The Essential Galileo (43 page)

S
AGR.
    Please, Simplicio, do not tell us any more, for I do not think it is worthwhile to take the time to recount them or waste words to confute them; if you give your assent to these or similar trifles, you do an injustice to your judgment, which we know to be very seasoned.

S
ALV.
    I am calmer than you, Sagredo, and so I will expend fifty words for the sake of Simplicio, should he perhaps think that there is any probability in the things he related. This is what I say. It is true, Simplicio, that waters whose exterior surface is higher displace those which are below them and lower; but this does not happen with those which are higher by reason of depth; and, once the higher ones have displaced the lower ones, they quickly calm down and level out. This Peripatetic of yours must be thinking that all the lakes in the world (which stay calm) and all the seas where the ebb and flow is unnoticeable have a bottom whose depth does not vary in the least; whereas I was so simple minded as to believe that, even without any other sounding, the islands which emerge above the waters are a very clear indication of the variability of the bottom. To that clergyman you can say that the moon every day comes over the whole Mediterranean, but that the waters rise only at its eastern end and here for us in Venice. To those who say that the moderate heat is capable of making the water swell, tell them to start a fire under a boiler full of water and keep their right hand in it until the water rises by a single inch due to the heat, and then to take it out and write about the swelling of the sea; or at least ask them to teach you how the moon manages to rarefy a certain part of the water and not the rest, namely, the one here in Venice and not that at Ancona, Naples, or Genoa. One is forced to say that poetical minds are of two kinds: some adept and inclined to invent fables, others disposed and accustomed to believe them.

S
IMP.
    I do not think anyone believes in fables while [447] knowing them to be such. In regard to the opinions about the causes of the ebb and flow (which are many), I know that the primary and true cause of an effect is only one, and so I understand very well and am sure that at most one can be true, and I know that all the rest are fictitious and false; and perhaps the true one is not even among those which have been produced so far. Indeed, I truly believe this is the way it is, for it would be strange if the truth produced so little light that nothing would appear among the darkness of so many falsehoods. However, taking the liberty which we allow among ourselves, I will say that to introduce the earth's motion and make it the cause of the ebb and flow seems to me already to be an idea no less fictitious than the others I have heard; and if I were not offered reasons more in accordance with the nature of things, then without any reluctance I would go on to believe that this is a supernatural effect and hence miraculous and inscrutable to the human intellect; this would be like the infinitely many others which are dependent directly on the omnipotent hand of God.

S
ALV.
    You speak very prudently and also in accordance with Aristotle's doctrine; as you know, at the beginning of his
Questions of Mechanics
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he attributes to a miracle things whose causes are unknowable. However, as to whether the true cause of the ebb and flow is one of the impenetrable ones, I think the strongest indication you have for this is your seeing that, of all the causes which have so far been advanced as true ones, there is none from which we can reproduce a similar effect, regardless of whatever artifice we employ; for by means of the light of the moon or sun or temperate heat or differences of depth, we will never make the water contained in a motionless vessel artificially run back and forth and go up and down at one place but not another. On the other hand, if by moving the vessel very simply and without any artifice I can represent to you exactly all those changes that are observed in seawater, why do you want to reject this cause and resort to a miracle?

S
IMP.
    I want to resort to a miracle if you do not convince me of natural causes other than the motion of the basins containing the waters of the sea, because I know that these basins do not move, the whole terrestrial globe being motionless by nature.

S
ALV.
    But do you not believe the terrestrial globe could be made to move supernaturally, namely, by the absolute power of God?

S
IMP.
    Who could doubt that?

[448] S
ALV.
    Therefore, Simplicio, since we must introduce a miracle to produce the ebb and flow of the sea, let us make the earth move miraculously, and then this motion will naturally make the sea move. This operation will be all the simpler (and, I shall say, the more natural, among the miraculous ones), inasmuch as giving a turning motion to one globe (of which we see so many moving
⁶⁵
) is less difficult than making an immense quantity of water go back and forth (in some places faster and in others slower) as well as rise and fall (more in some places, less in others, and not at all in still others), and having all these variations take place in the same containing vessel. Moreover, the latter involves many different miracles, the former only one. Finally, the miracle of making the water move implies another miracle as a consequence; that is, keeping the earth motionless against the impulses of the water, which are powerful enough to make it waver in this or in that direction unless it were miraculously restrained.

S
AGR.
    Please, Simplicio, let us suspend our judgment about declaring false the new view which Salviati wants to explain to us, and let us not be too quick to place it in a pigeonhole with the old ridiculous accounts. As regards the miracle, let us resort to it after we have listened to discussions confined within the limits of natural reason, although I am inclined to find miraculous all works of nature and of God.
⁶⁶

S
ALV.
    My judgment is the same; to say that the natural cause of the tides is the earth's motion does not prevent this process from being miraculous.

Now, to resume our reasoning, I repeat and reaffirm that so far it is not known how it can happen that the waters contained in our Mediterranean basin undergo the motions they are seen to have as long as the containing basin or vessel remains itself motionless; what generates the difficulty and renders this subject inextricable are the things I will mention below which are observed every day. So, listen.

We are here in Venice, where there is a low tide and the sea is tranquil and the air calm. The water begins to rise, and within five or six hours, it rises ten palms or more. Such a rise does not derive from the expansion of the water that was there before, but rather from new water that has come here, water of the same kind as the old, of the same salinity, of the same density, and of the same weight; [449] boats float on it, Simplicio, just as they did on the old water, without subsiding a hair lower; a barrel of this new water does not weigh a single grain more or less than an equal volume of the old water; it is as cold as the other, without any change; in short, it is new water which has visibly entered the bay through the narrows and mouth of the Lido.
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Now, you tell me whence and how it has come here.

Are there perhaps around here some openings and caves at the bottom of the sea through which the earth inhales and regurgitates the water, breathing as if it were an immense and enormous whale?
⁶⁸
If this is so, how is it that in a period of six hours the water does not rise likewise in Ancona, Dubrovnik, and Corfu, where the rise is very small and perhaps unobservable? Who will find a means of injecting new water into a motionless vessel and ensuring that it will rise only in a definite part of it and not elsewhere?

Will you say perhaps that the new water is supplied by the ocean, coming through the Strait of Gibraltar? This does not remove the difficulties already mentioned and carries with it some more serious ones. First, tell me what must be the speed of the water which enters the strait and in six hours reaches the extreme shores of the Mediterranean (covering a distance of two or three thousand miles), and which then again covers the same distance in the same time when it returns? What will happen to the various ships at sea? What will happen to those which might be in the strait, where there would be such a constant and impetuous flow of an immense quantity of water that, by using a channel no more than eight miles wide, it would provide enough water to flood in six hours an area hundreds of miles wide and thousands of miles long? What tiger or falcon ever ran or flew at such a speed? I mean a speed of four hundred and more miles per hour. There are indeed currents along the strait (I do not deny it), but they are so slow that rowboats outrun them, although not without a delay in their course. Furthermore, if this water comes through the strait, the other difficulty still remains; that is, how it manages to rise so much here in a region so remote, without first rising by a similar or greater height in the nearer regions.

In short, I do not think that either stubbornness or intellectual subtlety can ever find solutions to these difficulties and consequently uphold the earth's stability against them, as long as we confine ourselves within natural limits.

S
AGR.
    I comprehend this very well already and am eagerly waiting to hear how these puzzling phenomena can without hindrance follow from the motions already attributed to the earth.

[450] S
ALV.
    In regard to the manner in which these effects should follow as a consequence of the motions that naturally belong to the earth, not only must they find no repugnance or hindrance, but they must follow easily; indeed, not only must they follow with ease, but with necessity, so that it is impossible for them to happen otherwise; for such is the character or mark of true natural phenomena. We have established the impossibility of explaining the motions we see in the water while simultaneously maintaining the immobility of the containing vessel; so, let us go on to see whether the motion of the container can produce the effect and make it happen in the way it is observed to happen.

There are two kinds of motions which can be imparted to a vessel and from which the water contained in it can acquire the power to flow alternately toward one of its extremities and toward the other, and alternately to rise and fall there. The first would occur when either one of the extremities is lowered, for then the water (flowing toward the inclined point) would be alternately raised and lowered, now at this extremity and now at that one. However, this rising and falling are nothing but a motion away from and toward the center of the earth, and hence this kind of motion cannot be attributed to the basins in the earth itself which contain the water; regardless of any motion attributed to the terrestrial globe, the parts of these containing vessels can neither approach nor recede from its center.

The other kind of motion occurs when, without tilting in any way, the vessel moves with forward motion at a speed that is not uniform but changing, by sometimes accelerating and sometimes being retarded. The water contained in the vessel is not rigidly attached to it as its other solid parts are; instead, as a fluid, the water is almost separate, free, and not obliged to go along with all the changes of its container; it follows that, when the vessel is retarded, the water retains a part of the impetus already acquired and so flows toward the forward end, where it necessarily rises; on the contrary, if the vessel should acquire additional speed, the water would retain a part of its slowness and remain somewhat behind, and so (before getting used to the new impetus) it would flow toward the rear of the vessel, where it would rise by a certain amount. These effects can be more clearly explained and shown to the senses by means of the example of one of those boats that constantly come from Lizzafusina, full of the fresh water used by the city. [451] Let us then imagine such a boat moving at moderate speed across the lagoon and calmly carrying the water with which it is filled; suppose then that it is considerably retarded, either by running aground or due to some other obstacle in its way; the water contained in the boat will not thereby lose the already acquired impetus (as the boat itself will), but will conserve it and flow forward toward the bow, where it will noticeably rise while dropping astern; but if, on the contrary, while the same boat is on its quiet course it acquires additional speed by a noticeable amount, then before the contained water gets used to the new speed it will retain its slowness and remain behind, namely, toward the stern, where it will consequently rise while dropping at the bow.

This effect is indubitable and clear and can be experienced at any time. There are three particulars about it that I now want us to note. The first is that, in order to make the water rise at one end of the vessel, there is no need of any new water, nor need it flow there from the other end. The second is that the water in the middle does not noticeably rise or fall, unless the course of the boat is very fast and the collision or other restraining obstacle is sudden and very strong, in which case all the water could not only flow forward but even spill out of the boat for the most part; the same thing would also happen if, while going slowly, it should suddenly receive an extremely powerful impetus; but if its quiet motion undergoes a moderate retardation or acceleration, the water in the middle rises and falls imperceptibly (as I said), and for the rest, the closer it is to the middle the less it rises, and the farther it is the more it rises. The third is that, whereas the water near the middle undergoes little variation by rising and falling as compared with that at the end, on the contrary it flows a great deal forward and backward as compared with the same.

Now, gentlemen, what the boat does in relation to the water contained in it and what the contained water does in relation to the boat are exactly the same as what the Mediterranean basin does in relation to the water contained in it and what the contained water does in relation to the Mediterranean basin. Next, we need to demonstrate how and in what manner it happens that the Mediterranean and all the other basins (in short, all parts of the earth) move with a significantly nonuniform motion, although only motions that are regular and uniform are assigned to the whole globe.