« הקודםהמשך »
W E have before observed the utility of the various motions of the sea
in keeping its waters from putrefaction, but of all the motions which are observable in it, that of the tides is the most remarkable, both for its utility and regularity.
Probably it was of the tides which the sacred writer spake when he represented Jehovah as having set bounds to the sea and said, Hitherto shall thy proud waves come, and here shall they be stayed..
The importance of the tides to all commercial nations is such, that they have in all ages been much attended to. The cause of thein was not so soon known as their use. Ancient navigators took the advantage of them without being able to give a scientific account of their origin. In the same manner our mordern sailors take the advantage of the winds for the purposes of commerce; but very few even think of the cause of them, and even our most profound philosophers are not agreed on this point. - Homer is said to be the first profane author who speaks of the tides. Indeed it is not very clear that he alludes to them when he describes Charybdis as rising and retiring thrice in every day *. The Greeks, from their situation, had but little opportunity of observing the tides. The conquests and commerce of the Romans afforded better opportunity of acquaintance with them : and after some wild conjectures of the earliest philosophers, it became well known in the time of Pliny, that the tides were, in a small degree, under the influence of the sun; but in a much greater of the indon.
To describe the nature of the tides we inust observe, that there is a certain dep h of the waters of the ocean which would obtain, if they were perfectly at rest; but observation shows that they are constantly varying from this level, and that variation which we call the tides, is regular and periodical. It is found that on the shores of the ocean, and in bays, creeks, and harbours, which communicate freely with the ocean, the waters rise up above this mean height twice a day, and as often sink below it, forming what is called a flood and an ebb, a high and a low water. The whole interval between high and low water is called a tide; the water is said to flow and to ebb; and the rising is called the flood tide, and the falling is called the ebb tide.
Thus the sea is observed to flow for about six hours from south to north, so that entering the mouths of rivers, it drives back the river waters towards their sources. After this continual flux of six hours the sea seems to rest for about a quarter of an hour, and then begins to ebh, or retire back again 'from north to south, for six hours more; in which time the waters sinking, the rivers resume their natural course. After a seeming pause of a quarter of an hour the sea again begins to flow as before : and thus it has always continued to do ever since the creation. It is no wonder that this amazing appearance early excited both the attention and wonder of mankind.
The connection betwixt the moon and tides was long observed before tlie particular manner of the moon's operation was even guessed at. It was found that there was a flux and reflux of the sea, in the space of twelve hours fifty minutes, which is exactly the time of alunar day. It was observed also, that when the moon was in the meridian, in other words, as nearly as possible over any part of the sea, that the waters flowed to that part, and made atide there; on the contrary, it was found, that when the moon left the meridian, the sea began to flow back again from whence it canne. Thus far the waters of the sea seemed very regularly to attend the motions of the moon.But it appeared, likewise, that when the moon was in the opposite meridian, as far as possible on the other side of the globe, that there was a tide on this side also ; so that the moon produced two tides, one by her greatest approach to us, and another by her greatest distance from us : in other words, the moon in going round the earth, produced two tides, always at the same time; one on the part of the globe directly under her, and the other on the part of the globe directly opposite. * Kelper was the first who conjectured that attraction was the principal cause of those appearances; asserting that the sphere of the moon's operation extended to the earth, and drew up its waters. The precise manner in which this is done, was discovered by Newton. Heliid hold of this class of phenomena as the most incontestable proof of universal gravitation, and has given a beautiful and synoptical view of the whole subject; contenting himself, however, with meerly exhibiting the chief consequences of the general principle, and applying it to the phenomena with singular address. But the wide steps taken by this philosopher in his investigation leave ordinary readers frequeratly at fault: many of
his assumptions require the greatest mathematical knowledge to satisfy us of their truth. It cannot be expected of us, therefore, in this work, to follow him. . It will be enough that we give a plain account of the facts which he has elaborately proved. * The moon has been found, like all the rest of the planets, to attract, and to be attracted by the earth. This attraction prevails throughout our whole planetary system. The more inatter there is contained in any body, the more it attracts; and its influence decreases in proportion as the distance, when squared, increases. This being premised, let us see what must ensue upon supposing the moon to be in the meridian of any tract of the sea. The surface of the water immediately under the moon, is Nearer the moon than any other part of the globe is; and therefore, inust be more subject to its attraction than the waters any where else. The waters will, therefore, be attracted by the moon, and rise in an heap; whose enrinence will be highest where the attraction is greatest. "In order to form this eminence, it is obvious that the surface, as well as the depths, will be agitated; and wherever the water run froin one part, succeeding waters must run to fill up the space wnich they have left. Thus the waters of the sea, running from all parts, to attend the motions of the moon, produce the flowing of the tide : and it is high tide at that part wherever the moon comes over it, or to its ineridian.
But as the moon travels onward, and ceases to point over the place where the waters were just risen, the cause of their rising will cease to operate, and they will flow back by their natural gravity, into the lower parts from whence they had travelled; and this retiring of the waters will form the ebbing of the sea.
.. . - Thus the first part of our subject seems pretty obvious, since, in
general, it requires no great sagacity to conceive that the waters nearest the moon are most attracted, or raised highest by its influence. But the other part seems more difficult to be accounted for, namely, how there came to be high tides at the same time, on the opposite side of the globe, and where the waters are farthest from the moon..
To comprehend this, therefore, it inust be observed, that the part of the earth, and its waters, that are farthest from the moon, are the parts, of all others, that are least attračied by the moon : it must also be observed, that all the waters, when the moon is on the opposite side of the earth, inust be attracted by it in the same direction that the earth itself attracts them; that is, if we may so say, quite through the body of the earth, towards the moon itself. This, therefore, being conceived, it is plain that these waters which are farthest from the moon, will have less weight than those of any other part, on the same side of the globe; because the moon's attraction, which conspires with the earth's attraction, is there least. Now, therefore, the waters farthest from the moon, having less weight, and being lightest, will be pressed on all sides, by those that, having more attraction, are heavier; and the heavier waters flowing in, will make them swell and rise in an eminence directly opposite to ihat on the other side of the globe, caused by the more iminediate influence of the moon,
In this manner the moon, in one diurnal revolution, produces two tides, one raised immediately under the sphere of its influence, and the other directly opposite to it, As the moon travels, this vast body of waters rear upwards, as if to watch its motions, and pursues the same constant rotation. However, in this great work of raising the tides, the sun has no small share ; it produces its own tides constantly every day, just as the moon does, but in a much less degree, because the sun is at an immensely greater distance. Thus there are solar tides and lunar tides. When the force of the two luminaries concur, which they always do. when they are either in the same, or in opposite parts of the heavens, they joinily produce a much greater tide, than when they are so situated in the heavens, as each to make peculiar tides of its own. Thus their united attraction forms the high spring-tides; but their divided attraction forms the lower neap-tides. .
This theory being well understood, it may be readily brought to explain the various appearances of the tides, if the earth was covered with a deep sea, and the waters uninfluenced by shoals, currents, straits, or teinpests. But in every part of the sea, near the shores, the geographer must lend his aid to correct the calculations of the astronomer. For by reason of the shallowness of some places, and the narrowness of the straits in others, there arises a great diversity in the effect, not to be accounted for without an exact knowledge of all the circumstances of the place. In the great depths of the ocean, for instance, a vesy slow and imperceptible motion of the whole body of water will suffice to raise its surface several feet high; but if the same increase of water is to be conveyed through a narrow channel, it must rush through it with the inost imperuous rapidity. In the English Channel, and in the Gerinan Ocean, the tide is found to flow strongest in those places that are narrowest; the same quantity of water being, in this case, driven through a smaller passage. It is often seen, therefore, pouring through a strait with great force, and, by its rapidity, considerably raised above the surface of that part of the ocean into which it runs.
This shallowness and narrowness of many parts of the sea, give rise also to a peculiarity in the tides of some parts of the world. For in many places, and in our own seas in particular, the greatest swell of the tide is not while the moon iş at its meridian, and directly over the place, but some time after it has declined from thence. The sea, in this case, being obstructed, pursues the moon with what dispatch it can, but does not arrive with all its waters till long aftes the moon has ceased to operatę. From the same causes, also, we may account for the Mediteșranean, the Baltic, and especially the Black Sea, having hardly any sensible tides. These, though they seem to us very extensive, are not, however, large enough to be much affected by the influence of the moon ; and as to their communication with the ocean, through such parrow iplets, it is impossible, in a few hours time, that they should receive and return water enough to raise or depress them in any çonsiderable degree.
In general, therefore, we may observe, that all tides are much higher and more considerable in the torrid zone, than in the rest of the ocean; the sea in those parts. being generally deeper, and less affected by changeable winds, or winding shores. One of the greatest tides we know of, is that at the mouth of the river Indus, where the water rises thirty feet in height. How great, then, must have been the amazement of Alexander's soldiers at so strange an appearance! They who had always before been accustomed only to the almost imperceptible rising of ih: Mediterranean, when made at once spectators of a river rising and falling thirty feet in a few hours, must have felt a mixture of surprise and apprehension. The tides are also remarkably high on the coasts of Malay, in the straits of Sunda, in the Red Sea, along the coast of China, and Japan, at Panama in America, and in the Gulph of Bengal. But perhaps the highest tide which is known is in the bay Fundy, ia North America, where the water rises and falls at least sixty feet.
The tides at Tonquin are, however, the most remarkable in the world. In this part there is but one rise and one fall in the course of twenty four hours ; whereas, in other places there are two. Besides, there, twice in each month, there is no tide at all, when the moon is near the equinoctial, the water being for some time quite stagnant. These, with some other odd appearanoes attending the tides, were considered by many as inscrutable; but Sir Isaac Newton, with peculiar sagacity, adjudged them to arise from the concurrence of two tides, one from the South Sea, and the other from the Indian Ocean. Of each of these tides there come two every day; two at one time greater, and two at another that are less. The time betwen the arrival of the two greater, is considered by him as high tide; the time between the two lesser, as ebb. With this clue, that great man solved every appearance, and so established his theory as to silence every opposer. · This statement of the cause of the tides has been controverted by many: but nothing equally satisfactory has ever been brought forward in its place. Mr. Saint Pierre, in his book called Studies of Nature, has quite discarded the Newtonian theory of the tides. He says it is erroneous in princip., and quite insufficient for explaining the phenomena: and he substitutes the liquifaction of the ice and snow of the polar regions, and the greater length of the polar than of the equatorial axis of the earth. · The character of this gentleman is respectable. His constant attention to final causes, and the proofs he brings thence of the being, wisdom, and goodness of Deity, shew his piety. And we esteem him the more, because of his fortitude in resisting the influence of national example in the days of French infidelity. Piety has not been often found among French philosophers for near an hundred years; but since the revolution in France the senseless atheism of ancient Greece las again been brought forward without disguise. St. Pierre has resisted this impiety, and we admire him for it. But he has failed in proving the earth to be a prolate spheroid. Experience has proved it to be an oblate spheriod. And as for the quantity of ice and snow in the polar regions, if there was twenty times as much, and it was all to melt in a day, there could be no