Tides

Tides. The rise and fall of the water of the sea has naturally been observed during all time, and the cause of this ebb and flow has been the subject of much discussion. It is noticed that roUghly speaking, high and low water occur twice a day on the sea-coast, or, more accurately, high tide occurs 50 minutes later each day, the time between two high or two low tides being 12 hours 25 minutes. In about a week, then, the times of high and low water are reversed, while after a fortnight they are again as they were before. It is further found that the times of high water are connected with the position of the moon - that, in fact, high water occurs at a certain, but not quite constant, time after the moon has crossed the meridian. The greatest interval elapses between high water and the moon's meridian passage, when the moon is between her first and second quarter or between her third quarter and new moon. The least interval is a week before or after each of these. Not only does the water itself rise and fal1, but there is a difference in the height of the tide on different days. The highest tide obtains when the moon is new or full, and at this time low tide is also lowest. But a week later, when the moon is in her first or third quarter, the high tide reaches a far lower point, and the sea does not recede so far at low water. The maximum high tide is known as spring tide. and the minimum as neap tide; there is often a difference of many feet between the height of the water at these two different times. Although the interval between two successive high tides is 12 hours 25 minutes, this is not equally divided into the times of rising and falling. The sea takes longer to "go out" than to "come in," and this difference is more marked at spring than at neap tides, being more noticeable again in estuaries than at places overlooking the free ocean. The tide always extends for some distance up a river; it is felt on the rhames as far as Teddington, and, but for that lock, would be felt much higher; but the higher up we go the later is high water observed to occur. In some cases, too, the tide rises so fast that a tremendous up-rush of water takes place, often spreading over the river banks. This is exemplified by the Severn "bore." This naturally causes a higher range of tide; and we notice that at Chepstow the spring tide reaches 50 feet, while lower down, at the mouth of the channel, it is only 18 feet.

Two main theories have been propounded to account for the tides - one known as the equilibrium theory of Newton, and the other as the kinetic theory of Laplace. If the earth were completely covered by water, it would be heaped up in certain places by the action of the sun and moon. The moon attracts every drop of water to herself with a force proportional to her mass and inversely proportional to the cube of her distance from the partic1e. If M be the moon and A B C D the earth, the water at A will be more attracted to the moon than the earth, and will therefore become heaped up, while the earth will be more attracted than the water at C; so the water there will be left behind in another heap. At the points B and D the water will sink, so that if the earth and moon were at rest the water would have shaped itself into a prolate spheroid, the long axis of which would point towards the moon. At first sight, it would appear that the effect of the travelling moon would be to attract this liquid form after her, so that as she crossed the meridian of any place she would produce high water there and also on the opposite side of the earth. High tide would thus occur twice in every 24 hours 54 minutes - i.e. twice in every lunar day. But this high tide does not occur just when the moon crosses the meridian, and this alteration is due to the fact that the water does not simply follow the moon, but is thrown into a state of oscillation. The moon causes a wave, but she travels much faster than the wave can; she causes another, beats that in turn, and so on; thus a state of oscillation is set up in the ocean. In this case, however, there will not be the tendency for the water to be highest when the attraction to the moon is greatest. The oscillation has succeeded in inverting the apparently natural occurrence of the tides. It causes low tide to occur immediately under the moon and on the opposite side, while high tide occurs between. This is the reversal of the above figure, and we have now an oblate spheroid with the minor axis pointing to the rooon. The observation of actual fact seems to show that low water does more nearly agree with the time required by the oscillation theory than with that deduced from the theory of equilibrium. But great complications arise from the introduction of land into the problem, with its varied shape, and from our scanty knowledge of the effects of the varying depth of the sea.

The sun's influence is now to be considered. It may be roughly calculated that the influence of the moon on the tides is proportional to its mass, and inversely proportional to the cube of its distance. The influence of the sun can be expressed in the same way; but the sun's mass is about 2,700,000 times that of the moon, and his distance nearly 390 times as much. Hence his influence on the tides will be only 2,700,000/390³ times that of the moon. This is less than one-half. The sun, then, acting alone, would produce low water at noon and midnight. A combination of the two will give, as at new and full moon, the maximum effect - i.e. about 1-1/2 times what the moon would do alone; and at the first and third quarters the result will be a minimum, only about half the moon's single effect; for in this last case the sun and moon will be acting in opposition to each other, the moon trying to cause high water, while the sun is endeavouring to draw the tide out. This shows the origin of the spring and neap tides; only actual observation shows that the two do not differ as much as the above numbers would indicate. Another inequality is due to the fact that the moon does not travel in the equator; her zenith and nadir distances are not the same for consecutive passages of the meridian on the opposite sides of the earth, and this causes the tides to be of different heights. This alteration in the heights of swelling lunar tides is known as a diurnal tide, and vanishes every fortnight, when the moon crosses the equator. For the same reason, two consecutive solar tides differ in summer and winter, when the sun is farthest from the equator, but are of equal height, in spring and autumn. The ealth's different distances from the sun in different parts of her orbit give rise to a further modification known as the semi-annual tide. Local peculiarities naturally produce deviations from the ordinary laws of the tides. Thus, in the Mediterranean, a sea so shut in from the ocean, the tide is hardly felt at all, except in some of the long and narrow bays. Like the bore in the Severn's estuary, the sea rises tremendously in the Bay of Bengal. Colombo, in Ceylon, experiences four tides a day; while at Pampiete, in the Society Islands, high tide occurs regularly at 2 pm. every day. The average time of retardation of high water at any place is known as the "establishment of the port," and is equal to the time of high water when the moon is either new or full. When a large wave is started by an earthquake, it is often erroneously called a tide; and the word is often applied too, to the rise and fall of the sea, produced by definite land and sea breezes and other meteoric phenomena. These are however, not true tides, for they are not the result of the attraction of the sun and moon, to whose action the word should be strictly limited.

Tidal instruments are for noting and predicting the tides in different places. One form of tide-gauge consists of a float in a tank or pipe filled by water coming from the sea by an inlet below low water. Thus the water in the pipe rises and falls with the sea. The float is supported by a wire connected to a wheel, and the motion of the float is communicated to a fine pencil, which rests against and marks a drum, the latter being caused to rotate once in twenty-four hours; a record of the tide is thus made on the drum. Sir William Thomson's instrument for the prediction of tides consists of a drum, moving proportionally to mean solar time,and marked by a pencil. This pencil is connected to a number of pulleys, each pulley having an harmonic motion and corresponding to one of the lunar or solar tides which have been found by harmonic analysis of the curves given by the tide-gauge. The pencil then produces on the drum the added effect of these different components, and this is known as a tide-curve. From the tide-curve at any place the height of the tide on any day can be predicted, a result very useful in navigation.