IN the open expanse of the Southern Ocean, as well as over a large portion of the Pacific, the tidal wave 1 rarely exceeds five or six feet in height, and in the Indian and Atlantic Oceans perhaps eight or ten. In bays and gulfs, however, opening broadly to its course, and narrowing towards their interior recesses-such as the Bay of Bengal, the Bristol Channel, and the Bay of Fundy in America-it may rise to twenty, thirty, or, under favourable circumstances of wind and season, even to fifty or sixty feet in height! And where such seas terminate in river-estuaries, the wave, still converging, forms a high heador wall of water, termed a Bore , which ascends the river withsudden and destructive force.
Tidal bores of considerable magnitude occur in many rivers, such as the Severn, Garonne, Amazon, and Hoogly; but that of the Tsien-tang in China appears to excel them all in grandeur, especially at spring-tides and during strong easterly gales. Dr.Macgowan gives the following graphic account of it in the Transactions of the Royal Asiatic Society : -"Between the river and the city walls, which are a miledistant, dense suburbs extend for several miles along the banks.As the hour of flood-tide approached, crowds gathered in the streets running at right angles with the Tsien-tang, but at safe distances. My position was a terrace in front of the Tri- wave Temple, which afforded a good view of the entire scene.
"On a sudden all traffic in the thronged mart was suspended; porters cleared the front street of every description of merchandise, boatmen ceased lading and unlading their vessels, and put out into the middle of the stream, so that a few minutes sufficed to give a deserted appearance to the busiest part of one ofthe busiest cities of Asia. The centre of the river teemed with craft, from small boats to large barges, including the gay "flower-boats".
"Loud shouting from the fleet announced the appearance ofthe flood, which seemed like a glistening white cable stretched athwart the river at its mouth, as far down as the eye could reach. Its noise, compared by Chinese poets to that of thunder, speedily drowned that of the boatmen; and as it advanced with great rapidity-at the rate, I should judge, of twenty- five miles an hour-it assumed the appearance of an alabaster wall, or rather of a cataract four or five miles across, and about thirty feet high, moving bodily onward! Soon it reached the advanced guard of the immense assemblage of vessels awaiting its approach.
"Knowing that the Bore of the Hoogly-which scarcely deserves mention in connection with the one before me- invariably overturned boats which were not skilfully managed, I could not but feel apprehensive for the lives of the floating multitude. As the foaming wall of water dashed furiously onward they were silenced, all being intently occupied in keeping their prows towards the wave, which threatened to submerge everything afloat; but they all vaulted, as it were, to the summit with perfect safety.
"The spectacle was of greatest interest when the Bore had passed about half way among the craft. On one side they were quietly reposing on the surface of the unruffled stream, while those on the lower portion were pitching and heaving on the flood: others were scaling, with the agility of salmon, the formidable cascade.
"This grand and exciting scene was of but a moment"s duration, -it passed up the river in an instant; but, from this point, with gradually diminishing force, size, and velocity, until it ceased to be perceptible, at a distance, say the Chinese accounts, of eighty miles from the city. The change fromTHE TIDAL BORE OF THE TSIEN-TANGebb to flood-tide was almost instantaneous. A slight flood continued after the passage of the wave, but it soon began to ebb.
"A very short period elapsed between the passage of the Bore and the resumption of traffic. The vessels were soon attached to the shore again, and women and children were occupied in gathering articles which the careless or unskilful had lost in the confusion. The streets were drenched with spray, and a considerable volume of water splashed over the banks into the head of the Grand Canal? a few feet distant.""Tidal wave. -Tides are periodical swells in the ocean, produced by the attraction of the sun and the moon. The influence of the moon in producing the tides is seven times as great as that of the sun, owing to its nearness to the Earth. A great protuberance of water, forming a tidal wave, follows the moon round the Earth; and its crest marks flood-tide, or high- water, wherever it happens to be.
This would account for one tide each day; but there are two tides in twenty-four hours, -or rather in twenty-four hoursfifty minutes, the length of the lunar day; i.e. , the time that elapses between moon-rise and moon-rise at the same place. The two tidal waves are always at opposite sides of the globe, and as it were chase one another round the Earth.
The reason of this is, that while the moon draws the waters fromFIG. 1. -SPRING-TIDESFIG.2. -NEAP-TIDESthe Earth on the side nearest to it, it also draws the solid Earth away from the waters on the opposite side,and causes a corresponding protuberance there.
When the sun and the moon exert their influence in the same direction the tides are highest, and are called spring -tides. (See fig. 1) When they operateat right angles to each other, the force of the sun neutralizes tosome extent that of the moon, and the tides are lowest; then called neap-tides . (See fig. 2)QUESTIONSWhat is the usual height of the tidal wave in the open sea Where does it rise to a much greater height? What is it termed in narrow estuaries? Where do tidal bores occur? Which is the most remarkable? What change did its approach make on the streets? What did the boatmen do? At what rate did it advance? What appearance did it assume? When was the spectacle of greatest interest? How long did the scene last?
