Tides and Tidal Streams. - In navigating coasts where the tidal range is considerable, caution is always necessary. It should be
remembered that there are indraughts to all bays and bights, although the general run of the stream may be parallel to the shore.
The turn of the tidal stream off-shore is seldom co-incident with the time of high and low water on the shore. In open channels, the tidal
stream ordinarily overruns the turn of the vertical movement of the tide by about three hours, forming what is usually known as tide and half-tide,
the effect of which is that at high and low water by the shore the stream is running at its greatest velocity.
In crossing a bar or shallow flats, "Table B, for finding the height of tide at any intermediate hour between high and low water", and
diagrams, given in the Tide Tables, will be found of great assistance in calculating how much the water has risen or fallen at any hour of
the tide.
On coasts where there is much diurnal inequality in the tides, the amount of the rise and fall can never be depended upon, and additional
caution is necessary.
It should also be remembered that at times the tide falls below the level of low-water ordinary springs. This always occurs on the coasts of
Europe at the equinoxes, but in other parts of the world, and especially in the tropics, such periodic low tides may coincide more frequently
with the solstices. Wind or a high barometer may produce it at any time, and the amount varies with locality. When the moon's perigee co-incides
with the full or new moon the same effect is often produced.