would lie still in whatever place you should lay it.
SIMPLICIO.:So long as you make use of such suppositions, Salviatus, I shall cease to wonder if you conclude most absurd conclusions.
SALVIATI.:Are you assured, then, that it would really move towards the declivity?
SIMPLICIO:Who doubts it?
SALVIATI:And this you verily believe, not because I told you so (for I endeavoured to persuade you to think the contrary), but of yourself, and upon your natural judgment.
SIMPLICIO:Now I see your game; you did not say this really believing it, but to try me, and to wrest words out of my mouth with which to condemn me.
SALVIATI:You are right. And how long and with what velocity would that ball move? But take notice that I gave as the example a ball exactly round, and a plane exquisitely polished, so that all external and accidental impediments might be taken away. Also I would have you remove all obstructions caused by the air’s resistance and any other causal obstacles, if any other there can be.
SIMPLICIO:I understand your meaning very well and answer that the ball would continue to move in infinitum if the inclination of the plane should last so long, accelerating continually. Such is the nature of ponderous bodies that they acquire strength in going, and, the greater the declivity, the greater the velocity will be.
SALVIATI:But if one should require that that ball should move upwards on that same surface, do you believe that it would do so?
SIMPLICIO:Not spontaneously, but being drawn, or violently thrown, it may.
SALVIATI:And in case it were thrust forward by the impression of some violent impetus from without, what and how great would its motion be?
SIMPLICIO:The motion would be continually decreasing and retarding as being contrary to Nature and would be longer or shorter, according to the strength of the impulse and according to the degree of acclivity.
SALVIATI:It seems, then, that hitherto you have well explained to me the accidents of a body on two different planes. Now tell me, what would befall the same body upon a surface that had neither acclivity nor declivity?
SIMPLICIO:Here you must give me a little time to consider my answer. There being no declivity, there can be no natural inclination to motion; and there being no acclivity, there can be no resistance to being moved. There would then arise an indifference between propulsion and resistance; therefore, I think it ought naturally stand still. But I had forgot myself: it was not long ago that Sagredus gave me to understand that it would do so.
SAGREDO:So I think, provided one did lay it down gently; but if it had an impetus directing it towards any part, what would follow?
SIMPLICIO:That it should move towards that part.
SALVIATI:But with what kind of motion? Continually accelerated, as in declining planes; or successively retarded, as in those ascending?
SIMPLICIO:I cannot tell how to discover any cause of acceleration or retardation, there being no declivity or acclivity.
SALVIATI:Well, if there be no cause of retardation, even less should there be any cause of rest. How long therefore would you have the body move?
SIMPLICIO:As long as that surface, neither inclined nor declined, shall last.
SALVIATI:Therefore if such a space were interminate, the motion upon it would likewise have no termination, that is, would be perpetual.
SIMPLICIO:I think so, if the body is of a durable matter.
SALVIATI:That has been already supposed when it was said that all external and accidental impediments were removed, and the brittleness o£ the body in this case is one of those accidental impediments. Tell me now, what do you think is the cause that that same ball moves spontaneously upon the inclining plane, and does not, except with violence, upon the plane sloping upwards?
SIMPLICIO:Because the inclination of heavy bodies is to move towards the centre of the Earth and only by violence upwards towards the circumference.
SALVIATI:Therefore a surface which should he neither declining nor nor ascending ought in all its parts to be equally distant from the centre. But is there any such surface in the world?
SIMPLICIO:There is no want of it, such is our terrestrial globe, for example, if it were not rough and mountainous. But you have that of the water, at such time as it is calm and still.
SALVIATI:Then a ship which moves in a calm at sea is one of those bodies that run along one of those surfaces that are neither declining nor ascending and is therefore disposed, in case all obstacles external and accidental were removed, to move incessantly and uniformly with the impulses once imparted.
SIMPLICIO:It should seem to be so.
SALVIATI:And that stone which is on the round top, does it not move as being, together with the ship, carried about by the circumference of a circle about the centre; and therefore consequently by a motion indelible in it, if all external obstacles be removed? And is not this motion as swift as that of the ship?
SIMPLICIO:Up to now all is well. But what follows?
SALVIATI:Then I pray you recant with good grace your last conclusion, if you are satisfied with the truth of all the premises.
...
SALVIATI:For a final proof of the nullity of all the experiments before alleged, I conceive it now a convenient time and place to demonstrate a way how to make an exact trial of them all. Shut yourself up with some friend in the largest room below decks of some large ship and there procure gnats, flies, and such other small winged creatures. Also get a great tub full of water and within it put certain fishes; let also a certain bottle be hung up which, drop by drop, lets forth its water into another narrow necked bottle placed underneath. Then, the ship lying still, observe how those small winged animals fly with like velocity towards all parts of the room; how the fishes swim indifferently towards all sides; and how the distilling drops all fall into the bottle placed underneath. And casting anything towards your friend, you need not throw it with more force one way than another, provided the distances be equal; and jumping broad, you will reach as far one way as another. Having observed all these particulars, though no man doubts that, so long as the vessel stands still, they ought to take place in this manner, make the ship move with what velocity you please, so long as the motion is uniform and not fluctuating this way and that. You shall not be able to discern the least alteration in all the forenamed effects, nor can you gather by any of them whether the ship moves or stands still.
Translated by Thomas Salusbury
Reading and Discussion Questions
1.What advantages might Galileo see in presenting his work in a dialogue format? What role does each of these characters play in the dialogue?
2.What arguments are offered against Aristotle and/or Aristotelianism?
3.How does Galileo answer the ancient objection that the Earth could not be moving because birds would be left behind?
4.What is the point of the thought experiment about dropping a ball from the mast of a ship at rest or in motion?
5.Explain why Galileo asks readers to imagine taking butterflies into the cabin of a ship.