I. ON THE NATURE OF TIDES (cont.)
2. General Description of Tidal Phenomena [Footnote 353-2]
High Water Retarded by 50m Per Diem
If we live by the sea or on an estuary, we see that the water rises and falls nearly twice a day; speaking more exactly the average interval from high water to high water is about 12h 25m, so that the average retardation from day to day is about 50m. The times of high water are then found to bear an intimate relation with the moons position. Thus at Ipswich high water occurs when the moon is nearly south, at London Bridge when it is south-west, and at Bristol when it is east-south-east. For a very rough determination of the time of high water it is sufficient to add the solar time of high water on the days of new and full moon (called the "establishment of the port") to the time of the moons passage over the meridian, either visibly above or invisibly below the horizon.
Variability of Interval After Moon's Transit
The interval between the moons passage over the meridian and high water varies sensibly with the moons age. From new moon to first quarter, and from full moon to third quarter (or rather from and to a day later than each of these phases), the interval diminishes from its average to a minimum, and then increases again to the average; and in the other two quarters it increases from the average to a maximum, and then diminishes again to the average.
Spring and Neap
The range of the rise and fall of water is also subject to great variability. On the days after new and full moon the range of tide is at its maximum, and on the day after the first and third quarter at its minimum. The maximum is called "spring tide" and the minimum "neap tide," and the range of spring tide is usually between two and three times as great as that of neap tide. At many ports, however, especially non-European ones, two successive high waters are of unequal heights, and the law of variability of the difference is somewhat complex; a statement of that law will be easier when we come to consider tidal theories. In considering any tide we find, especially in estuaries, that the interval from high to low water is longer than that from low to high water, and the difference between the intervals is greater at spring than at neap.
In a river the current continues to run up stream for some considerable time after high water is attained and to run down similarly after low water. Much confusion has been occasioned by the indiscriminate use of the term "tide" to denote a tidal current and a rise of water, and it has often been incorrectly inferred that high water must have been attained at the moment of cessation of the upward current.
Distinction of Rise and Fall from Flood and Ebb
The distinction between "rising and falling" and "flowing and ebbing" must be carefully maintained in rivers, whilst it vanishes at the seaboard. If we examine the progress of the tide-wave up a river, we find that high water occurs at the sea earlier than higher up. If, for instance, on a certain day it is high water at Margate at noon, it is high water at Gravesend at a quarter past two, and at London Bridge a few minutes before three. The interval from low to high water diminishes also as we go up the river; and at some distance up certain riversas, for example, the Severnthe rising water spreads over the flat sands in a roaring surf and travels up the river almost like a wall of water. This kind of sudden rise is called a "bore." [Footnote 353-3] In other cases where the difference between the periods of rising and falling is considerable, there are, in each high water, two or three rises and falls. A double high water exists at Southampton.
When an estuary contracts considerably, the range of tide becomes largely magnified as it narrows; for example, at the entrance of the Bristol Channel the range of spring tides is about 18 feet, and at Chepstow about 50 feet.
Augmentation of Height in Estuaries
This augmentation of the height of the tide-wave is due to the concentration of the energy of motion of a large mass of water into a narrow space. At oceanic ports the tidal phenomena are much less marked, the range of tide being usually only 2 or 3 feet, and the interval from high to low water sensibly equal to that from low to high water. The changes from spring to neap tide and the relation of the time of high water to the moons transit remain, however, the same as in the case of the river tides.
In long and narrow seas, such as the English Channel, the tide in mid-channel follows the same law as at a station near the mouth of a river, rising and falling in equal times; the current runs in the direction analogous to up stream for three hours before and after high water, and down stream for the same period before and after low water. But near the sides of channels and near the mouths of bays the changes of the currents are very complex ; and near the headlands separating two bays there is usually at certain times a very swift current, termed a "race."
In inland seas, such as the Mediterranean, the tides are nearly insensible except at the ends of long bays. Thus at Malta the tides are not noticed by the ordinary observer, whilst at Venice they are conspicuous.
The effect of a strong wind on the height of tide is generally supposed to be very marked, especially in estuaries. In the case of an exceptional gale, when the wind veered round appropriately, Airy states [Footnote 354-1] that the water has been known to depart from its predicted height at London by as much as 5 feet. The effect of wind will certainly be different at each port. The discrepancy of opinion on this subject appears to be great,so much so that we hear of some observers concluding that the effect of the wind is insensible.
Variations in barometric pressure also cause departures from the predicted height of water, high barometer corresponding to decrease of height of water. Roughly speaking, an inch of the mercury column will correspond to something less than a foot of water, but the effect seems to vary much at different ports. [Footnote 354-2]
353-2 Founded on Airy's "Tides and Waves," in Ency. Metrop.
353-3 See a series of papers bearing on this kind of wave by Sir W. Thomson, in Phil. Mag., 1886-87.
354-1 Airy, "Tides and Waves."
354-2 Airy, op. cit., §§ 572-573.
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