TIDES, two periodical motions of the waters of the sea, called the flux and re flux, or the flow and ebb. The cause of the tides is the attraction of the sun and moon, but chiefly of the latter; the waters of the immense ocean, forgetful, as it were, of their natural quietus, move and roll in tides, obsequious to the strong at tractive power of the moon, and weaker influence of the sun. See ASTRONOMIC. That the tides may have their full motion, the ocean inwhich they are produced ought to be' extended from east to west 90°, or a quarter of a great circle of the earth, at least; because the places where the moon rises most, and most depresses the water, are at that distance from one an other. Hence it appears, that it is only in the great oceans that such tides can be produced; and why, in the large Pacific ocean, they exceed those in the Atlantic ocean; hence also it is obvious, why the tides are not so great in the torrid zone, between Africa and America, where the ocean is narrower, as in the temperate zones on either side; and from this, also, we may understand why the titles are so small in islands that arc very far distant from the shores. It is manifest, that, in the Atlantic ocean, the water cannot rise on one shore but by descending on the other; so that, at the intermediate distant islands, it must continue at about a mean height between its elevation on the one and on the other shore. As the tides pass over shoals, and run through straights into bays of the sea, their motion becomes more va rious, and their height depends on a great many circumstances. The tide that is pro duced on the western coast of Europe corresponds to the theory above describ ed: thus, it is high water on the coast of Spain, Portugal, and the West of Ireland, about the third hour after the moon has passed the meridian: from thence it flows into the adjacent channels, as it finds the easiest passage. One current from it, for example, runs up 'by the south of England, and another comes in by the north of Scot land: they take a considerable time to move all this way, and it is high water sooner in the places to which they first come; and it begins to fall in those places, while the two currents are yet going on to others that are further in their course As they return, they are not able to raise a tide; because the water rams faster off than it returns, till, by a new tide propa gated from the ocean, the return of the cm-rent is stopped, and the water begins to rise again. The tide takes twelve hours
to collie trot n the ocean to London bridge; so that, when it is high water there, a new tide is already come to its height in the ocean; and, in some intermediate place, it most be low water at the same time. In channels, therefore, and narrow seas, the progress of the tides may be, in some respects, compared to the motion of the waves of the sea. It may be observed, that when the tide runs over shoals, and flows upon fiat shores, the water is raised to a greater height than in the open and deep oceans that have steep banks; be cause the force of its motion cannot be broken, upon these level shores, till the water rises to a greater height. If a place communicates with two oceans (or two dif ferent with the same oceim, one of which is a readier and easier passage) two tides may arrive at that place in different times, which, interfering with each other, may produce a greater variety of pheno mena.
An extraordinary instance of this kind is mentioned at Bathsha, a port in the kingdom of Tonquin in the East Indies, of northern latitude 20° 50'. The day the moon passes the equator, the water stagnates there without any motion: as the moon removes from the equator, the water begins to rise and fall once a day; and it is high water at the setting of the moon, and low water at her rising. This daily tide increases for about seven or eight days, and then decreases for as many days by the same degrees, till this motion ceases when the moon has return ed to the equator. When she has passed the equator, and declines towards the south pole, the water rises and tails again, as before; but it is high water now at the rising, and low water at the setting of the moon.
Ms tables, are those which set forth the times of high water at sundry places, as they fall on the days of the full and change of the moon. These are common in many almanacs, particularly in White's Ephemeris, Nautical Almanac, &c.
Tina waiters, or TmssmEN, are inferior officers belonging to the custom-house, whose employment it is to watch or attend upon ships, until the customs be paid: they get this name from their going on board ships, on their arrival in the mouth of the Thames or other port, and so come up with the tide.