Oceanic tides are true waves, not currents. Coast tides are currents. The momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height--in some places up to 50 or 60 feet, or even more. Early observed connections between moon and tides would be these:-- 1st. Spring tides at new and full moon. 2nd. Average interval between tide and tide is half a lunar, not a solar, day--a lunar day being the interval between two successive returns of the moon to the meridian: 24 hours and 50 minutes. 3rd. The tides of a given place at new and full moon occur always at the same time of day whatever the season of the year. LECTURE XVII THE TIDES Persons accustomed to make use of the Mersey landing-stages can hardly fail to have been struck with two obvious phenomena. One is that the gangways thereto are sometimes almost level, and at other times very steep; another is that the water often rushes past the stage rather violently, sometimes south towards Garston, sometimes north towards the sea. They observe, in fact, that the water has two periodic motions--one up and down, the other to and fro--a vertical and a horizontal motion. They may further observe, if they take the trouble, that a complete swing of the water, up and down, or to and fro, takes place about every twelve and a half hours; moreover, that soon after high and low water there is no current--the water is stationary, whereas about half-way between high and low it is rushing with maximum speed either up or down the river. To both these motions of the water the name _tide_ is given, and both are extremely important. Sailors usually pay most attention to the horizontal motion, and on charts you find the tide-races marked; and the places where there is but a small horizontal rush of the water are labelled "very little tide here." Landsmen, or, at any rate, such of the more philosophic sort as pay any attention to the matter at all, think most of the vertical motion of the water--its amount of rise and fall. Dwellers in some low-lying districts in London are compelled to pay attention to the extra high tides of the Thames, because it is, or was, very liable to overflow its banks and inundate their basements. Sailors, however, on nearing a port are also greatly affected by the time and amount of high water there, especially when they are in a big ship; and we know well enough how frequently Atlantic liners, after having accomplished their voyage with good speed, have to hang around for hours waiting till there is enough water to lift them over the Bar--that standing obstruction, one feels inclined to say disgrace, to the Liverpool harbour. [Illustration: FIG. 108.--The Mersey]