CHAPTER I.

INTRODUCTION.—THE FORMATION OF THE TIDES CONSIDERED, THEIR VARIATION, AND EFFECTS.

For, lo! the sea that fleets about the land, And like a girdle clips her solid waste, Music and measure both doth understand: For his great crystal eye is always cast Up to the moon, and on her fixed fast: And as she danceth in her pallid sphere, So danceth he about the centre here.

The above lines, so beautifully expressed by one of our earlier poets, introduces a subject generally understood, but the important object connected with our present inquiry cannot be maintained without a thorough knowledge of cause and effect. A minute acquaintance, therefore, with the formation of the tides and currents, their variation and effects, transmitted to us by the observations, experiments, and discoveries of the earlier, and confirmed by the researches of the modern philosophers, will not be deemed altogether superfluous, as they will tend to remove any obstacle that might otherwise present itself on the consideration of so difficult a subject.

By the term tide is meant that regular motion of the sea, according to which it ebbs and flows twice in the twenty-four hours.

After some wild conjectures of the earliest philosophers, observes Goldsmith, it became well known in the time of Pliny that the tides were entirely under the influence in a small degree of the sun, but in a much greater of the moon. It was found that there was a flux and reflux of the sea in the space of twelve hours and fifty minutes, which is exactly the time of a lunar day. It was observed that whenever the moon was in the meridian, or in other words, as nearly as possible over any part of the sea, that the sea flowed to that part, and made a tide there; on the contrary, it was found that when the moon left the meridian, the sea began to flow back again from whence it came, and there might be said to ebb. Thus far the waters of the sea seemed very regularly to attend the motions of the moon. But as it appeared, likewise, that when the moon was in the opposite meridian, as far off 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 once 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.

Kepler was the first who conjectured that attraction was the principal cause; asserting, that the sphere of the moon’s operation extended to the earth, and drew up its waters. But what Kepler only hinted, has been completely developed and demonstrated by Sir Isaac Newton.

After his great discovery of the law of gravitation, he found it an easy matter to account for the whole phenomena of the tides. The moon, like all the rest of the planets, has been found to attract and to be attracted by the earth. This attraction prevails throughout our whole planetary system; the more matter 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 in the meridian of any tract of the sea. The surface of the water immediately under the moon, is nearer the moon than any part of the globe is, and, therefore, must be more subject to its attraction than the waters anywhere else. The waters will there be attracted by the moon, and rise in a heap, whose eminence will be the highest where the attraction is greatest. In order to form this eminence, it is obvious that its surface, as well as the depths, will be agitated, and that wherever the water runs from one part, succeeding waters must run to fill up the space it has left. Thus the waters of the sea, running from all parts to attend the motion 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 meridian. {11}

But when the moon travels onward, and ceases to point over the place where the waters were just risen, the cause of their rising ceasing to operate, 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. {12a}

Thus the first part of the demonstration is obvious, since in general it requires no great sagacity to conceive that the waters nearest the moon are most attracted or raised highest by the moon. But the other part of the demonstration, namely, how there come to be high tides at the same time on the other side of the globe is not so easy to conceive. To comprehend this, it must be observed, that the part of the earth and its waters farthest from the moon, are the parts of all others that are least attracted by the moon; it must also be observed, that all the waters, when the moon is on the opposite side of the earth, must be attracted in the same direction that the earth itself attracts them; that is apparently quite through the body of the earth, towards the moon itself. This, therefore, being conceived, it is plain that those 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 that on the other side of the globe, caused by the more immediate influence of the moon. {12b}

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 rears upward, 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 tide 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 forces of these two great luminaries concur, which they always do when they are either in the same or in the opposite parts of the heavens, they jointly produce a much greater tide, than when they are so situated in the heavens as each to make peculiar tides of their own; in the former, the attraction of the sun conspires with the attraction of the moon, by which means the high spring tides are formed; in the latter, the action of the sun is opposed to that of the moon, consequently the effect must be to depress the waters where the moon’s action has a tendency to raise them, and hence the production of the lower neap tides. {13a}

The spring tides {13b} do not take place on the very day of the new and full moon, nor the neap tides on the very day of the quadratures, but a day or two after; the effect is neither greatest nor least when the immediate influence of the cause is greatest or least: as the greatest heat, for example, is not on the solstitial day, when the immediate action of the sun is greatest, but some time after it.—And although the action of the sun and moon were to cease, yet the ocean would continue to ebb and flow for some time, as its waves continue in violent motion for some time after a storm. {14a}

Sir Isaac Newton has shown that the tides increase as the cube of the distances decrease, so that the moon, at half her present distance, would produce a tide eight times greater. Now the moon describes an ellipse about the earth, and of course must be once in every revolution nearer the earth than in any other part of her orbit; consequently she must produce a much higher tide when in this point of her orbit than in the opposite point. {14b}

This is the reason that two great spring tides never take place immediately after each other; for if the moon be at her least distance at the time of new moon, she must be at her greatest distance at the time of full moon, having performed half a revolution in the intervening time; and, therefore, the spring tide at the full will be much less than at the preceding change. For the same reason, if a great spring tide happens at the time of full moon, the tide at the following change will be less. {14c}

The spring tides are highest and the neap tides lowest about the beginning of the year; for the earth being nearest the sun about the first of January, must be more strongly attracted by that body than at any other time of the year: hence the spring tides which happen about that time, will be greater than at any other time, and should the moon be new or full in that part of her orbit, which is nearest to the earth at the same time, the tides will be considerably higher than at any other time of the year.

The tide which happens at any time while the moon is above the horizon, is called the superior tide, and when below the horizon, the inferior. When the moon is in the equinoctial, the superior and inferior tides are of the same height, but when the moon declines towards the elevated pole, the superior tide is higher than the inferior. If the latitude of the place and the declination of the moon are of contrary names, the inferior tides will be the highest. But the highest tide at any particular place is when the moon’s declination is equal to the latitude of the place, and of the same name, and the height of the tide diminishes as the differences between the latitude and declination increases, therefore the nearer any place is to that parallel whose latitude is equal to the moon’s declination and of the same name, the higher will be the tide at that place. In comparing the height of tides at different places, it is supposed that the sun and moon are at the same distances from the earth, and in the same position with respect to the meridian of these places. {15a}

The above observations relative to the regularity of the tides could only result by supposing the earth to be covered with the waters of the ocean to a great depth, but as this is not the case, it is only at places situated on the shores of large oceans where such tides exist. {15b}

From local circumstances the tides are subject to great irregularities, such as meeting with islands, headlands, passing through straits, &c. In order that they may have their full motion, the ocean in which they are produced ought to extend 90° from east to west, because that is the distance between the greatest elevation and the greatest depression produced in the waters by the moon.

Hence it is that the tides in the Pacific Ocean exceed those of the Atlantic, and that they are less in that part of the Atlantic which is within the torrid zone between Africa and America, than on the temperate zones on either side of it where the ocean is much broader. {16a}

Tides are not perceptible in lakes and most inland seas, and deep and extensive as is the Mediterranean, are scarcely sensible to ordinary observation, their effects being quite subordinate to the winds and currents. In some places, however, as in the Straits of Messina, there is an ebb and flow to the amount of two feet and upwards; at Naples and at the Euripus, of twelve and thirteen inches, and Rennell informs us, at Venice, of five feet. {16b}

The ebb and flow of the ocean is very slight in islands remote from any continent, as for example, at St. Helena, where it seldom exceeds three feet. Tides are remarkably high on the coasts of Malay, in the Straits of Sunda, on the open coast of Patagonia, along the coasts of China and Japan, at Panama, in the Gulph of Bengal, and at the mouth of the Indus, where the water rises thirty feet in height. Tides are greatest in any given line of coast, in narrow bays and estuaries; and are least in the intervening tracts where the land is prominent. {16c}

On the authority of the late Captain Hewett, R.N., at the entrance of the estuary of the Thames, the rise of the spring tides is eighteen feet; but when we follow our eastern coast from thence northward; towards Lowestoft and Yarmouth, we find a gradual diminution, until at the place last mentioned the highest rise is only seven or eight feet. From this point there begins again to be an increase, so that at Cromer, where the coast again retires towards the west, the rise is sixteen feet; and towards the extremity of the gulph called “the Wash,” as at Lynn and in Boston Deeps, it is from twenty-two to twenty-four, and in some extraordinary cases, twenty-six feet. From thence again there is a decrease towards the north; the elevation at the Spurn Point being from nineteen to twenty feet, and at Flamborough Head, on the Yorkshire coast, from fourteen to sixteen feet.

It is also recorded, on the authority of Captain Beaufort, R.N., that at Milford Haven, in Pembrokeshire, at the mouth of the Bristol Channel, the tides rise thirty-six feet, and at King-road, near Bristol, forty-two feet. At Chepstow, on the Wye, a small river which opens into the estuary of the Severn, they reach fifty feet, sometimes sixty-nine, and even seventy-two feet. {17}

The tides at Tonquin are the most remarkable in the world. In this part there is but one tide and one ebb every twenty-four hours; whereas in other places there are two. Besides 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 other peculiar appearances attending the same phenomena, were considered by many as inscrutable; but Sir Isaac Newton 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 successively two every day; two at one time greater, and two at another that are less. The time between the arrival of the two greater is considered by him as high tide; the time between the two lesser as ebb. In short, with this clue that great mathematician solved every appearance, and so established his theory as to silence every opposer. This fluctuation of the sea from the tides, observes the same author, produces another and more constant rotation of its waters from the east to the west, in this respect following the course of the moon.

This may be considered as one great and general current of the waters of the sea; and although it be not every where distinguishable, it is nevertheless every where existent, except when opposed by some particular current or eddy produced by partial and local causes. This tendency of the sea towards the west is plainly perceivable in all the great straits of the ocean; as for instance, in those of Magellan, in South America, where the tide running in from the east nearly twenty feet high, and continues flowing six hours, whereas the ebb continues but two hours, and the current is directed to the west. This proves that the flux is not equal to the reflux, and that from both results a motion of the sea westward, which is more powerful during the time of the flux than the reflux. This motion westward has been sensibly observed by navigators in their passage back from India to Madagascar, and so on to Africa. In the great Pacific, also, it is very perceivable; but the places where it is most obvious are, as it was said, in those straits which join one ocean to another. In the straits between the Maldivia Islands, in the gulph of Mexico, between Cuba and Jucatan. In the straits in the gulph of Paria, the motion is so violent, that it has received the appellation of the Dragon’s Mouth. Northward, in the sea of Canada, in Waigat’s straits, in the straits of Java, and in short, where the ocean on one part pours into the ocean on the other. In this manner is the sea carried with an unceasing circulation round the globe, and at the same time that its waters are pushed backward and forward with the tide; they have thus a progressive current to the west, which, though less observable, is not the less real. {19}