1. =Tides.=--The tides or “tidal waves” are caused by the attraction of the moon and the sun. The phenomena are complex, and a full discussion of their causes need not be given here. When the tide rises it is said to “flow,” and it is called the flood tide; when it falls it is called the ebb tide. The period between one tide and the next, _e.g._ from high water to high water, is about twelve hours, twenty-five minutes. At a spring tide the range of the tide is greater than usual; at a neap tide less. Where there are channels, as, for instance, the seas which surround the British Isles, the tidal waves run up them as the tide rises in the neighbouring ocean, and run back as it falls. At some places, as Southampton, the tide comes in from two directions, and there is a double tide. The times and levels of high and low water at various places have been ascertained by observation, and are recorded. The levels are, however, liable to be affected by winds. A wind blowing towards the shore raises the level of both high and low water; a wind blowing off shore lowers both levels. A severe storm in the North Sea has caused a double tide at London Docks, by accelerating the North Sea tidal wave. In a funnel-shaped estuary, especially if it faces the direction of the tidal wave in the sea, the tide in going up the channel increases in velocity, and the momentum of the water causes it to rise higher and higher as the width decreases. At the upper end of the Bristol Channel the range of the tide is double the range in the sea outside the channel. The Bay of Fundy is another place where a similar phenomenon occurs. When a river or estuary is shallow and the range of the tide is great, so that its rise is rapid, the flood tide in some cases advances in the form of a wave or “bore,” causing a sudden rise in the water-level and a sudden reversal of the flow of the stream. A bore is most pronounced at spring tides. That of the Severn is well known. In the case of a tide running along a coast or up an estuary, the water of the flood tide, after it has ceased to rise, continues for a short time, owing to its momentum, to flow in the same direction as before. The same thing happens when the ebb tide ceases to fall. The tide also acquires special velocity, just as a river does, round any projecting headland. The rise and fall of the tide are least rapid near the turns of the tide. If the time from the beginning to the end of the flow be divided into six equal parts, the proportional rise of the water will be approximately as follows. And similarly with the fall during the ebb. Time 1 2 3 4 5 6 Rise of water ·067 ·25 ·5 ·75 ·933 1