The Essential Galileo (45 page)

S
ALV.
    After these things have been clarified, it is time for us to go on and examine the variety of particular phenomena which experience enables us to observe in regard to the tides. First, there will be no difficulty understanding why it happens that there are no noticeable tides in ponds, lakes, and even small seas; this has two very effective causes.

One is that, as the basin acquires different degrees of speed at different hours of the day, because of its smallness they are acquired with little difference by all its parts, and the forward as well as the backward parts (namely, the eastern and the western) are accelerated and retarded almost in the same way; moreover, since this change occurs gradually, and not by a sudden obstacle and retardation or an immediate and large acceleration in the motion of the containing basin, it as well as all its parts receive equally and slowly the same degrees of speed; from this uniformity it follows that the contained water too receives the same action with little resistance, and consequently it gives very little sign of rising and falling and of flowing toward this or the other end. This effect is also clearly seen in small artificial containers, in which the water acquires the same degrees of speed whenever the acceleration or the retardation takes place in a relatively slow and uniform manner. However, in sea basins that extend for a great distance from east to west the acceleration or retardation is much more noticeable and unequal, for while one end is undergoing very retarded motion the other is still moving very rapidly.

The other cause is the reciprocal vibration of the water stemming from the impetus it also receives from the container, which vibration has very frequent oscillations in small vessels, as we have seen: for [458] the earth's motions can cause agitation in the waters only at twelve-hour intervals, since the motion of the containing basins is retarded and is accelerated the maximum amount only once a day, respectively; but the second cause depends on the weight of the water while in the process of reaching equilibrium, and it has its oscillations at intervals of one hour, or two, or three, etc., depending on the length of the basin; now, mixed with the former cause, which is very small in small vessels, the latter renders it completely imperceptible; for before the end of the operation of the primary cause with the twelve-hour period, the secondary one due to the weight of the water comes about, and with its period of one hour, two, three, or four, etc. (depending on the size and depth of the basin), it perturbs and removes the first, without allowing it to reach the maximum or the middle of its effect. From this contraposition, any sign of tides remains completely annihilated or much obscured.

I say nothing of the constant alterations due to air; disturbing the water, they would not allow us to ascertain a very small rise or fall of half an inch or less, which might actually be taking place in water basins that are no longer than a degree
⁷¹
or two.

Second, I come to resolving the difficulty of how tidal periods can commonly appear to be six hours, even though the primary cause embodies a principle for moving the water only at twelve-hour intervals, that is, once for the maximum speed of motion and once for maximum slowness. To this I answer that such a determination cannot in any way result from the primary cause alone; instead we must add the secondary ones, namely, the greater or lesser length of the vessels and the greater or lesser depth of the water contained in them. Although these causes do not act to bring about the motions of the water (since this action originates only from the primary cause), nevertheless they have a key role in determining the periods of the oscillations, and this role is so powerful that the primary cause remains subject to them. Thus, the six-hour interval is no more proper or natural than other time intervals, although it is perhaps the one most commonly observed since it occurs in our Mediterranean, which for many centuries was the only accessible sea; however, such a period is not [459] observed in all its regions, for in some of the more narrow areas such as the Hellespont and the Aegean, the periods are much shorter and also much different from each other. Some say that Aristotle long observed these variations from some cliffs in Euboea and found their causes incomprehensible, and that because of this, (overcome with despair) he jumped into the sea and drowned himself.
⁷²

Third, we can quickly explain why it happens that although some seas are very long—for example, the Red Sea—nevertheless they are almost entirely lacking in tides. This occurs because its length does not extend from east to west, but from southeast to northwest. For, the earth's motions being from west to east, the impulses received by the water always cross the meridians and do not move from one parallel to another; so, in seas that extend transversely in the direction of the poles and that are narrow in the other direction, no cause of tides remains but the contribution of some other sea with which they are connected and which is subject to large motions.

Fourth, we can very easily understand the reason why, in regard to the rise and fall of the water, tides are greatest at the extremities of gulfs and smallest in the middle. This is shown by daily experience here in Venice, which is located at the end of the Adriatic and where this variation amounts to five or six feet; but in areas of the Mediterranean far from the extremities, such a variation is very small, as is the case in the islands of Corsica and Sardinia and on the shores of Rome and Leghorn, where it does not exceed half a foot. We can also understand how, on the contrary, in places where the rise and the fall are very small, the flow back and forth is large. I say it is easy to understand the cause of these phenomena because we can make clear tests in all sorts of vessels we can artificially build; here the same effects are seen to follow naturally from our making them move with a motion which is nonuniform, namely, sometimes accelerated and sometimes retarded.

Fifth, considering how the same quantity of water that moves slowly through a wide area must flow with great impetus when passing through a narrow place, we shall have no difficulty in understanding the cause of the immense currents which flow in the narrow channel that separates Calabria from [460] Sicily; for although all the water contained in the eastern Mediterranean and bound by the width of the island and the Ionian Gulf may slowly flow into it toward the west, nevertheless, when constricted into this strait between Scylla and Charybdis,
⁷³
it flows rapidly and undergoes very great agitation. Similar to this and much greater we understand are the currents between Africa and the large island of Madagascar, as the waters of the North and South Indian Ocean, which surround it, flow and become constricted in the smaller channel between it and the South African coast. Very great must be the currents in the Strait of Magellan, which connects the extremely vast South Atlantic and South Pacific Oceans.

Sixth, to account for some more obscure and implausible phenomena observed in this subject, we now have to make another important consideration about the two principal causes of tides and then mix them together. The first and simpler of these is (as we have said several times) the definite acceleration and retardation of the earth's parts, from which the water would acquire a definite tendency to flow toward the east and to go back toward the west within a period of twenty-four hours. The other is the one that depends on the mere weight of water: once stirred by the primary cause, it then tries to reach equilibrium by repeated oscillations; these are not determined by a single period in advance, but they have as many temporal differences as the different lengths and depths of sea basins; and insofar as they depend on this second principle, some oscillations might flow back and forth in one hour, others in two, four, six, eight, ten, etc. Now, let us begin to join together the primary cause, whose fixed period is twelve hours, with one of the secondary causes whose period is, for example, five hours: sometimes it will happen that the primary and secondary causes agree by both producing impulses in the same direction, and with such a combination (a unanimous consent, so to speak) the tides are large; other times, the primary impulse being somehow opposite to that of the secondary cause, and thus one principle taking away what the other one gives, the watery motions will weaken and the sea will reduce to a very calm and almost motionless state; finally, [461] on still other occasions, when the same two causes neither oppose nor reinforce each other, there will be other variations in the increase or decrease of the tides. It may also happen that of two very large seas connected by a narrow channel, due to the mixture of the two causes of motion, one sea has tidal motions in one direction while the other has them in the opposite; in this case, in the channel where the two seas meet there are extraordinary agitations with contrary motions, vortices, and very dangerous boilings, as it is in fact constantly observed and reported. These conflicting motions, dependent on the different positions and lengths of interconnected seas and on their different depths, give rise sometimes to those irregular disturbances of the water whose causes have worried and continue to worry sailors, who experience them without seeing winds or any other serious atmospheric disturbance that might produce them.

These atmospheric disturbances must be significantly taken into account in other cases, and we must regard them as a tertiary accidental cause, capable of significantly altering the occurrence of the effects produced by the primary and more important causes. For example, there is no doubt that very strong winds from the east can support the water and prevent it from ebbing; then, when at the appropriate time there is a second wave of flow (and a third), it will rise a great deal; thus, if sustained for a few days by the power of the wind, it will rise more than usual and produce extraordinary flooding.

Seventh, we must also note another cause of motion dependent on the great quantity of river water flowing into seas that are not very large. Here, in channels or straits connected with such seas, the water is seen flowing always in the same direction; for example, this happens at the Bosporus near Constantinople, where the water always flows from the Black Sea to the Sea of Marmara. For, in the Black Sea, because of its smallness, the principal causes of the tides are of little effect; on the contrary, very large rivers flow into it, and so with such a superabundance of water having to go through the strait, here the flow is very noticeable and always southward. Moreover, we must note that although this strait is very narrow, it is not [462] subject to perturbations like those in the strait of Scylla and Charybdis. For, the former has the Black Sea on the north and the Sea of Marmara, the Aegean, and the Mediterranean on the south; and, as we have already noted, insofar as a sea extends in a north-south direction it is not subject to tides; on the contrary, because the strait of Sicily is interposed between two parts of the Mediterranean that extend for long distances in an east-west direction, namely, in the direction of tidal currents, in the latter the disturbances are very large. Similarly, they would be larger between the Pillars of Hercules,
⁷⁴
if the Strait of Gibraltar were less wide; and they are reported to be very large in the Strait of Magellan.

For now, this is all I can think of telling you about the causes of this first (diurnal) period of the tides and related phenomena; if you want to advance any comments, you can do it now, so that we can then go on to discuss the two other (monthly and annual) periods.
⁷⁵

[§8.8 Day IV: Ending]
⁷⁶

[485] S
AGR.
    I think you have done a great deal in opening up for us the first door to such a lofty speculation. Even if you had given us [486] only the first basic proposition, in my opinion that alone so greatly surpasses the inanities introduced by so many others that merely thinking of them nauseates me; I mean the proposition (which seems unobjectionable to me) declaring very convincingly that if the vessels containing the seawater stood still, it would be impossible by the common course of nature for it to exhibit the motions we see, and on the contrary, given the motions attributed for other reasons by Copernicus to the terrestrial globe, such changes in the seas must necessarily follow. I am very surprised that among men of sublime intellect (of whom there have been many) no one has seen the incompatibility between the reciprocal motion of the contained water and the immobility of the containing vessel; this incompatibility seems very evident to me now.

S
ALV.
    What is more surprising is that, while some have thought of finding the cause of the tides in the earth's motion (thus showing greater perspicacity than is common), when they then came to the point, they grasped nothing; they did not understand that it is not enough to have a single uniform motion (such as, for example, the mere diurnal motion of the terrestrial globe), but that we need an unequal motion, sometimes accelerated and sometimes retarded; for when the motion of a vessel is uniform, the water contained therein will get used to it and will never undergo any change. Moreover, it is totally useless to say (as an ancient mathematician
⁷⁷
is reported to have said) that when the earth's motion encounters the motion of the lunar orb, such a contrast causes the tides; for it is neither explained nor selfevident how this is supposed to happen, but rather we can see its manifest falsity, given that the earth's rotation is not contrary to the moon's motion but in the same direction. Thus, what has been stated and thought so far by others is, in my opinion, completely invalid. However, of all great men who have philosophized on such a puzzling effect of nature, I am more surprised about Kepler than about anyone else; although he had a free and penetrating intellect and grasped the motions attributed to the earth, he lent his ear and gave his assent to the dominion of the moon over the water, to occult properties, and to similar childish ideas.